<PAGE>   1



                                                               EXHIBIT NO.10.43


                               OFFICE SPACE LEASE

                                 by and between

                                MONUMENT ONE LLC

                                  as Landlord,

                                      and

                          E.SPIRE COMMUNICATIONS, INC.

                                   as Tenant



                            Fairfax County, Virginia





                                        
<PAGE>   2
TABLE OF CONTENTS


<TABLE>
<S>     <C>                                                                                    <C>
SCHEDULE  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   1

1.       DEMISE AND TERM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
         A.      Term.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
         B.      Delays.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   2
         C.      Partial Substantial Completion.  . . . . . . . . . . . . . . . . . . . . . . .   3
         D.      Extended Term. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
         E.      Prior Occupancy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5

2.       RENT.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
         A.      Definitions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
         B.      Components of Rent.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
         C.      Payment of Rent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
         D.      Allocation of Rent Abatement for Tax Purposes. . . . . . . . . . . . . . . . .  11

3.       USE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  11

4.       CONDITION OF PREMISES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  12

5.       BUILDING SERVICES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
         A.      Basic Services.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
         B.      Electricity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
         C.      Telephones.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
         D.      Additional Services. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
         E.      Failure or Delay in Furnishing Services. . . . . . . . . . . . . . . . . . . .  13
         F.      Tenant's Option to Provide Electricity and Char Service. . . . . . . . . . . .  13
         G.      Excess Use of Utilities. . . . . . . . . . . . . . . . . . . . . . . . . . . .  14

6.       RULES AND REGULATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  15

7.       CERTAIN RIGHTS RESERVED TO LANDLORD. . . . . . . . . . . . . . . . . . . . . . . . . .  15

8.       MAINTENANCE AND REPAIRS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
         A.      General Obligations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
         B.      Noise and Vibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  16

9.       ALTERATIONS; SIGNS.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  16
         A.      Requirements.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  16
         B.      Liens. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
         C.      Signs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
         D.      Building Signage.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  17

10.      INSURANCE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
         A.      Tenant's Insurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
         B.      Landlord's Insurance.  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  19
         C.      Risk of Loss.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  19

11.      TENANT'S AND LANDLORD'S RESPONSIBILITIES.  . . . . . . . . . . . . . . . . . . . . . .  19
         A.      Tenant's Responsibilities  . . . . . . . . . . . . . . . . . . . . . . . . . .  19
         B.      Landlord's Responsibilities. . . . . . . . . . . . . . . . . . . . . . . . . .  20

12.      FIRE OR OTHER CASUALTY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
</TABLE>

                                        
                                       i
                                         
<PAGE>   3

<TABLE>
<S>      <C>                                                                                     <C>
         A.      Destruction of the Building 20
         B.      Destruction of the Premises. . . . . . . . . . . . . . . . . . . . . . . . . .  20

13.      CONDEMNATION.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  21

14.      ASSIGNMENT AND SUBLETTING  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
         A.      Landlord's Consent.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
         B.      Standards for Consent. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
         C.      Recapture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  23
         D.      Assignment or Subletting to Affiliates.  . . . . . . . . . . . . . . . . . . .  23
         E.      Miscellaneous. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  23

15.      SURRENDER. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  24

16.      DEFAULTS AND REMEDIES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  24
         A.      Default. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  24
         B.      Right of Re-Entry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  24
         C.      Termination of Right to Possession.  . . . . . . . . . . . . . . . . . . . . .  25
         D.      Termination of Lease.  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  25
         E.      Other Remedies.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  25
         F.      Bankruptcy.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  25
         G.      Waiver of Trial by Jury. . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
         H.      Venue. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  26

17.      HOLDING OVER.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  26

18.      SECURITY DEPOSIT.    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
         A.      Amount.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
         B.      Security.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
         C.      Form.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
         D.      Right to Draw. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  27
         E.      Right to Pledge or Assign. . . . . . . . . . . . . . . . . . . . . . . . . . .  28
         F.      Reservation of Rights. . . . . . . . . . . . . . . . . . . . . . . . . . . . .  28
         G.      Reduction of Security Deposit. . . . . . . . . . . . . . . . . . . . . . . . .  28
         H.      Return of Security Deposit.  . . . . . . . . . . . . . . . . . . . . . . . . .  28

19.      INTENTIONALLY OMITTED. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  28

20.      ESTOPPEL CERTIFICATE.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  29

21.      FINANCING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  29
         A.      Subordination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  29
         B.      Mortgagee Requirements.  . . . . . . . . . . . . . . . . . . . . . . . . . . .  29

22.      QUIET ENJOYMENT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  30

23.      BROKER.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  30

24.      NOTICES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  30

25.      PARKING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  31

26.      MISCELLANEOUS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  31
         A.      Successors and Assigns.  . . . . . . . . . . . . . . . . . . . . . . . . . . .  31
         B.      Entire Agreement.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  31
</TABLE>



                                       ii
<PAGE>   4

<TABLE>
<S>      <C>                                                                                     <C>
         C.      Time of Essence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  31
         D.      Execution, Delivery and Authority. . . . . . . . . . . . . . . . . . . . . . .  31
         E.      Severability.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
         F.      Governing Law. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
         G.      Attorneys' Fees. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
         H.      Joint and Several Liability. . . . . . . . . . . . . . . . . . . . . . . . . .  32
         I.      Force Majeure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
         J.      Captions.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
         K.      No Waiver. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  32
         L.      Limitation of Liability; Effect of Sale. . . . . . . . . . . . . . . . . . . .  32
         M.      No Partnership.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  33
         N.      Tenant's Duty to Mitigate. . . . . . . . . . . . . . . . . . . . . . . . . . .  33
         O.      Compliance with Laws as of Commencement Date.  . . . . . . . . . . . . . . . .  33
         P.      Environmental Representation.  . . . . . . . . . . . . . . . . . . . . . . . .  33

27.      ACCESS RIGHTS TO ADDITIONAL MONUMENT BUILDINGS.  . . . . . . . . . . . . . . . . . . .  33

28.      AVAILABLE SPACE IN ADDITIONAL MONUMENT BUILDINGS.  . . . . . . . . . . . . . . . . . .  34

29.      ANTENNA RIGHTS.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
         A.      Scope of Rights. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
         B.      Compliance with Law. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
         C.      Installation Procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . .  34
         D.      Interference.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  36
         E.      Maintenance and Removal of the Antenna Equipment.  . . . . . . . . . . . . . .  36
         F.      Indemnification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  37
         G.      Taxes  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  37
         H.      Miscellaneous  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  37

30.      ACCESS RIGHTS FOR CABLING COMMUNICATIONS EQUIPMENT.  . . . . . . . . . . . . . . . . .  37
</TABLE>


                                      iii
<PAGE>   5
                               OFFICE SPACE LEASE

         THIS OFFICE SPACE LEASE ("Lease"), a deed of lease, is made as of the
_____ day of _____, 1998, between MONUMENT ONE LLC, A DELAWARE LIMITED
LIABILITY COMPANY ("Landlord"), and E.SPIRE COMMUNICATIONS, INC. ("Tenant"),
for space in the building commonly known as Monument I which is located at
12975 Worldgate Drive, Herndon, Fairfax County, Virginia 20765 (such building,
together with the land upon which it is situated (which land is depicted on
EXHIBIT A-1) and common areas, including, without limitation, all sidewalks,
parking areas and landscaped areas, being herein referred to as the
"Building").  The following schedule (the "Schedule") sets forth certain basic
terms of this Lease:

                                    SCHEDULE
<TABLE>
<S>                                       <C>  
1.  PREMISES - FLOORS:                     The entire rentable square
                                           feet on each floor of the
                                           Building (i.e., floors 1
                                           through 7).
   
2.  COMMENCEMENT DATE:                     March 1, 1999.

2a. PHASE I COMMENCEMENT DATE:             March 1, 1999.

2b. PHASE II COMMENCEMENT DATE:            April 1, 1999.

3.  EXPIRATION DATE:                       February 28, 2009, subject to 
                                           adjustment as set forth in Section 
                                           1.D hereof.

4.  RENTABLE SQUARE FEET OF THE PREMISES:  167,285, as determined in accordance
                                           with the BOMA Method of Measurement.

5.  RENTABLE SQUARE FEET OF THE BUILDING:  167,285, as determined in accordance
                                           with the BOMA Method of Measurement.

6.  BASE RENT:                             The base rent ("Base Rent")for each 
                                           Lease Year (being defined as each 
                                           consecutive twelve-month period 
                                           beginning on the Commencement Date)
                                           during the Term shall be as follows:
</TABLE>


<TABLE>
<CAPTION>
                                                       ANNUAL                                       
                                           LEASE     BASE RENT        ANNUAL           MONTHLY      
                                           YEAR      PER SQ FT       BASE RENT        BASE RENT     
                                            <S>        <C>         <C>               <C>
                                             1         $28.38      $4,747,548.36     $395,629.03    
                                                                                                    
                                             2         $29.09      $4,866,320.64     $405,526.72    
                                                                                                    
                                             3         $29.82      $4,988,438.76     $415,703.23    

                                             4         $30.56      $5,112,229.56     $426,019.13    
                                                                                                    
                                             5         $31.33      $5,241,039.00     $436,753.25    
                                                                                                    
                                             6         $32.11      $5,371,521.36     $447,626.78    

                                             7         $32.91      $5,505,349.32     $458,779.11    
                                                                                                 
                                             8         $33.73      $5,642,523.00     $470,210.25
                                                                                                    
                                             9         $34.58      $5,784,715.32     $482,059.61    

                                            10         $35.44      $5,928,580.44     $494,048.37    
</TABLE>                                   


7.  TENANT'S PROPORTIONATE SHARE:          100%
<PAGE>   6
8.  SECURITY DEPOSIT:                      $791,258.06.

9.  BROKER(S):            LANDLORD'S:      Millennium Realty Advisors LLC
                          TENANT'S:        CB Richard Ellis, Inc.

10.  EXHIBITS:                             A.     Floor Plans
                                           A-1    Description of Land
                                           B.     Work Agreement
                                           B-1    Modified Shell Definition
                                           C.     Rules and Regulations
                                           D.     Estoppel Certificate
                                           E.     Intentionally Omitted
                                           F.     HVAC Specifications
                                           G.     Letter of Credit
                                           H.     Antenna Equipment and Location

11.  COMMENCEMENT DATE OF EXTENSION TERM:  First Extension Term - The first day
                                           after the Expiration Date (being
                                           March 1, 2009 if the Expiration Date
                                           is February 28, 2009).

                                           Second Extension Term - The first day
                                           after the expiration of the first 
                                           Extension Term (being March 1, 2012 
                                           if the Expiration Date of the First
                                           Extension Term is February 29, 2012).

12.  EXPIRATION DATE OF EXTENSION TERM:    First Extension Term - Three (3)
                                           years after the Commencement Date of
                                           the first Extension Term (being
                                           February 29, 2012 if the
                                           Commencement Date of the first
                                           Extension Term is March 1, 2009).

                                           Second Extension Term - Five (5) 
                                           years after the Commencement Date of 
                                           the second Extension Term (being 
                                           February 28, 2017 if the Commencement
                                           Date of the second Extension is Term 
                                           March 1, 2012).

1.       DEMISE AND TERM.

         A.      TERM.  This Lease shall be effective as of the date hereof.
Landlord leases to Tenant and Tenant leases from Landlord the premises (the
"Premises") described in ITEM 1 of the Schedule and shown on the plans attached
hereto as EXHIBIT A, subject to the covenants and conditions set forth in this
Lease, for a term (as it may be extended in accordance with the terms of this
Lease, the "Term") commencing on the date (the "Commencement Date") described
in ITEM 2 of the Schedule and expiring on the date (the "Expiration Date")
described in ITEM 3 of the Schedule, as may be extended in accordance with the
terms of Section 1.D below, unless terminated earlier as otherwise provided in
this Lease.  The Tenant Work (as defined in EXHIBIT B) to be constructed in the
Premises may be constructed in two (2) phases: Phase I (as hereinafter defined)
and Phase II (as hereinafter defined).  References herein to "any phase" shall
refer to either Phase I or Phase II, and references herein to "each phase"
shall refer to Phase I and Phase II.  The portion of the Premises located on
the 3rd, 4th, 5th 6th and 7th floors of the Building shall hereinafter be
referred to as "Phase I" and the portion of the Premises located on the 1st and
2nd floors of the Building shall hereinafter be referred to as "Phase II."

         B.      DELAYS.  If (i) the Phase I portion of the Tenant Work to be
built by Landlord pursuant to EXHIBIT B is not substantially completed by
Landlord on or before the Phase I Commencement Date or Landlord is otherwise
unable to deliver possession of the Phase I portion of the Premises to Tenant on
or before the Phase I Commencement Date for any reason or cause, or (ii) the
Phase II portion of the Tenant





                                       2
<PAGE>   7
Work to be built by Landlord pursuant to EXHIBIT B is not substantially
completed by Landlord on or before the Phase II Commencement Date or Landlord is
otherwise unable to deliver possession of the Phase II portion of the Premises
to Tenant on or before the Phase II Commencement Date, this Lease shall not be
void or voidable, nor shall Landlord or Landlord's agents and employees be
liable to Tenant for any loss or damage resulting therefrom, and, subject to the
abatement set forth in this Section 1.B, all the obligations of Tenant
hereunder, including, but not limited to, the obligations of Tenant to pay Base
Rent, Adjustment Rent and any other Rent payable hereunder shall not be delayed
and shall begin on the Commencement Date. Notwithstanding the foregoing, in the
event that (i) substantial completion of the Tenant Work on any floor of the
Phase I portion of the Premises is delayed beyond the Phase I Commencement Date
as a result of a Landlord Delay, or (ii) substantial completion of the Tenant
Work on any floor of the Phase II portion of the Premises is delayed beyond the
Phase II Commencement Date as a result of a Landlord Delay, then Tenant shall be
entitled to an abatement of one (1) day of Base Rent applicable to such floor
(the "Per Diem Base Rent") for each day of Landlord Delay applicable to such
floor (but in no event shall such number of days be more than the number of days
that substantial completion of such floor is delayed beyond the applicable
commencement date for such floor (i.e., the Phase I Commencement Date for any
floor in the Phase I portion of the Premises and the Phase II Commencement Date
for any floor in the Phase II portion of the Premises) as a result of a Landlord
Delay). The Per Diem Base Rent for any floor shall be (i) the rentable square
footage of such floor(s), times (ii) the per square foot rate payable hereunder
during Lease Year 1, as set forth in ITEM 6 of the Schedule, divided by (iii)
three hundred sixty-five (365). Substantial Completion of the Premises, or any
floor thereof, shall be determined as set forth in Paragraph 7.1 of the Work
Agreement.

         C.      PARTIAL SUBSTANTIAL COMPLETION.  In the event that any portion
of any floor(s) of the Premises is occupied by Tenant for the purpose of
conducting business therein (as opposed to preparing the Premises for Tenant's
use) prior to the applicable commencement date for such floor (i.e., the Phase
I Commencement Date for any floor in the Phase I portion of the Premises and
the Phase II Commencement Date for any floor in the Phase II portion of the
Premises), then with respect to such floor(s), Tenant's tenancy thereof shall
be deemed to be by the day and all of Tenant's obligations hereunder shall
commence as of the first day of such occupancy; provided, however, that for
each day prior to such applicable commencement date that Tenant occupies any
floor(s) of the Premises, Tenant shall be responsible for paying Base Rent only
for such floor(s) (such Base Rent to be calculated as the rentable square feet
of such floor(s) times the per square foot rate payable hereunder during Lease
Year 1 as set forth in ITEM 6 of the Schedule, pro rated for the number of days
in such tenancy).  The foregoing sentence shall not be deemed to imply a right
of Tenant to occupy any floor of the Premises prior to the date that the Tenant
Work for such floor is substantially complete and Landlord notifies Tenant
thereof nor an obligation that Tenant occupy any portion of the Premises prior
to the applicable commencement date for such portion of the Premises, nor shall
it be deemed an obligation of Landlord to deliver any portion of the Premises
prior to the applicable commencement date for such portion of the Premises.

         D.      EXTENDED TERM.

                 (i)      Landlord grants Tenant the options (together referred
         to as the "Extension Options," and individually sometimes referred to
         as an "Extension Option") to extend the Term for up to two (2)
         consecutive periods the first of which (the "First Extension Option")
         shall be for three (3) years (the "First Extension Term") and the
         second of which (the "Second Extension Option") shall be for five (5)
         years (the "Second Extension Term") (together sometimes referred to as
         the "Extension Terms").  The First Extension Term and the Second
         Extension Term shall commence upon the dates described in ITEM 11 of
         the Schedule and shall expire on the dates described in ITEM 12 of the
         Schedule subject to the covenants and conditions set forth in this
         Section.  Tenant shall have no right to an extension of the Term if at
         the time Tenant seeks to exercise the applicable Extension Option, or
         at the time the applicable Extension Term would have otherwise
         commenced, (a) Tenant has then assigned this Lease or is then
         subletting more than fifty percent (50%) of the Premises (other than
         to any Qualified Tenant Affiliates); and (b) Tenant's financial
         strength and general creditworthiness is less than the financial
         strength and general creditworthiness of Tenant as of the date hereof;
         provided, however, that the occurrence of the foregoing condition set
         forth in this Section 1.D(i) shall not void Tenant's exercise of the
         Extension Option if Tenant delivers to Landlord with its Exercise





                                       3
<PAGE>   8
         Notice (hereinafter defined) an additional Security Deposit in an
         amount equal to four (4) times the monthly Base Rent then in effect
         (which additional Security Deposit may be delivered in the form of
         cash or Letter of Credit (in the form required in Section 18) and
         shall be added to the original Security Deposit and be governed by the
         provisions of Section 18 hereof), or (iii) Tenant is in default under
         this Lease.  In addition, Tenant shall have no right to exercise the
         Second Extension Option unless the First Extension Option has been
         exercised and the Lease is otherwise in full force and effect.  To
         exercise the First Extension Option, Tenant shall give notice of its
         exercise ("Exercise Notice") to Landlord not earlier than twenty-four
         (24) months prior to the Expiration Date and not later than nine (9)
         months prior to the Expiration Date, and to exercise the Second
         Extension Option, Tenant shall give its Exercise Notice to Landlord
         not earlier than twenty-four (24) months prior to the expiration of
         the First Extension Term and not later than nine (9) months prior to
         the expiration of the First Extension Term.  If Tenant is entitled to
         and gives Landlord notice in accordance with the terms of this
         Section, the Term shall be extended for the period commencing on the
         Commencement Date of the applicable Extension Term and expiring on the
         Expiration Date of the applicable Extension Term and, except as set
         forth below in this Section, shall be on the same terms and condition
         as are set forth in this Lease.  Base Rent during the extended Term
         shall be the then-current (i.e., as of the commencement of the
         applicable Extension Term) market rent for first-class office
         properties in Herndon and Reston, Virginia of comparable quality and
         character to the Building built no later than the Building (the
         "Market Rate"), with subsequent escalations in Base Rent thereafter to
         be determined by market practice with respect to comparable space, as
         such Base Rent is reasonably determined by Landlord (and notice
         thereof delivered to Tenant); provided, however, Landlord shall have
         no obligation to deliver to Tenant notice of such Base Rent
         determination earlier than the later of the following dates (i) the
         date that is twelve (12) months prior to the Expiration Date in the
         case of a Base Rent determination in connection with the First
         Extension Option and twelve (12) months prior to the expiration of the
         First Extension Term in the case of a Base Rent determination in
         connection with the Second Extension Option, or (ii) the date that is
         forty-five (45) days after the date that Tenant delivers to Landlord
         the applicable Exercise Notice.  Further, the Base Year, as such term
         is hereinafter defined in Section 2.B.(ii) hereof, for the First
         Extension Term shall be the calendar year during which the
         commencement of the First Extension Term occurs and the Base Year for
         the Second Extension term shall be the calendar year during which the
         commencement of the Second Extension Term occurs.  The new Base Year
         shall be taken in consideration in determining the Market Rent.

                 (ii)     If Tenant disagrees with Landlord's determination of
         Base Rent for any Extension Term, Tenant shall give Landlord notice of
         objection within five (5) days after Landlord delivers to Tenant
         notice of Landlord's Base Rent determination; otherwise Landlord's
         determination shall be deemed conclusive.  If Tenant timely delivers
         to Landlord such notice of objection as provided above, then Landlord
         and Tenant shall negotiate in good faith to determine the amount of
         Base Rent within ten (10) days of the date of Landlord's receipt of
         Tenant's written notice of objection (the "Negotiation Period").

                 (iii)    In the event Landlord and Tenant are unable to agree
         upon the Base Rent for the applicable Extension Term within the
         Negotiation Period, then Tenant shall be entitled to elect to proceed
         with the binding arbitration process set forth below by delivering to
         Landlord written notice of such election within fifteen (15) days
         after the expiration of the Negotiation Period. If Tenant timely
         elects to proceed with binding arbitration, then the Base Rent for the
         applicable Extension Term shall be based upon the Market Rate for
         first-class office properties of comparable quality and character to
         the Building built no later than the Building in Reston and Herndon,
         Virginia, as determined by binding arbitration in accordance with the
         following procedures.  Within fifteen (15) days after Tenant delivers
         to Landlord notice of its election to proceed to binding arbitration,
         Landlord and Tenant shall each select a real estate broker (based on
         the criteria set forth in subsection 1.D. (iv) hereof).  Within twenty
         (20) days of their selection, each broker shall make a written
         determination of the Market Rate for the Extension Term.  All
         determinations of the Market Rate shall be in writing.  The party
         appointing each broker shall be obligated, promptly after receipt of
         the valuation report prepared by the broker appointed by such party,
         to deliver a copy of such valuation report to the other party.  If





                                       4
<PAGE>   9
         the Market Rate determination of the broker designated by Landlord is
         within five percent (5%) of the Market Rate determination of the
         broker designated by Tenant, then the Base Rent for the applicable
         Extension Term shall be the average of the two Base Rent
         determinations for the Extension Term.  If the Market Rate
         determinations of these two brokers vary by more than five percent
         (5%), then a third broker shall be selected by the initial two brokers
         within fifteen (15) business days after the initial two valuation
         reports have been delivered to the parties (the third broker also
         having the qualifications set forth in subsection 1.D. (iv) below).
         If a third broker is appointed, the third broker shall review the
         valuation reports of the initial two brokers and select the one of the
         initial two valuation reports that most closely reflects the Market
         Rate for the applicable Extension Term.  The third broker shall
         promptly deliver a report of his determination to each of the parties.
         The determination of the Market Rate for the applicable Extension Term
         pursuant to this subsection 1.D. (iv) hereof shall be final and
         binding upon Landlord and Tenant.  The expenses of each of the first
         two brokers appointed under this subsection 1.D. (iv) shall be borne
         by the party appointing such broker.  The expenses of the third broker
         appointed under this subsection 1.D. (iv) shall be paid one-half 
         (1/2) by Landlord and one-half ( 1/2) by Tenant.

                 (iv)     The real estate brokers selected by Landlord and
         Tenant shall have the following qualifications:  (i) must be a
         independent and licensed real estate broker in the Commonwealth of
         Virginia; (ii) must have a minimum of ten (10) years' experience in
         commercial office leasing in the Fairfax, Virginia area; (iii) must be
         an active broker in the Northern Virginia area and known for
         commercial office expertise; (iv) must have experience representing
         both landlords and tenants; (v) in the case of the third broker only,
         is not then representing either Landlord or Tenant; and (vi) in the
         case of the third broker only, shall not have been involved in any
         disputes with Landlord, Tenant or any of the other brokers.  In the
         event that real estate brokers with the qualifications described in
         this subsection 1.D. (iv) are unavailable, qualified consultants with
         similar qualifications may be substitutes.

                 (v)      An amendment modifying the Lease to set forth the
         Base Rent and new Base Year for the Premises during the applicable
         Extension Term shall be executed by Landlord and Tenant within ten
         (10) days of Landlord's determination thereof, of the parties'
         agreement thereto (if applicable) or of the determination of the Base
         Rent by the brokers pursuant to subsection 1.D. (iii) hereof.  In the
         event that (i) Tenant and Landlord fail to agree on the Base Rent for
         the Extension Term within the Negotiation Period, and Tenant does not
         timely elects to proceed with binding arbitration, or (ii) any of the
         conditions set forth in subsection 1.D. (i) are not satisfied, then
         this extension option shall be null, void and of no further force or
         effect and the Lease shall end on the date otherwise scheduled for
         expiration thereof, unless earlier terminated in accordance with the
         terms thereof. Tenant shall have no further right or option to extend
         the Term.  Time is of the essence with respect to this Section 1.D.

         E.      PRIOR OCCUPANCY.  Tenant and Tenant's contractors shall have
joint access (24 hours per day) to the Premises with Landlord and Landlord's
contractor prior to Landlord's delivery of possession of the Premises to Tenant
for the purpose of installing telephone, computer, other special equipment and
furniture, optic fibers, a telecommunications switch and to inspect the
progress of the Tenant Work in the Premises, provided that in Landlord's
judgement (i) such joint access will not cause unreasonable interference with
the work of Landlord's contractor; (ii) there are no delays created thereby in
the construction process of the improvements to be constructed by Landlord in
the Premises or the Building; (iii) an acceptable schedule of work is agreed
upon between Landlord's contractor and Tenant's contractors.  Any delay in
substantial completion of the Premises that results therefrom shall be deemed a
Tenant Delay.  Tenant shall indemnify, hold harmless, and defend Landlord from
any loss, damage, cost or expense (including attorneys' fees and court costs)
incurred by Landlord, whether before or after the applicable commencement date,
as a result of the performance of such work by Tenant or Tenant's contractor,
and in addition to any other rights Landlord may have as a result thereof,
Landlord shall have the right to apply all or a portion of the Improvement
Allowance (hereinafter defined) against any such loss,damage, cost or expense
incurred by Landlord as a result thereof.  In addition, Tenant shall not be
permitted to enter the Premises until Landlord has approved all furniture,
equipment and other items to be installed by Tenant or its contractor, as well
as the plans for such





                                       5
<PAGE>   10
installation, and Landlord and Tenant have agreed in writing to a schedule for
all of the foregoing work to be completed by Tenant or its contractor in the
Premises.  Access, if any, to the Premises by Tenant in accordance with the
foregoing and solely for the purposes set forth above shall not be deemed to
advance the applicable commencement date.

         F.      EXTENDED DELAY IN COMMENCEMENT DATE.  Notwithstanding anything
to the contrary contained in this Section 1 above, if (i) the Phase I portion
of the Tenant Work (as defined in EXHIBIT B) or the Phase II portion of the
Tenant Work to be built by Landlord pursuant to EXHIBIT B is not substantially
completed by Landlord on or before the date that is three hundred forty-eight
(348) days after the Award Date (as defined in EXHIBIT B hereof) applicable to
such phase for any reason or cause other than due to an event of Tenant Delay
(as defined in EXHIBIT B hereof) or Termination Force Majeure (as defined
below) or (ii) the Phase I portion of the Tenant Work or the Phase II portion
of the Tenant Work to be built by Landlord pursuant to EXHIBIT B is not
substantially completed by Landlord on or before the date that is five hundred
thirty-three (533) days after the Award Date applicable to such phase for any
reason or cause other than due to an event of Tenant Delay, then Tenant shall
have the right to terminate this Lease by delivering to Landlord, no later than
ten (10) days after such date, fifteen (15) days prior written notice of such
termination.  In the event Landlord substantially completes the applicable
phase of the Tenant Work on or before the expiration of such fifteen (15)-day
period, such right of termination shall be deemed to be void and without
effect.  In the event the applicable phase of the Tenant Work is not
substantially complete on or before the expiration of such fifteen (15)-day
period, this Lease shall immediately terminate, and all rights, obligations and
liabilities of the parties hereunder shall be released and discharged.  If any
phase of the Tenant Work (i.e., Phase I or Phase II) to be built by Landlord
pursuant to EXHIBIT B is not substantially completed by Landlord on or before
the date that is three hundred forty-eight (348) days after the Award Date
applicable to such phase, Landlord shall deliver to Tenant a good faith
estimate of the date that Landlord then anticipates that such phase of the
Tenant Work will be substantially completed.  The term "Termination Force
Majeure" shall mean acts of God, strikes, sabotage, accidents, acts of war,
fire and casualty, legal requirements, government restrictions or controls on
construction, insurance reimbursement problems or delays, emergencies,
shortages or inability to obtain labor, materials or equipment, energy
shortage, the failure of the applicable governmental authority to issue
building permits for the Tenant Work within twenty-one (21) days after Landlord
has submitted to the appropriate authority an application for such building
permit with the required documentation, the failure of the applicable
governmental authority to conduct inspections required in connection with the
Tenant Work (and issue its approval or disapproval thereof) within three (3)
business days after Landlord has submitted to the appropriate authority a
request for such inspection or the failure of the applicable governmental
authority to issue a certificate of occupancy for the Premises within five (5)
business days after Landlord has submitted to the appropriate authority an
application (which application shall have been submitted after receipt of the
approvals from all inspections that are required as a condition precedent to
submitting such application) for such certificate of occupancy, or any causes
beyond the reasonable control of Landlord.

2.       RENT.

         A.      DEFINITIONS.  For purposes of this Lease, the following terms
shall have the following meanings:

                 (i)      INTENTIONALLY OMITTED.





                                       6
<PAGE>   11
                 (ii)     "EXPENSES" shall mean all expenses, costs and
         disbursements (other than Taxes) paid or incurred by Landlord in
         connection with the ownership, management, maintenance, operation,
         replacement and repair of the Building, including, without limitation,
         exterior common areas.  Tenant acknowledges that the Building is
         located in the Worldgate Corporate Park and that certain expenses
         ("Worldgate CAM Charges") associated with the maintenance, security
         and/or operation of certain portions of the Worldgate Corporate Park
         will be shared by various owners of property in the Worldgate
         Corporate Park.  Accordingly, for purposes of this Lease "Expenses"
         shall also include the Landlord's or the Building's share of any
         Worldgate CAM Charges and any other costs, charges or assessments
         incurred by Landlord or charged against the Building for the common
         maintenance, security or operations of one or more buildings in the
         Worldgate Corporate Park or any other expenses shared by the owners of
         one or more buildings in the Worldgate Corporate Park.  Expenses shall
         not include: (a) costs of tenant alterations; (b) costs of capital
         improvements (except for costs of any capital improvements ("Permitted
         Capital Improvements") (1) made or installed for the purpose of
         reducing Expenses (provided, however, if any such capital improvement
         is anticipated to cost more than $500,000, then Landlord shall give
         Tenant notice thereof and if Tenant delivers to Landlord within
         fifteen (15) days after such notice a report from a Virginia licensed
         engineer demonstrating that such capital improvement is not likely to
         reduce Expenses, then such improvement shall be considered a Permitted
         Capital Improvement only if Landlord obtains and delivers to Tenant a
         report from an engineer approved by Landlord and Tenant stating that
         it is likely that such capital improvement will reduce Expenses), or
         (2) made or installed pursuant to governmental requirement or
         insurance requirement (provided, however, in no event shall Expenses
         include in any calendar year more than $40,000.00 for the amortization
         of any Permitted Capital Improvement made pursuant to any governmental
         requirement or any insurance requirement), which Permitted Capital
         Improvements shall be amortized by Landlord in accordance with any
         method permitted for federal tax purposes or generally accepted
         accounting principles); (c) interest and principal payments on
         mortgages (except interest on the cost of any capital improvements for
         which amortization may be included in the definition of Expenses) and
         penalties and late charges related thereto; (d) advertising expenses
         and leasing commissions; (e) any cost or expenditure for which
         Landlord is reimbursed, whether by insurance proceeds or otherwise,
         except through Adjustment Rent (hereinafter defined); (f) the cost of
         any kind of service furnished to any other tenant in the Building
         which Landlord does not generally make available to all tenants in the
         Building; (g) legal expenses of negotiating leases; (h) salaries and
         fringe benefits of employees above the grade of building manager; (i)
         management fees in excess of those customarily charged for similar
         types of first-class buildings in Fairfax County, Virginia; (j)
         depreciation expenses on any fixed assets (except as set forth above
         in connection with costs of capital improvements which may be included
         in the definition of Expenses); (k) cost of correcting any defect
         (excluding standard repairs and replacements resulting from ordinary
         wear and tear, use, fire, casualty, or vandalism) in the physical
         condition of the Building resulting from the original construction of
         the Building, provided that, except with respect to a Latent Building
         Defect, such corrections are made, or Tenant delivers to Landlord
         written notice thereof, within the first Lease Year; (l) Taxes; (m)
         Landlord's general corporate overhead and general administrative
         expenses relating exclusively to maintaining Landlord's existence,
         either as a corporation, partnership or other entity, such as annual
         fees, partnership organization and administration expenses and deed
         recordation expenses; (n) the cost of any repair made by Landlord
         because of a condemnation of the Building (or any part thereof), but
         only to the extent Landlord is reimbursed therefor from condemnation
         proceeds; (o) the profit portion of any amount paid to a corporation,
         entity, or person which controls, is controlled by, or is in common
         control with Landlord for goods or services, to the extent such amount
         is not reasonably comparable to the amount paid for similar goods or
         services provided to first-class office buildings in the Reston and
         Herndon, Virginia area providing services similar to, and to the same
         level as, those provided for the Building (for purposes of this item,
         "control" shall be deemed to be ownership of more than twenty-five
         percent (25%) of the legal and equitable interest of the controlled
         corporation or other business entity); (p) ground rent payments on any
         ground lease, except for that portion of any ground rent payments made
         by Landlord that represent the pass-through of real estate taxes and
         operating expenses from any ground lessor to Landlord that are
         otherwise permitted hereunder; (q) the cost of any capital
         expenditures as determined for federal income tax purposes, except for
         any





                                       7
<PAGE>   12
         Permitted Capital Improvements; (r) fines or penalties incurred by
         Landlord as a result of Landlord's failure to pay when due Taxes, but
         only if Tenant is current on all payments of Rent due hereunder at the
         time such payments are due and at the time any such fines or penalties
         are incurred by Landlord; (s) any compensation paid to clerks,
         attendants or other persons in commercial concessions operated by
         Landlord in the Building for a profit, with respect to which tenants
         of the Building receive no benefit without paying a separate fee
         therefor; (t) any penalties or finesincurred as a result of Landlord's
         negligent violation of any federal, state or local law or regulation,
         unless the violation results in whole or in part from any act or
         omission of Tenant, any other tenant of the Building (to the extent
         not recovered directly from such tenant) or any other cause not within
         Landlord's exclusive control; (u) the cost of any repair, restoration,
         replacement or other item, to the extent Landlord is actually
         reimbursed therefor by insurance, warranties or condemnation proceeds
         or directly by any tenant in the Building (other than through the
         pass-through of operating expenses); (v) costs incurred for any items
         to the extent Landlord is reimbursed therefor from the proceeds of a
         manufacturer's materialman's, vendor's or contractor's warranty; (w)
         costs (including any construction allowance or similar consideration
         paid to a tenant) incurred in renovating or otherwise improving,
         decorating, painting or redecorating space leased to specific tenants
         or vacant rentable space leased or available to lease to prospective
         tenants of the Building (specifically excluding any common areas),
         unless such items are similarly provided to, or benefit generally,
         Tenant; or (x) costs of HVAC outside normal business hours sold to
         tenants of the Building by Landlord, but only if and to the extent
         Landlord receives reimbursement therefor from such tenants as an
         additional charge.  Expenses shall be determined on a cash or accrual
         basis, as Landlord may elect, based on generally accepted accounting
         principles, consistently applied.  Tenant shall promptly give Landlord
         notice of any defect in the Building of which Tenant is aware.  As
         used herein, the term "Latent Building Defect" shall mean a defect in
         the condition of the Building existing as of the date that the
         Premises is delivered to Tenant that could not have reasonably been
         discovered within the first Lease Year by a reasonable visual
         inspection of the Premises and the common area of the Building or in
         the ordinary course of Tenant's business.

                 (iii)    "RENT" shall mean Base Rent, Adjustment Rent and any
         other sums or charges due by Tenant hereunder.

                 (iv)     "TAXES" shall mean all taxes, assessments and fees,
         general or special, extraordinary or ordinary or foreseen or
         unforeseen, now or hereafter assessed, imposed or levied upon the
         Building, the property of Landlord located therein or the rents
         collected therefrom, by any governmental entity based upon the
         ownership, leasing, renting or operation of the Building, including,
         without limitation, those related to school, public betterment,
         general or local improvements and operations or imposed in connection
         with any business improvement or special taxing district, storm water
         management taxes, assessments or fees and all costs and expenses of
         protesting any such taxes, assessments or fees.  Taxes shall not
         include any net income, capital stock, succession, transfer,
         franchise, gift, estate or inheritance taxes; provided, however, if at
         any time during the Term, a tax or excise on income is levied or
         assessed by any governmental entity, in lieu of or as a substitute for
         or in addition to, in whole or in part, real estate taxes or other ad
         valorem taxes, such tax shall constitute and be included in Taxes.
         For the purpose of determining Taxes for any given year, the amount to
         be included for such year shall be Taxes which are assessed for,
         become a lien during, accrued for or allocated to such year rather
         than Taxes which are due for payment or paid during such year.

                 (v)      "TENANT'S PROPORTIONATE SHARE" shall mean the
         percentage set forth in ITEM 7 of the Schedule which has been
         determined by dividing the Rentable Square Feet of the Premises (as
         defined in ITEM 4 of the Schedule) by the Rentable Square Feet of the
         Building (as defined in ITEM 5 of the Schedule).  Such rentable square
         feet have been determined using the modified Building Owners and
         Managers Association method of measurement (1996 edition).

         B.      COMPONENTS OF RENT.  Tenant agrees to pay the following
amounts to Landlord at the office of the Building or at such other place as
Landlord designates:





                                       8
<PAGE>   13
                 (i)      Commencing on the Commencement Date, Base Rent to be
         paid in monthly installments in the amount set forth in ITEM 6 of the
         Schedule in advance on or before the first day of each month of the
         Term, without demand. Base Rent for any partial month during the Term
         shall be prorated on a daily basis based upon a thirty (30)-day month
         and shall be paid in advance.  Notwithstanding the foregoing, the Base
         Rent applicable to Phase II of the Premises shall be abated for the
         period beginning with the Commencement Date and ending with the
         earlier of (i) the Phase II Commencement Date, or (ii) the date any
         portion of Phase II of the Premises is occupied by Tenant for the
         purpose of conducting business therein.  The per diem Base Rent for
         each floor in Phase II shall be calculated as set forth in Section 1.B
         hereof.

                 (ii)     Commencing one year after the Commencement Date,
         Adjustment rent ("Adjustment Rent") in an amount equal to the sum of
         (a) Tenant's Proportionate Share of the amount by which Expenses for
         each calendar year (or portion thereof in the event of a partial
         calendar year during the Term) exceeds Expenses for calendar year 1999
         (the "Base Year"), plus (b) Tenant's Proportionate Share of the amount
         by which Taxes for each calendar year (or portion thereof in the event
         of a partial calendar year during the Term) exceeds Taxes for the Base
         Year.  Prior to each calendar year during the Term, or as soon as
         reasonably possible, Landlord shall estimate, in good faith, and
         notify Tenant of the amount of Adjustment Rent due for such year, and
         Tenant shall pay Landlord one-twelfth of such estimate on the first
         day of each month during such year.  Such estimate may be revised by
         Landlord whenever it obtains information relevant to making such
         estimate more accurate, provided such revisions shall not be made more
         frequently than twice in any twelve month period.  After the end of
         each calendar year, Landlord shall deliver to Tenant a report setting
         forth the actual Expenses and Taxes for such calendar year and a
         statement of the amount of Adjustment Rent that Tenant has paid and is
         payable for such year (the "Expense Statement").  Tenant acknowledges
         that actual Taxes for a calendar year may not be determined until
         after actual Expenses for such calendar year are determined.
         Accordingly, Tenant acknowledges that Landlord may report the actual
         Expenses and actual Taxes for a calendar year separately.  Within
         thirty (30) days after receipt of such Expense Statement or Expense
         Statements, Tenant shall pay to Landlord the excess of Adjustment Rent
         due for such calendar year over any payments of Adjustment Rent made
         by Tenant for such year, it being acknowledged by Tenant that in the
         event Landlord separately reports actual Expenses and actual Taxes for
         a calendar year, Landlord may reasonably allocate Adjustment Rent paid
         by Tenant for such calendar year between Expenses and Taxes for such
         calendar year.  If Tenant's estimated payments of Adjustment Rent
         exceed the amount due Landlord for such calendar year, Landlord shall
         promptly refund such excess to Tenant, provided Tenant is not then in
         default hereunder, in either case without interest to Tenant.  For any
         partial Adjustment Year (i.e., less than a full calendar year), the
         Expenses for the Base Year and the Taxes for the Base Year shall be
         pro rated based upon the number of days in such Adjustment Year and
         the resulting amounts shall be compared to the Expenses and the Taxes,
         respectively, in determining the Adjustment Rent for such partial
         Adjustment Year.

                 (iii)   Within ninety (90) days of receipt of any Expense
         Statement, Tenant shall be entitled to the following audit right with
         respect to such Expense Statement.  Such audit right shall be
         exercisable by Tenant providing Landlord with a written notice of its
         exercise of such audit right.  If within sixty (60) days after
         Landlord's receipt of Tenant's written notice and statement, Landlord
         and Tenant are unable to resolve Tenant's objections, then not later
         than fifteen (15) days after the expiration of such sixty (60)-day
         period Tenant shall deliver to Landlord written notice (the "Audit
         Notice") that it wishes to employ an independent certified public
         accounting firm reasonably acceptable to Landlord to inspect and audit
         Landlord's books and records relating to the objections raised in
         Tenant's statement.  If Tenant elects to employ such accountant as set
         forth above, then Tenant shall deliver to Landlord a confidentiality
         and nondisclosure agreement satisfactory to Landlord executed by such
         accountant, and provide Landlord not less than fifteen (15) days
         notice of the date on which the accountant desires to examine
         Landlord's books and records during regular business hours; provided,
         however, that such date shall be between thirty (30) and ninety (90)
         days after Tenant delivers to Landlord the Audit Notice.  Such audit
         shall be limited to a determination of





                                       9
<PAGE>   14
         whether Landlord calculated the Expense Statement in accordance with
         the terms and conditions of this Lease.  All costs and expenses of any
         such audit shall be paid by Tenant.  Any audit performed pursuant to
         the terms of this section shall be conducted only by an independent
         certified public accounting firm reasonably acceptable to Landlord.
         Notwithstanding anything contained herein to the contrary, Tenant
         shall be entitled to exercise its right to audit pursuant to this
         Section 2.B only in strict accordance with the foregoing procedures
         and each suchaudit shall relate only to the most recent calendar year
         covered by the audited Expense Statement; provided, however, if (i)
         such audit indicates that there was a demonstrated error in the
         calculation of a component of Expenses resulting in an overstatement
         of Expenses in the Expense Statement, and (ii) such component is a
         reoccurring Expense and because of the nature of the component it is
         likely that such component was similarly overstated in any prior
         calendar year during the Term, then Tenant shall have the right to
         audit the books and records relating solely to such component for each
         such prior year but such audit shall be limited to determining whether
         or not the same component was similarly in error in such prior years.
         Such limited audit right must be exercise only by Tenant delivering to
         Landlord written notice thereof within thirty (30) days after the
         earlier of (i) the date Tenant receives the audit report from the
         independent certified public accounting firm, or (ii) one hundred
         twenty (120) days after the date that Tenant delivers to Landlord the
         Audit Notice in connection with the Expense Statement in which the
         error was first discovered.  The audit rights pursuant to this Section
         2.B shall not transfer or apply to any subtenant or any other person
         or entity other than the "tenant" hereunder.  If on account of any
         demonstrated errors in the Expense Statement under audit, Tenant is
         entitled to a refund of the amount paid by Tenant for Tenant's
         Proportionate Share of Expenses for the calendar year or years under
         audit because such Expense Statement overstated the amounts to which
         Landlord was entitled hereunder by more than five percent (5%) of the
         amount of Expenses for the applicable calendar year, then Landlord
         shall promptly reimburse Tenant for the reasonable costs and expenses
         incurred in such audit, but in no event more than Twelve Thousand
         Dollars ($12,000.00).

                 (iv)     If Taxes paid by Landlord for any calendar year
         during the Term, or any part thereof, for which Tenant has paid
         Tenant's Proportionate Share of Expenses, are refunded to Landlord as
         a result of a final determination of such Taxes, then, provided Tenant
         is not then in default under this Lease, Tenant shall be entitled to a
         refund of Tenant's Proportionate Share of Expenses in an amount equal
         to Tenant's share of such refund (net of expenses incurred to obtain
         the refund).

         C.      PAYMENT OF RENT.  The following provisions shall govern the
payment of Rent: (i) if this Lease commences or ends on a day other than the
first day or last day of a calendar year, respectively, the Rent for the year
in which this Lease so begins or ends shall be prorated on a daily basis (based
upon a thirty (30) day month) and the monthly installments shall be adjusted
accordingly; (ii) all Rent shall be paid to Landlord without offset or
deduction except any rent abatement provided for in Section 12 hereof, and the
covenant to pay Rent shall be independent of every other covenant in this
Lease; (iii) if during all or any portion of any year the Building is not fully
rented and occupied (fully rented and occupied shall mean that one-hundred
percent (100%) of the Rentable Square Feet of the Building is occupied by
tenants under lease), Landlord shall make an appropriate adjustment of Taxes
and variable Expenses for such year to determine the Taxes and Expenses that
would have been paid or incurred by Landlord had the Building been fully rented
and occupied for the entire year and the amount so determined shall be deemed
to have been the Taxes and Expenses for such year; (iv) any sum due from Tenant
to Landlord which is not paid when due shall bear interest from the date due
until the date paid at the annual rate of two percentage (2%) points above the
rate then most recently announced by The First National Bank of Chicago as its
corporate base lending rate, from time to time in effect, but in no event
higher than the maximum rate permitted by law (the "Default Rate"); and, in
addition, Tenant shall pay Landlord a late charge for any Rent payment which is
paid more than five (5) business days after its due date equal to five percent
(5%) of such payment; provided, however, that on the first occasion of the late
payment of Rent in any calendar year, and no more than once in any calendar
year, Landlord agrees to waive its right to collect interest and late charges
on such payment of Rent if such payment is made no later than the fifth (5th)
business day after Landlord delivers to Tenant written notice of such late
payment; (v) if changes are made to this Lease or the Building, changing the





                                       10
<PAGE>   15
number of square feet contained in the Premises or in the Building, Landlord
shall make an appropriate adjustment to Tenant's Proportionate Share; (vi) in
the event of the expiration or termination of this Lease prior to the
determination of any Adjustment Rent, Tenant's agreement to pay any such sums
and Landlord's obligation to refund any such sums (provided Tenant is not in
default hereunder) shall survive the expiration or termination of this Lease;
(vii) INTENTIONALLY DELETED; (viii) Landlord may at any time change the fiscal
year of the Building upon prior written notice to Tenant; (ix) each amount owed
to Landlord under this Lease for which the date of payment is not expressly
fixed shall be due on the date listed on the statement showing such amount is
due (which date shall not be earlier than thirty (30) days after the date
Tenant receives such statement); (x) if Landlord fails to give Tenant an
estimate of Adjustment Rent prior to the beginning of any calendar year, Tenant
shall continue to pay Adjustment Rent at the rate for the previous calendar
year until Landlord delivers such estimate or notice, at which time Tenant
shall pay retroactively the increased amount for all previous months of such
calendar year; and (xi) if Landlord permits Tenant to take possession of any
phase of the Premises (other than for the purposes set forth in Section 1.E
hereof) prior to the commencement date applicable to such phase, such tenancy
shall be by the day and Tenant shall be responsible for payment of Rent, in
advance, at the rate of one-thirtieth (1/30th) of the monthly Rent as set forth
above for each day of such occupancy prior to the commencement date applicable
to such phase, and Tenant shall comply fully with all other terms and
provisions of this Lease upon Tenant's possession of any portion of the
Premises.

         D.      ALLOCATION OF RENT ABATEMENT FOR TAX PURPOSES.  Landlord and
Tenant agree that no portion of the Base Rent paid by Tenant during the portion
of the Term occurring after the expiration of any period during which such rent
was abated shall be allocated, for income tax purposes, nor is such rent
intended by the parties to be allocable, for income tax purposes, to any
abatement period.

3.       USE.  Tenant will occupy and use the Premises solely for first-class
non-governmental business offices (including the Specific Authorized Uses
(hereinafter defined), to the extent ancillary and accessory to such office
use) in accordance with applicable zoning regulations.  The Premises will not
be used for any other purposes.  "Specific Authorized Uses" shall mean the
following uses: (i) operation, installation, maintenance, repair and
replacement of the communications equipment of Tenant and its customers, (ii)
the location and operation of Network Operations Center Equipment (hereinafter
defined) and (iii) to serve pre-prepared food and/or non-alcoholic beverages in
an area of the Premises that also contains computers, display equipment and/or
network or Internet connections.  "Network Operation Center Equipment" shall
mean the following equipment of Tenant (i) telecommunications switching
equipment and transport nodes and telecommunications equipment of Tenant's
customers, (ii) AC and DC power equipment and sealed batteries, (iii)
projection and computer equipment to facilitate network management and
monitoring, and (iv) computer and data processing equipment.  The term "Network
Operations Center" shall mean an area of the Premises used for the location and
operation of the Network Operations Center Equipment.  The term "Cyber Cafe"
shall mean an area of the Premises used to serve pre-prepared food and/or
non-alcoholic beverages, which area will also contain computers, display
equipment, and/or network or Internet connections.  Landlord represents that it
is not bound by any proffer or any private covenants, conditions or
restrictions that prohibit the use of the Premises for any of the Specific
Authorized Uses; provided, however, Tenant shall be entitled to use the
Premises for any of the Specific Authorized Uses only if, and to the extent
that, such uses are permitted under and in compliance with all federal, state
and municipal laws, ordinances, rules and regulations (including without
limitation all zoning laws, ordinances, rules and regulations) applicable to
the Building, and Landlord makes no representation that any such Specific
Authorized Uses are permitted thereunder or in compliance therewith.  Tenant
shall, at its own cost and expense, comply with all federal, state and
municipal laws, ordinances, rules and regulations issued by any governmental
authority which relate to the condition, use or occupancy of the Premises and
all covenants, conditions and restrictions of record as of the date of this
Lease and those covenants, conditions and restrictions which are created after
the date of this Lease to the extent the Tenant is given written notice of same
and to the extent same do not unreasonably increase Tenant's cost to comply
therewith, materially impair any rights of Tenant hereunder, or unreasonably
interfere with the Tenant's access to or use of the Premises which relate to
the condition, use or occupancy of the Premises.  Without limiting the
foregoing, Tenant shall not cause, nor permit, any hazardous or toxic
substances to be brought upon, produced, stored, used, discharged or disposed
of in, on or about the Premises without the prior written consent of Landlord
and then only in compliance with all





                                       11
<PAGE>   16
applicable environmental laws, except that Tenant shall be permitted to use and
keep in the Premises such cleaning, copier and other supplies as are reasonable
and customary for office use, provided that Tenant uses, stores and disposes of
same in accordance with all applicable environmental laws.  Tenant shall not
commit waste or use the Premises in any way as to constitute a nuisance.  All
common areas within the Building shall be designated as "non-smoking" and
Tenant shall cooperate with Landlord in enforcing such non-smoking rule.  To
the extent Tenant desires to allow people to smoke within the Premises Tenant
shall designate specific smoking areas within the Premises provided however
that such areas must be properly ventilated and Tenant shall cause any smokers
in the Premises to use such smoking areas.  Except as may otherwise be provided
in this Lease, Tenant shall not be required to make any alteration outside of
the Premises or to any base Building systems to comply with any federal, state
and municipal laws, ordinances, rules and regulations issued by any
governmental authority except that Tenant shall be responsible for the cost of
any alteration made by Landlord in or to any part of the Building to the extent
such alteration is required: (i) to comply with the requirement of any federal,
state and municipal laws, ordinances, rules and regulations issued by any
governmental authority and such requirement is a result of Tenant's particular
business or its particular use of the Premises, or (ii) as a result of any
damage caused by Tenant in the Building or any alteration made by Tenant.
Notwithstanding the foregoing, Tenant shall not be required to modify any
sprinkler heads required by any applicable legal requirement to be modified,
but shall be required to relocate such sprinkler heads if required by any
applicable legal requirement.  For purposes of this Section 3, the base
Building HVAC ducts, VAV boxes, central air handlers, that portion of the
electrical system the supplies power to the main electrical closet serving the
Premises, and that portion of the plumbing system that supplies water and
sewage service to the existing bathrooms and wet stacks serving the Premises
shall be part of the base Building systems in the Premises.  All other
electrical, mechanical, plumbing and any other systems within the Premises or
exclusively servicing the Premises, including without limitation, any
supplemental HVAC systems exclusively servicing the Premises and the sprinkler
system (including without limitation any Halon system that may be installed for
the Premises) for the Premises (the "Premises Systems") shall not be part of
the base Building systems and shall be the sole responsibility of Tenant;
provided, however if any such system is located outside the Premises it shall
be considered a part of the Premises Systems only if it exclusively services
the Premises and is not a base Building system, such as any telecommunications
cabling and any generators used in connection with any supplemental HVAC or
other equipment of Tenant.

4.       CONDITION OF PREMISES.  No agreement of Landlord to alter, remodel,
decorate, clean or improve the Premises or the Building (or to provide Tenant
with any credit or allowance for the same), and no representation regarding the
condition of the Premises or the Building, have been made by or on behalf of
Landlord or relied upon by Tenant, except as expressly stated in this Lease
(including any exhibits hereto).

5.       BUILDING SERVICES.

         A.      BASIC SERVICES.  Landlord shall furnish the following services
in a manner reasonably comparable with other first-class office buildings in
Herndon and Reston, Virginia of similar size, location and age:  (i) heating,
ventilating and air conditioning to provide a temperature condition required,
in Landlord's reasonable judgment, for comfortable occupancy of the Premises
under normal business operations, daily from 8:00 A.M. to 6:00 P.M. (Saturday
from 9:00 A.M. to 1:00 P.M.), Sundays and holidays excepted; (ii) water for
drinking and for any restrooms and office kitchens requested by Tenant; (iii)
men's and women's restrooms at locations designated by Landlord, in common with
other tenants of the Building; (iv) janitor service in the Premises and common
areas of the Building, weekends and holidays excepted, including periodic
outside window washing of the perimeter windows in the Premises not less than
twice per year; (v) maintenance of interior and exterior common areas of the
Building, including snow removal as necessary and maintenance of the landscaped
areas; (vi) passenger elevator service in common with Landlord and other
tenants of the Building, 24 hours a day, 7 days a week; provided, however, that
Landlord shall have the right to remove passenger and freight elevators from
service as the same shall be required for moving freight or for servicing or
maintaining the elevators and/or the Building; and (vii) freight elevator
service daily, subject to scheduling with Landlord.  Tenant shall have access
to the Premises 24 hours per day, 365 days per year.  The base Building
heating, ventilating and air conditioning ("HVAC") system is designed to comply
with the HVAC specifications set forth on EXHIBIT E attached hereto.





                                       12
<PAGE>   17
         B.      ELECTRICITY.  The Building shall be metered or submetered for
electrical use.  Electricity shall be distributed to the Premises and the
remainder of the Building either by the electric utility company serving the
Building or, at Landlord's option, by Landlord; and Landlord shall permit
Landlord's wire and conduits, to the extent available, suitable and safely
capable, to be used for such distribution.  If and so long as Landlord is
distributing electricity to the Premises, Tenant shall obtain all of its
electricity from Landlord and shall pay all of Landlord's charges, which
charges shall be based on meter readings, provided such charges are competitive
with charges of third party vendors, including, without limitation, the
Virginia Electric Power Company.  If, pursuant to Section 5.F hereof, Tenant
elects to provide its own electricity, then Tenant shall make all necessary
arrangements with the electric utility company for providing, metering and
paying for electric current furnished to the Building and Landlord shall have
no liability or obligations with respect thereto.  All electricity used during
the performance of repairs in the Premises, or the operation of any special air
conditioning systems serving the Premises shall be included in Expenses.

         C.      TELEPHONES.  Tenant shall arrange for telephone service
directly and shall be solely responsible for paying for such telephone service.
If Landlord acquires ownership of the telephone cables in the Building, at any
time, Landlord shall permit Tenant to connect to such cables on such terms and
conditions as Landlord may prescribe.  In no event does Landlord make any
representation or warranty with respect to the provision of telephone service
in the Building and Landlord shall have no liability with respect thereto.

         D.      ADDITIONAL SERVICES.  Landlord shall not be obligated to
furnish any services other than those stated above.  If Landlord elects to
furnish services requested by Tenant in addition to those stated above
(including, without limitation, services at times other than those stated
above), Tenant shall pay Landlord's actual cost to furnish such services.  If
Tenant shall fail to make any such payment, Landlord may, without notice to
Tenant and in addition to all other remedies available to Landlord, discontinue
any additional services.  No discontinuance of any such service shall result in
any liability of Landlord to Tenant or be considered as an eviction or a
disturbance of Tenant's use of the Premises.  In addition, if Tenant's
concentration of personnel or equipment materially and adversely affects the
temperature or humidity in the Premises or the Building, Landlord may install
supplementary air conditioning units in the Premises, and Tenant shall pay one
hundred fifteen percent (115%) of the cost of such air conditioning units and
of the installation, operation and maintenance thereof; provided, however,
that, except in the case of the risk of immediate harm to any person or the
Building or any other emergency, Landlord shall give Tenant written notice of
the problem and an opportunity to cure the problem within ten (10) days after
receipt of such notice, and if Tenant is the only tenant of the Building and
such problem cannot be reasonably cured within such ten (10) day period, Tenant
shall have such additional time (not to exceed thirty (30) days) as may be
reasonably necessary to cure such problem, provided that Tenant commences such
cure within such ten (10) day period and diligently and continuously pursues
such cure.

         E.      FAILURE OR DELAY IN FURNISHING SERVICES.  Except as set forth
in Section 11.B hereof, Tenant agrees that Landlord shall not be liable for
damages for failure or delay in furnishing any service stated above if such
failure or delay is caused, in whole or in part, by any one or more of the
events stated in Section 26.I. below, nor shall any such failure or delay be
considered to be an eviction or disturbance of Tenant's use of the Premises,
or, except as set forth in this Section 5.E, relieve Tenant from its obligation
to pay any Rent when due or from any other obligations of Tenant under this
Lease.  Landlord agrees to use its reasonable efforts to promptly cure such
failure or delay after Tenant has notified Landlord thereof.  Notwithstanding
the forgoing, if for reasons within the control of Landlord and not caused by
Tenant, or any of its employees or agents, or caused by any one or more of the
events stated in Section 26.I, any interruption or stoppage of any services
Landlord is required hereunder to provide to the Building shall occur and such
interruption or stoppage shall continue for more than three (3) consecutive
business days and shall render a substantial portion of the Premises
untenantable and Tenant shall actually cease to conduct business in such
portion of the Premises, then, provided that Tenant provides Landlord with
written notice thereof promptly upon the occurrence thereof, the Base Rent
payable hereunder for such untenantable portion of the Premises shall be abated
for the period beginning on the fourth (4th) consecutive business day of such
failure and shall continue until the earlier of the date that (i) Tenant again
uses such portion of the Premises, or (ii)





                                       13
<PAGE>   18
such portion of the Premises is again tenantable.

         F.      TENANT'S OPTION TO PROVIDE ELECTRICITY AND CHAR SERVICE.
Tenant shall have the option to provide, at its sole cost, its own char service
and/or electrical service to the Building, provided that Tenant exercises such
option by delivering to Landlord notice thereof (i) ninety (90) days prior to
the Commencement Date if the exercise of such option is to be effective on the
Commencement Date, or (ii) ninety (90) days prior to the commencement of any
calendar year if such option is to be effective on the first day of such
calendar year, it being understood that if such option is not effective as of
the Commencement Date then such option can become effective only on the first
day of a calendar year. If such election is made, Landlord shall have no further
obligation hereunder to provide such services after the effective date of such
election. In the event Tenant does in fact exercise such option to provide its
own char service and/or electrical service, then (i) Expenses for the Base Year
shall be adjusted to exclude the amount of char service and/or electricity
expenses (as the case may be) otherwise included in the Base Year, and (ii) the
Base Rent payable by Tenant for each Lease Year during the initial Term
(excluding any Extension Terms) as shown in ITEM 6 of the Schedule shall be
adjusted as follows to take into account the fact that Landlord is not required
to provide such services to Tenant and the calculation of Expenses shall take
into account the fact that Tenant is providing such services. For each month
during the initial Term (excluding any Extension Terms) commencing on the date
that Tenant's election to provide the foregoing services becomes effective, the
monthly installment of Base Rent otherwise payable hereunder (as set forth in
ITEM 6 of the Schedule) shall be adjusted by reducing such monthly amount by the
following: (i) in the event Tenant elects to provide only its own electricity
service, an amount equal to (a) One and 53/100 Dollars ($1.53), times (b) the
number of square feet in the Premises, divided by (c) twelve (12), (ii) in the
event Tenant elects to provide only its own contract char service, an amount
equal to (a) Eighty-four Cents ($ .84), times (b) the number of square feet in
the Premises, divided by (c) twelve (12), and (iii) in the event Tenant elects
to provide both its own electricity service and contract char service, an amount
equal to (a) Two Dollars and 37/100 ($2.37), times (b) the number of square feet
in the Premises, divided by (c) twelve (12). In the event Tenant does in fact
exercise such option to provide its own char service and/or electrical service,
Tenant shall be responsible for (i) providing such services to the entire
Building (including without limitation the Premises and the common areas of the
Building) in a first-class manner, (ii) contracting with the appropriate vendor
or utility company to provide such services and (iii) paying the appropriate
vendor or utility company for such services. In addition, if Tenant reasonably
requests that it be permitted to provide other service otherwise provided by
Landlord, Landlord shall consider permitting (without in any way being obligated
to so permit) Tenant to provide such services; in which case, Expenses for the
Base Year and the Base Rent shall be adjusted by Landlord for such items in
manner similar to the forgoing adjustment as reasonably determined by Landlord.
Tenant acknowledges and agrees that it may be appropriate for Landlord to
control any such other services for various reasons and Landlord shall have sole
discretion to withhold its consent to permitting Tenant to provide any such
services otherwise provided by Landlord. If Tenant exercises its option to
provide any services otherwise provided by Landlord hereunder, Tenant shall be
entitled to cancel such option, provided that Tenant exercises its right to
cancel such option by delivering to Landlord notice thereof at least ninety (90)
days prior to the commencement of the calendar year for which such cancellation
is to be effective, it being understood that such cancellation can become
effective only on the first day of a calendar year. If such cancellation is
made, then as of the effective date of such cancellation (i) Tenant shall no
longer be required to provide such services and Landlord shall provide such
services in the manner otherwise required hereunder, (ii) Expenses for the Base
Year shall be calculated in the manner set forth herein as if Tenant had not
exercised its option to provide the services that have been cancelled, (iii) the
Base Rent payable by Tenant for each Lease Year during the Term shall be as
shown in ITEM 6 of the Schedule (without any reduction for the services that
have been cancelled) as if Tenant had not exercised its option to provide the
services that have been cancelled, and (iv) the calculation of Expenses shall
take into account the fact that Landlord is providing such services.

         G.      EXCESS USE OF UTILITIES.  If the quantity or kind of utilities
(including without limitation electricity) or services furnished by Landlord to
the Premises to meet Tenant's requirements is in excess of the utilities and
services typically and customarily consumed by other office tenants of first
class office buildings in Reston and Herndon, Virginia, then Tenant shall
directly reimburse Landlord upon demand for the additional cost resulting from
such excessive consumption; provided, however, for purposes of this



                                       14
<PAGE>   19

provision Tenant's electricity consumption shall be deemed to be excessive if
the cost of electricity provided to the Building exceeds an amount equal to One
and 53/100 Dollars ($1.53) times the number of rentable square feet in the
Building, and all costs in excess of such amount shall be excessive consumption
and shall be directly reimburse by Tenant to Landlord.

6.       RULES AND REGULATIONS.  Tenant shall observe and comply, and shall
cause its subtenants, assignees, invitees, employees, contractors and agents to
observe and comply, with the Rules and Regulations listed on EXHIBIT C attached
hereto and with such reasonable modifications and additions thereto as Landlord
may make from time to time of which Landlord notifies Tenant.  Landlord shall
not be liable for failure of any person to obey the Rules and Regulations.
Landlord shall not be obligated to enforce the Rules and Regulations against
any person, and the failure of Landlord to enforce any such Rules and
Regulations shall not constitute a waiver thereof or relieve Tenant from
compliance therewith, provided, however, that Landlord shall not discriminate
against Tenant in the enforcement of such Rules and Regulations.  To the extent
the terms of this Lease conflict with the Rules and Regulations the terms of
this Lease shall prevail.

7.       CERTAIN RIGHTS RESERVED TO LANDLORD.  Landlord reserves the following
rights, each of which Landlord may exercise without notice to Tenant and
without liability to Tenant, and the exercise of any such rights shall not be
deemed to constitute an eviction or disturbance of Tenant's use or possession
of the Premises and shall not give rise to any claim for set-off or abatement
of Rent or any other claim:  (a) to change the name or street address of the
Building or the suite number of the Premises, provided that unless such change
is required by law or any governmental or quasi-governmental authority,
Landlord shall use reasonable efforts to provide Tenant with notice thereof (if
possible) at least six (6) months prior to such change; (b) to install, affix
and maintain any and all signs on the exterior or interior of the Building,
subject to the restrictions set forth in Section 9.D hereof; (c) to make
repairs, decorations, alterations, additions or improvements, whether
structural or otherwise, in and about the Building (without materially
diminishing the first class nature of the Building unless such repairs,
decorations, alterations, additions or improvements are necessary for the
operation of the Building, are required by law or any governmental or
quasi-governmental authority or are requested by Tenant), including, without
limitation, reconfiguring parking areas, driveways, walkways and other exterior
common areas, and for such purposes to enter upon the Premises (upon reasonable
notice except in the case of emergencies), temporarily close doors, corridors
and other areas of the Building and interrupt or temporarily suspend services
or use of common areas, provided however that Landlord shall use reasonable
efforts not to interfere with the operation of Tenant's business in the
Premises; provided, further, that Tenant agrees to pay Landlord for overtime
and similar expenses incurred if such work is done other than during ordinary
business hours at Tenant's request; (d) to retain at all times, and to use in
appropriate instances and with reasonable notice (except in the case of
emergencies), keys to all doors within and into the Premises; (e) to grant to
any person or to reserve unto itself the exclusive right to conduct any
business or render any service in the Building, to the extent such grant of
exclusivity does not restrict Tenant's right hereunder to use the Premises for
the purposes permitted hereunder; (f) to inspect the Premises at reasonable
times and upon reasonable notice (except in the case of emergencies) and, if
vacated or abandoned and Tenant indicates in writing that it does not intend to
reoccupy the Premises, to prepare the Premises for reoccupancy; (g) to install,
use and maintain in and through the Premises pipes, conduits, wires and ducts
serving the Building, provided that such installation, use and maintenance does
not unreasonably interfere with Tenant's use of the Premises; (h) to take any
other action which Landlord reasonably deems necessary in connection with the
operation, maintenance, marketing, improvement or preservation of the Building;
and (i) to reasonably approve the weight, size and location of safes or other
heavy equipment or articles, which articles may be moved in, about or out of
the Building or Premises only at such times and in such manner as Landlord
shall reasonably direct, at Tenant's sole risk and responsibility.

8.       MAINTENANCE AND REPAIRS.

         A.      GENERAL OBLIGATIONS. Tenant, at its expense, shall maintain
and keep the Premises in clean, safe, sanitary and good order, condition and
repair at all times during the Term, normal wear and tear excepted and subject
to the casualty and condemnation provisions of this Lease as specifically
provided in Sections 12 and 13 hereof.  Except as otherwise provided in this
Lease (including without limitation the




                                       15
<PAGE>   20

immediately preceding sentence) and except for ordinary wear and tear, Landlord
shall maintain and repair the common areas of the Building, the base Building
structure, the base Building systems (but not the Premises Systems, as
heretofore defined) and the exterior of the Building (including the Building's
roof) in a manner that is reasonably comparable to other similar first class
office buildings in the Herndon and Reston, Virginia area of a similar
location, size and age and shall maintain, repair, replace and paint, as
necessary, all building-standard common area elements installed by Landlord.
Notwithstanding the foregoing, except as provided in Section 12 hereof,
Landlord shall have no obligation to make any repairs brought about by any act
or omission of Tenant, its agents, employees or invitees.  All building
standard bulbs, tubes and lighting fixtures for the Premises shall be provided
and installed by Landlord, which cost shall be part of Expenses.

         B.      NOISE AND VIBRATION.  Business machines and mechanical
equipment belonging to Tenant which cause noise or vibration that may be
transmitted to the structure of the Building or to any space therein to such a
degree as to be objectionable to Landlord or to any tenant in the Building
shall be installed and maintained by Tenant, at Tenant's expense, on vibration
eliminators or vibration reducers or other devices sufficient to eliminate such
noise and vibration or reduce the same to a level acceptable to Landlord in
Landlord's sole judgment, and if such noise and/or vibration is not so
eliminated or reduced to a level acceptable to Landlord in its sole judgment,
Landlord shall have the right to require Tenant to remove such machines and/or
equipment from the Premises.

9.       ALTERATIONS; SIGNS.

         A.      REQUIREMENTS.  Tenant shall not make any replacement,
alteration, improvement or addition to or removal from the Premises
(collectively an "alteration") without the prior written consent of Landlord,
which consent shall not be unreasonably withheld.  In the event Tenant proposes
to make any alteration requiring Landlord's consent, Tenant shall, prior to
commencing such alteration, submit to Landlord for prior written approval: (i)
detailed plans and specifications; (ii) the names, addresses and copies of
contracts for all contractors on alterations which will cost $100,000 or more
and the name, address and a copy of the contract for the general contractor
only on alterations costing less than $100,000; (iii) all necessary permits
evidencing compliance with all applicable governmental rules, regulations and
requirements; (iv) certificates of insurance in form and amounts required by
Landlord, naming Landlord, its managing agent and any other parties designated
by Landlord (that have any insurable interest in the Building or any part
thereof) as additional insureds; and (v) all other documents and information as
Landlord may reasonably request in connection with such alteration.  Tenant
agrees to pay Landlord's reasonable charges for review of all such items, such
charges not to exceed one hundred ten percent (110%) of Landlord's actual costs
for such review.  Notwithstanding anything contained in this Section 9.A,
Tenant shall have the right to make Permitted Alterations (hereinafter defined)
in the Premises, without Landlord's consent (but with fifteen (15) days prior
written notice (the "Permitted Alteration Notice"), which notice shall contain
a description of the Permitted Alterations proposed to be undertaken by Tenant
and state that the alterations are Permitted Alterations).  A Permitted
Alteration shall mean any of the following Alterations in the Premises,
provided that such Alterations are not visible from outside the Premises, do
not require a permit or any government approval to install or construct, are
consistent with the design standards of the Building, cost less than Ten
Thousand Dollars ($10,000.00), and do not affect the structure of, or any of
the systems in, the Building: (i) paint and install wall coverings and other
Alterations that are purely cosmetic irrespective of the aggregate cost thereof
(i.e., the $10,000.00 limitation shall not apply to this item (i)); (ii)
install and remove office furniture; (iii) install normal and customary office
equipment that are not affixed to the Premises and are designed to be
free-standing (i.e., not "built-in"), subject to the other provisions of this
Lease, (iv) relocate 120 volt electrical outlets; (v) install and remove
Tenant's computer and telecommunication equipment and perform cable pulls in
connection therewith; (vi) install and remove carpeting and other floor
coverings. In the event that, within ten (10) days after receiving the
Permitted Alterations Notice, Landlord determines, in its reasonable
discretion, that the proposed Alterations are not Permitted Alterations, and so
notifies Tenant, Tenant shall apply for Landlord's consent for such Alterations
in accordance with the provisions of this Section 9.A.  Neither approval of the
plans and specifications nor supervision of the alteration by Landlord shall
constitute a representation or warranty by Landlord as to the accuracy,
adequacy, sufficiency or propriety of such plans and specifications or the
quality of workmanship or the compliance of such alteration with applicable
law.  Tenant shall pay the entire cost of the alteration and, if requested by
Landlord, shall



                                       16
<PAGE>   21


deposit with Landlord, prior to the commencement of any alteration costing
Fifty Thousand Dollars ($50,000) or more, payment and completion bonds payable
to Landlord in the full amount of the cost of the alteration, including both
labor and materials.  Each alteration shall be performed in a good and
workmanlike manner, in accordance with the plans and specifications approved by
Landlord where Landlord approval is required, and shall meet or exceed the
standards for construction and quality of materials for a first-class office
building in Fairfax County, Virginia.  In addition, each alteration shall be
performed in compliance with all applicable governmental and insurance company
laws, regulations and requirements.  Each alteration shall be performed by
Landlord or under Landlord's supervision, and in harmony with Landlord's
employees, contractors and other tenants.  Each alteration, whether temporary
or permanent in character, made by Landlord or Tenant in or upon the Premises
(excepting only Tenant's furniture, equipment and trade fixtures) shall become
Landlord's property and shall remain upon the Premises at the expiration or
termination of this Lease without compensation to Tenant; provided, however,
that Tenant shall have the right to remove at Tenant's expense, prior to the
expiration or termination of the Term, or any Extension Term if Tenant
exercises its options under Section 1 D. hereof, all movable furniture,
furnishings or equipment if such items (i) are not affixed to or in the
Premises, or (ii) temporarily affixed to or in the Premises by bolts or screws,
but only to the extent such items can be removed without any damage to the
Premises or the Building.  With respect to all other alterations, whether
temporary or permanent in nature, Landlord shall have the right to require
Tenant to remove such alteration at Tenant's sole cost and expense in
accordance with the provisions of Section 15 of this Lease, which required
removal shall be specified by Landlord when Landlord consents to Tenant's
requested alterations.  If Tenant installs any alterations without the consent
of Landlord as set forth above (if such consent is required) then Landlord
shall have the right to remove such alterations and Tenant shall reimburse
Landlord therefor as additional Rent.

         B.      LIENS.  Upon completion of any alteration, Tenant shall
promptly furnish Landlord with sworn owner's and contractors' statements and
full and final waivers of lien from the general contractor and all
subcontractors providing $5,000 or more in labor or material costs, covering
all labor and materials included in such alteration.  Tenant shall use best
efforts not to permit any mechanic's lien to be filed against the Building, or
any part thereof, arising out of any alteration performed, or alleged to have
been performed, by or on behalf of Tenant.  If any such lien is filed, Tenant
shall within ten (10) days after notice of such lien, have such lien released
of record or deliver to Landlord a bond in form, amount, and issued by a surety
satisfactory to Landlord, indemnifying Landlord against all costs and
liabilities resulting from such lien and the foreclosure or attempted
foreclosure thereof.  If Tenant fails to have such lien so released or to
deliver such bond to Landlord, Landlord, without investigating the validity of
such lien, may pay or discharge the same, and Tenant shall reimburse Landlord
upon demand for the amount so paid by Landlord, including Landlord's expenses
and reasonable attorneys' fees.

         C.      SIGNS.  Except as set forth in Section 9.D hereof, no sign,
advertisement or notice shall be inscribed, painted, affixed or otherwise
displayed on any part of the exterior or interior of the Building except on the
interior of the Premises so long as the same is not visible from outside the
Premises, on the directories and the doors of offices, and then only in such
place, number, size, color and style as is harmonious with the design of the
Building and its furnishings and is approved by Landlord in writing, such
approval not to be unreasonably withheld, conditioned or delayed.  All signs
approved by Landlord as aforesaid shall be provided by Landlord at Tenant's
expense, except that Landlord shall bear the cost of placing Tenant's name in
the directory.  If any sign, advertisement or notice which does not conform to
the foregoing is nevertheless exhibited by Tenant, Landlord shall have the
right to remove the same and Tenant shall be liable as additional Rent for any
and all expenses incurred by Landlord in said removal.

         D.      BUILDING SIGNAGE.  Tenant shall have the right to install
signage at multiple locations on the interior and exterior of the Building (the
"Building Signage") identifying the Tenant as a tenant of the Building,
provided that (i) such Building Signage is permitted under the laws, rules and
regulations of the Fairfax County, Virginia and any other governmental or
quasi-governmental authorities having appropriate jurisdiction over the
Building and under any covenants affecting the Building, (ii) such Building
Signage conforms to all such laws, rules, regulations and covenants, and to the
terms and conditions hereinafter set forth, and (iii) Tenant has obtained all
permits, licenses and approvals that may be required in order to install such
Building Signage.  The exact number, style, dimensions, design and location of
the Building Signage





                                       17
<PAGE>   22
shall be subject to Landlord's prior review and prior written approval, which
shall not be unreasonably withheld, conditioned or delayed, provided that the
foregoing conditions have been met and Landlord determines that such Building
Signage and the amount thereof is appropriate for first-class office buildings
in Herndon, Virginia and in Fairfax County, Virginia.  In order to obtain
Landlord's approval, Tenant must submit to Landlord for Landlord's approval
samples of materials to be used for the Building Signage (showing, among other
things, the thickness thereof), samples of any colors used for the Building
Signage, complete shop drawings of the Building Signage and plans and
specifications for the actual construction and attachment of the Building
Signage.  Any such Building Signage shall be installed by a contractor
reasonably approved by Landlord and maintained by a contractor reasonably
acceptable to Landlord.  On or before the end of the Term, Tenant shall, at its
expense, have a contractor selected by Landlord remove the Building Signage and
repair the Building affected thereby to the condition such part of the Building
was in at the time such Building Signage was installed.  Tenant hereby agrees
to indemnify and hold Landlord and its agents, officers, directors and
employees harmless from and against any cost, damage, claim, liability or
expense (including reasonable attorneys' fees) incurred by or claimed against
Landlord and its agents, officers, directors and employees, directly or
indirectly, as a result of or in any way arising from the installation and
maintenance of such Building Signage.  Tenant shall obtain insurance coverage
for such Building Signage which coverage shall be included in Tenant's
comprehensive liability insurance required pursuant to Section 10 of this
Lease.  Tenant's rights under this Section 9.D are personal to Tenant and to
any assignee of the Lease (including any subtenant of the entire Premises for
the entire remaining Term), but no sublessee of less than the entire Building
for the entire remaining Term shall have any signage rights hereunder;
provided, however, Tenant shall be entitled to assign all or a portion of its
signage rights hereunder to any subtenant for the term of its sublease if such
subtenant subleases more than fifty thousand (50,000) rentable square feet of
space in the Building from Tenant, but in no event shall Tenant be entitled to
assign such rights to more than one (1) subtenant exercising such rights at the
same time (i.e., in no event shall more than one (1) subtenant of Tenant have
such signage rights).  During the Term, provided that Tenant is not in default
of any provision of this Lease and is leasing one hundred percent (100%) of the
Building, Landlord shall not place on the exterior of the Building any signage,
unless such signage is required by law or relates to parking, directions,
safety or other operations and/or maintenance concerns of the Building or
relates to the sale or leasing of any part of the Building.  Tenant
acknowledges and agrees that any and all rights that Tenant may have to install
on or around, affix to, or otherwise affect the outside of the Building or
landscaping or outside common areas thereof (including without limitation,
signs, antennas, generators and fuel tanks) shall be subject to architectural
and design review and approval pursuant to certain covenants, conditions and
restrictions affecting the Building.

10.      INSURANCE.  Landlord and Tenant agree to provide insurance and
allocate the risks of loss as follows:

         A.      TENANT'S INSURANCE

                 (i)      Tenant, at its sole cost and expense but for the
         mutual benefit of Landlord (when used in this Section 10.A. the term
         "Landlord" shall include Landlord's managing agent, partners,
         beneficiaries, officers, agents, servants and employees and the term
         "Tenant" shall include Tenant's partners, beneficiaries, officers,
         agents, servants and employees), agrees to purchase and keep in force
         and effect during the Term hereof, insurance under an "all risk" fire
         and extended coverage policy on all alterations, additions, and
         improvements in the Premises and on all personal property located in
         the Premises and any Antenna Equipment (hereinafter defined),
         protecting Tenant from damage or other loss caused by fire or other
         casualty, including but not limited to vandalism and malicious
         mischief, perils covered by extended coverage, theft, sprinkler
         leakage, water damage (however caused), explosion malfunction or
         failure of heating, and cooling or other apparatus, and other similar
         risks in amounts not less than the full insurable replacement value of
         such property.  Such property insurance (except that which insures
         only Tenant's moveable personal property located in, on or about the
         Premises and not including any alterations) shall name Landlord as a
         "loss payee" (to the extent of Landlord's interest in such property)
         and shall provide that it is specific and non-contributory and shall
         contain a replacement cost endorsement.  Such insurance shall also
         contain a clause pursuant to which the insurance carriers waive all
         rights of subrogation against





                                       18
<PAGE>   23
         Landlord with respect to losses payable under such policies.

                 (ii)     Tenant also agrees to maintain commercial general
         liability insurance covering Tenant as the insured party against
         claims for bodily injury and death and property damage occurring in or
         about the Premises, with limits of not less than One Million Dollars
         ($1,000,000.00) per occurrence and Three Million Dollars
         ($3,000,000.00) general aggregate.  Such insurance shall include
         contractual liability insurance.

                 (iii)    All policies required to be carried by Tenant under
         this Lease shall be issued by insurers of recognized responsibility
         licensed to do business in the Commonwealth of Virginia with a Best's
         rating of A-/VIII or better and shall name Landlord as an additional
         insured.  Tenant shall, prior to commencement of the Term, furnish to
         Landlord certificates evidencing such coverage, which certificates
         shall state that such insurance coverage may not be changed or
         canceled without at least thirty (30) days' prior written notice to
         Landlord and Tenant.  In the event Tenant shall fail to procure such
         insurance, Landlord may at its option after giving, Tenant no less
         than ten (10) days' prior written notice of its election to do so
         procure the same for the account of Tenant and the cost thereof shall
         be paid to Landlord as additional Rent upon receipt by Tenant of bills
         therefor.

         B.      LANDLORD'S INSURANCE.  Landlord agrees to purchase and keep in
force and effect commercial general liability insurance in an amount not less
than Three Million Dollars ($3,000,000.00) and "all risk" fire and extended
insurance on the Building improvements (not including, however, any
improvements, alterations or additions in the Tenant's Premises or other
tenants' premises) against fire or other casualty, including but not limited to
vandalism and malicious mischief, perils covered by extended coverage, theft,
sprinkler leakage, water damage (however caused), explosion, malfunction or
failure of heating and cooling or other apparatus, and other similar risks in a
commercially reasonable amount, but in no event less than the amount required
by Landlord's insurance company to prevent the application of any coinsurance
provision.  Such property insurance shall also contain a clause pursuant to
which the insurance carriers waive all rights of subrogation against Tenant
with respect to losses payable under such policies and losses which would have
been payable if Landlord had carried such all risk insurance at one hundred
percent of the replacement cost of the Building improvements exclusive of any
improvements, alterations or additions in the Tenant's Premises or other
tenants' premises.

         C.      RISK OF LOSS.  By this Section 10, Landlord and Tenant intend
that the risk of loss or damage as described above be borne by responsible
insurance carriers to the extent above provided, and Landlord and Tenant hereby
agree to look solely to, and to seek recovery only from, their respective
insurance carriers in the event of a loss of a type described above to the
extent that such coverage is agreed to be provided hereunder.  For this
purpose, any applicable deductible amount shall be treated as though it were
recoverable under such policies.  Landlord and Tenant agree that applicable
portions of all monies collected from such insurance shall be used toward the
full compliance with the obligations of Landlord and Tenant under this Lease in
connection with damage resulting from fire or other casualty.

11.      TENANT'S AND LANDLORD'S RESPONSIBILITIES.

         A.      TENANT'S RESPONSIBILITIES.  Subject to the provisions of
Section 10.C hereof and to the extent permitted by law, Tenant shall assume the
risk of responsibility for, have the obligation to insure against, and
indemnify Landlord, its members, partners, and their officers, directors,
agents and employees (collectively, the "Landlord Indemnified Parties") and
hold the Indemnified Parties harmless from, any and all liability for any loss,
damage, injury, cost or expense incurred by the Indemnified Parties and
occasioned by or in any way related to or connected with (i) the use and
occupancy of the Premises or the Building by Tenant, its agents, employees,
invitees, and any other persons who gain access to the Premises including,
without limitation, any violation of the Americans with Disabilities Act and
any zoning, health, environmental or other law, ordinance, order, rule or
regulation of any governmental body or agency, (ii) the negligence or the
intentionally wrongful acts or omissions of Tenant, its agents, employees and
invitees, and (iii) injury or death to individuals or damage to property
sustained in or about the Premises, except, with respect to the above, to the
extent such loss, damage or injury results from the negligence or willful
misconduct of such Landlord





                                       19
<PAGE>   24
Indemnified Parties.  Tenant's obligation to indemnify Landlord hereunder shall
include the duty to defend against any claims asserted by reason of such loss,
damage or injury and to pay any judgments, settlements, costs, fees and
expenses, including attorneys' fees, incurred in connection therewith. Tenant
shall not be responsible to Landlord for any consequential damages except in
connection with Tenant's breach of any provision of Section 17 which continues
in excess of thirty successive calendar days, or breach of any provision of
Sections 20 or 21 hereof.

         B.      LANDLORD'S RESPONSIBILITIES.  Subject to the provisions of
Section 10.C hereof and to the extent permitted by law, Landlord shall assume
the risk of responsibility for, have the obligation to insure against, and
indemnify, Tenant, its members, partners, and their officers, directors, agents
and employees (collectively, the "Tenant Indemnified Parties") and hold the
Tenant Indemnified Parties harmless from, any and all liability for any loss,
damage, injury to persons (including, without limitation, death resulting
therefrom), injury to property, cost or expense incurred by Tenant Indemnified
Parties to the extent resulting directly from any negligence or willful
misconduct of Landlord or any agent or employee of Landlord in connection with
the management, repair, maintenance or use of the common areas of the Building,
except to the extent any such loss, damage injury results from the negligence
or willful misconduct of such Tenant Indemnified Parties.  Landlord's
obligation to indemnify Tenant hereunder shall include the duty to defend
against any claims asserted by reason of such loss, damage or injury and to pay
any judgments, settlements, costs, fees and expenses, including attorneys'
fees, incurred in connection therewith.

12.      FIRE OR OTHER CASUALTY

         A.      DESTRUCTION OF THE BUILDING.  If the Building should be
substantially destroyed (which, as used herein, means destruction or damage to
at least 75% of the Building) by fire or other casualty, either party hereto
may, at its option, terminate this Lease by giving written notice thereof to
the other party within (30) days after such casualty.  In such event, the Rent
shall be apportioned to and shall cease to accrue as of the date of such
casualty.  In the event neither party exercises this option, then the Premises
shall be reconstructed and restored, at Landlord's expense, to substantially
the same condition as they were prior to the casualty.

         B.      DESTRUCTION OF THE PREMISES.  If the Premises are damaged, in
whole or in part, by fire or other casualty, but the Building is not
substantially destroyed as provided above or if neither Landlord nor Tenant
terminates this Lease in accordance with Section 12.A, then the parties hereto
shall have the following options:

                 (i)      If, in Landlord's reasonable judgment, the Premises
         cannot be reconstructed or restored within one hundred twenty (120)
         days after such casualty to substantially the same condition as they
         were in prior to such casualty, Landlord may terminate this Lease by
         written notice given to Tenant within thirty (30) days after the
         casualty.  If, in Landlord's reasonable judgement, the Premises cannot
         be reconstructed or restored within one hundred twenty (120) days
         after such casualty to substantially the same condition as they were
         in prior to such casualty, but nonetheless Landlord does not so elect
         to terminate this Lease, then Landlord shall notify Tenant, within
         thirty (30) days after the casualty, of the amount of time necessary,
         as reasonably estimated by Landlord, to reconstruct or restore the
         Premises.  After receipt of such notice from Landlord, Tenant may
         elect to terminate this Lease.  This election shall be made by Tenant
         by giving written notice to Landlord within fifteen (15) business days
         after the date of Landlord's notice.  If neither party terminates this
         Lease pursuant to the foregoing, Landlord shall proceed to reconstruct
         and restore the Premises to substantially the same condition as they
         were in prior to the casualty.  In such event this Lease shall
         continue in full force and effect to the balance of the Term, upon the
         same terms, conditions and covenants as are contained herein;
         provided, however, that the Rent shall be abated in the proportion
         which the approximate area of the portion of the Premises that is
         untenantable (as a result of such casualty) bears to the total area in
         the Premises, from the date of the casualty until substantial
         completion of the reconstruction of the Premises.

                 (ii)     Notwithstanding the above, if the casualty occurs 
         during the last twelve (12) months





                                       20
<PAGE>   25
         of the Term, either party hereto shall have the right to terminate
         this Lease as of the date of the casualty, which right shall be
         exercised by written notice to be given by either party to the other
         party within thirty (30) days therefrom.  If this right is exercised,
         Rent shall be apportionedto and shall cease as of the date of the
         casualty.  If a casualty occurs during the last twelve (12) months of
         the Term and after the expiration of the thirty (30) day period set
         forth above, Tenant shall have no right to exercise any previously
         unexercised renewal options without first obtaining Landlord's written
         consent.  Notwithstanding the foregoing sentence, nothing stated
         herein, however, shall impair or impede the Landlord's right to
         terminate this Lease under Section 12. B. (i) above.

                 (iii)    If, in Landlord's reasonable judgment, the Premises
         are able to be restored within one hundred twenty (120) days after
         such casualty to substantially the same condition as they were prior
         to such casualty, Landlord shall so notify Tenant within thirty (30)
         days after the casualty, and Landlord shall then proceed to
         reconstruct and restore the damaged portion of the Premises, at
         Landlord's expense, to substantially the same condition as it was
         prior to the casualty, Rent shall be abated in the proportion which
         the approximate area of the portion of the Premises that is
         untenantable (as a result of such casualty) bears to the total area in
         the Premises from the date of the casualty until substantial
         completion of the reconstruction repairs, and this Lease shall
         continue in full force and effect for the balance of the Term, upon
         the same terms, conditions and covenants as are contained herein.

                 (iv)     In the event Landlord undertakes reconstruction or
         restoration of the Premises pursuant to subsections (i), (ii) or (iii)
         above, Landlord shall use reasonable diligence in completing such
         reconstruction repairs, but in the event Landlord fails to
         substantially complete the same within one hundred eighty (180) days
         after the date of the casualty (except however if under subsection (i)
         above Landlord notified Tenant that it would take longer than one
         hundred twenty (120) days to reconstruct or restore the Premises, but
         Tenant nonetheless elected not to terminate the Lease but required
         Landlord to reconstruct or restore the Premises, then the foregoing
         one hundred and eighty (180) day period shall be extended to the time
         period set forth in Landlord's notice plus sixty (60) days), except as
         a result of any of the occurrences set forth in Section 26.J. below,
         Tenant may, at its option, terminate this Lease as of a date ten (10)
         days after receipt of its notice by Landlord upon giving Landlord
         written notice to that effect, whereupon both parties shall be
         released from all further obligations and liability hereunder, except
         if in such ten (10) day period Landlord substantially completes such
         reconstruction or restoration then such termination shall be deemed
         null and void.

                 (v)      Notwithstanding anything contained herein to the
         contrary, Landlord shall have no duty to repair or restore the
         Premises or Building if the damage is due to an uninsurable casualty
         or if insurance proceeds are insufficient to pay for such repair or
         restoration or if the holder of any mortgage, deed of trust or similar
         instrument applies proceeds of insurance to reduce its loan balance
         and the remaining proceeds, if any, available to Landlord are not
         sufficient to pay for such repair or restoration (not taking into
         consideration any deductible).  If Landlord notifies Tenant within
         ninety (90) days after the casualty that Landlord elects not to repair
         or restore the Premises or Building due to any of the foregoing
         circumstances, each of the parties shall have a right to terminate
         this Lease as of the date of the casualty, which right shall be
         exercised by written notice to be given by either party to the other
         party within thirty (30) days of receipt by Tenant of Landlord's
         notice set forth in this subsection.  If this right is exercised, Rent
         shall be apportioned to and shall cease as of the date of the
         casualty.

13.      CONDEMNATION.  If the Building (or more than fifty percent (50%) of
the rentable square footage thereof) or the Premises (or any portion of the
Premises located on the first or second floor of the Building) is rendered
untenantable by reason of a condemnation (or by a deed given in lieu thereof)
and Landlord does not restore or replace such untenantable portion within one
hundred twenty days of such condemnation or deed given in lieu thereof, then
either party may terminate this Lease by giving, written notice of termination
to the other party within thirty (30) days after such condemnation, in which
event this Lease shall terminate effective as of the date of such condemnation.
If this Lease so terminates, Rent shall be paid through and apportioned as of
the date on which Tenant is legally obligated to vacate the Premises.  If such





                                       21
<PAGE>   26
condemnation does not render the Premises or the Building untenantable, this
Lease shall continue in effect and Landlord shall promptly restore the portion
not condemned to the extent reasonably possible to the condition in which they
were delivered to Tenant.  In such event, however, Landlord shall not be
required to expend an amount in excess of the proceeds received by Landlord
from the condemning authority less any amounts applied by a holder of any
mortgage, deed of trust or similar instrument against its loan balance.  If
only a portion of the Premises are so taken and cannot be restored, then this
Lease shall terminate as to the portion of the Premises actually taken by the
condemning authority as of the date when title vests in such governmental or
quasi-governmental authority, and Base Rent shall be ratably reduced as of such
date.  Landlord reserves all rights to compensation for any condemnation.
Tenant hereby assigns to Landlord any right Tenant may have to such
compensation, and Tenant shall make no claim against Landlord or the condemning
authority for compensation for termination of Tenant's leasehold interest under
this Lease or interference with Tenant's business.  The foregoing
notwithstanding, as long as Landlord's award is not thereby reduced, Tenant
shall be entitled to claim, prove and receive in the condemnation proceedings,
such awards as may be allowed by statute for its relocation expenses, but only
if such awards shall be made by the condemning authority in addition to, and
stated separately from, the award made by it for the Building or the part
thereof so taken.  In no event shall Tenant be entitled to any award for the
unexpired portion of the Term.

14.      ASSIGNMENT AND SUBLETTING

         A.      LANDLORD'S CONSENT.  Tenant shall not, without the prior
written consent of Landlord: (i) assign, convey, mortgage, sublease or
otherwise transfer, directly or indirectly, this Lease or any interest
hereunder, or sublease the Premises, or any part thereof, whether voluntarily
or by operation of law; or (ii) permit the use or occupancy of the Premises by
any person other than Tenant and its employees or consultants, business
partners and clients of Tenant provided that as to such consultants, business
partners or clients such use or occupancy is for a period not in excess of
twelve (12) continuous months.  Any such assignment, conveyance, mortgage,
sublease or other transfer or use described in the preceding, sentence (a
"Transfer") occurring without the prior written consent of Landlord shall be
void and of no effect and shall constitute a Default.  Landlord's consent to
any Transfer shall not constitute a waiver of Landlord's right to withhold its
consent to any future Transfer.  Landlord's consent to any Transfer or
acceptance of rent from any party other than Tenant shall not release Tenant
from any covenant or obligation under this Lease.  Landlord may require as a
condition to its consent to any assignment of this Lease that the assignee
execute an instrument in which such assignee assumes the obligations of Tenant
hereunder arising after the date of such assignment.

         B.      STANDARDS FOR CONSENT.  If Tenant desires the consent of
Landlord to a Transfer, Tenant shall submit to Landlord, at least fifteen (15)
days (or 10 days in the case of a subletting during the first Lease Year) prior
to the proposed effective date of the Transfer, a written notice ("Tenant's
Transfer Notice") which includes the business terms of the assignment or
subletting, financial information and statements concerning the proposed
transferee and such other information as Landlord may reasonably require about
the proposed Transfer and the transferee, together with a non-refundable
processing fee in the amount of five hundred dollars ($500.00), which fee shall
be waived in the case of a subletting during the first Lease Year.  If Landlord
does not terminate this Lease, in whole or in part, pursuant to Section 14.C,
Landlord shall not unreasonably withhold, condition or delay its consent to any
assignment or sublease.  Landlord shall not be deemed to have unreasonably
withheld its consent if, in the reasonable judgment of Landlord: (i) the
transferee is of a character or engaged in a business which is not consistent
with the first-class nature of the Building; (ii) the transferee is a tenant of
the Building or any building in the Worldgate Office Park (i.e., the area of
Herndon, Virginia commonly known as Worldgate Office Park) which is owned by
Landlord or an affiliate of Landlord, (iii) the transferee is negotiating
(i.e., Landlord has at least executed a letter of intent with such transferee)
for any space in the Building or any building in the Worldgate Office Park
which is owned by Landlord or an affiliate of Landlord; provided, however, that
this item (iii) shall not apply to any Transfer that is a sublease for an
aggregate period of four (4) years or less (including any possible renewal or
extension periods pursuant to any renewal or extension options or other similar
rights provided to the subtenant); (iv) the transferee is a governmental
agency, entity or unit; or (v) Tenant is in Default under this Lease. The
immediately preceding sentence shall not be deemed to be a limitation of the
bases upon which Landlord





                                       22
<PAGE>   27
may withhold its consent to a proposed transfer.  If Landlord wrongfully
withholds its consent to any Transfer, Tenant's sole and exclusive remedy
therefor shall be to seek specific performance of Landlord's obligation to
consent to such Transfer.  Landlord shall have fifteen (15) days (or 10 days in
the case of a subletting during the first Lease Year) from the receipt of
Tenant's Transfer Notice (together with the current financial statements of the
proposed assignee or subtenant and other information required hereunder) to
review Tenant's request and to notify Tenant (by certified mail, return receipt
requested) whether it will consent to such proposed assignee or subtenant. If
(i) Landlord fails to notify Tenant whether or not it will consent to such
proposed assignee or subtenant within such fifteen (15) day period (or 10 day
period in the case of a subletting during the first Lease Year), and,
thereafter, Tenant delivers notice ("Transfer Response Failure Notice") to
Landlord by certified mail, return receipt requested, of such failure and (ii)
Landlord fails to respond to such request within 5 business days after
Landlord's receipt of the Transfer Response Failure Notice, then Landlord's
consent to the proposed assignee or subtenant shall be deemed given.

         C.      RECAPTURE.  Except with respect to a proposed subletting or
assignment to a Qualified Tenant Affiliate, Landlord shall have the right to
terminate this Lease upon a proposed assignment of this Lease or a subletting
of all or substantially all of the Premises; provided, that in the event of a
subletting for less than the entire remaining Term, Landlord shall be entitled
to terminate this Lease with respect to the portion of the Premises proposed to
be sublet only for the period of the proposed sublease.  Landlord may exercise
such right to terminate by giving notice to Tenant at any time within thirty
(30) days after the date on which Tenant has furnished to Landlord all of the
items required under Section 14.B.  If Landlord exercises such right to
terminate, Landlord shall be entitled to recover possession of, and Tenant
shall surrender to Landlord the Premises on the later of (i) the effective date
of the proposed Transfer, or (ii) sixty (60) days after the date of Landlord's
notice of termination.  In the event Landlord exercises such right to
terminate, Landlord shall have the right to enter into a lease with the
proposed transferee without incurring any liability to Tenant on account
thereof.

         D.      ASSIGNMENT OR SUBLETTING TO AFFILIATES.  Notwithstanding
anything to the contrary contained in Section 14.A hereof, Tenant may upon at
least twenty-one (21) days prior written notice to Landlord (the "Affiliate
Assignment Notice"), but without Landlord's prior written consent, assign this
Lease, or sublet all or a portion of the Premises to a Qualified Tenant
Affiliate (hereinafter defined), provided, that the business operations of the
proposed assignee or subtenant (which shall be disclosed in the Affiliate
Assignment Notice") does not conflict with any exclusivity or other limitation
that may be imposed upon Landlord, and no default exists hereunder and no event
exists which event with notice and/or the passage of time would constitute a
default hereunder if not cured within the applicable cure period.  A "Qualified
Tenant Affiliate" shall mean a corporation or other entity which (i) shall
control, be controlled by or be under common control with Tenant or which
results from a merger or consolidation with Tenant or succeeds to all or
substantially all of the business and assets of Tenant, (ii) is of a type and
quality consistent with the first-class nature of the Building, (iii) is not a
party by whom any suit or action could be defended on the ground of sovereign
immunity.  For purposes of the immediately preceding sentence, "control" shall
be deemed to be ownership of more than fifty percent (50%) of the legal and
equitable interest of the controlled corporation or other business entity.  In
the event of any assignment to a Qualified Tenant Affiliate, Tenant shall
remain fully liable to perform the obligations of the Tenant under this Lease,
such obligations to be joint and several with the obligations of the Qualified
Tenant Affiliate as tenant under this Lease.  Notwithstanding any provision
contained in this Lease to the contrary, Landlord's prior written consent shall
be required to (a) any merger, consolidation or acquisition involving Tenant or
the assets or ownership interest of Tenant if in connection therewith, any of
the assets of Tenant are transferred, granted or pledged as security for the
purchase price (or other consideration) for such merger, consolidation or
acquisition, and (b) any sale, conveyance or transfer of all or substantially
all of Tenant's assets to an entity that does not assume all of the obligations
of Tenant under this Lease.

         E.      MISCELLANEOUS.

                 (i)      In the event Tenant fails to execute and deliver the
         assignment or sublease to which Landlord consented within ninety (90)
         days after the giving of such consent, then Tenant shall again comply
         with all of the provisions and conditions of this Section 14 before
         assigning its interest in this





                                       23
<PAGE>   28
         Lease or subletting any portion of the Premises.

                 (ii)     Tenant shall remain fully liable for the performance
         of all of Tenant's obligations hereunder jointly and severally with
         any assignee or subtenant (as a primary obligor) notwithstanding any
         subletting or assignment provided for herein.

                 (iii)    In addition to the processing fee described in
         Section 16.B, Tenant shall pay to Landlord any third party reasonable
         attorneys' or other fees and expenses incurred by Landlord in
         connection with any proposed Transfer, whether or not Landlord
         consents to such Transfer, such payment to Landlord not to exceed
         $1,500 for each such consent request.

15.      SURRENDER.  Upon termination of the Term or Tenant's right to
possession of the Premises, Tenant shall return the Premises to Landlord in
good order and condition, ordinary wear and tear and damage by fire or other
casualty excepted; provided, however, that if Tenant is insured (or required
pursuant to this Lease to be insured) for any such damage, Tenant shall, at the
election of Landlord, either (i) apply the insurance proceeds (or the amount
Tenant would have received as insurance proceeds had Tenant maintained the
insurance required pursuant to this Lease) to repair such damage or (ii)
surrender such insurance proceeds (or the amount Tenant would have received as
insurance proceeds had Tenant maintained the insurance required by this Lease)
to Landlord upon surrender of the Premises.  If Landlord requires Tenant to
remove any alterations pursuant to Section 9, then such removal shall be done
in a good and workmanlike manner, and upon such removal Tenant shall restore
the Premises to its condition prior to the installation of such alterations.
If Tenant does not remove such alterations after request to do so by Landlord,
Landlord may remove the same and restore the Premises, and Tenant shall pay the
cost of such removal and restoration to Landlord upon demand.  Tenant shall
also remove its furniture, equipment, trade fixtures and all other items of
personal property from the Premises prior to termination of the Term or
Tenant's right to possession of the Premises.  If Tenant does not remove such
items, Tenant shall be conclusively presumed to have conveyed the same to
Landlord without further payment or credit by Landlord to Tenant, or at
Landlord's sole option such items shall be deemed abandoned, in which event
Landlord may cause such items to be removed and disposed of at Tenant's
expense, which shall be one hundred fifteen percent (115%) of Landlord's actual
cost of removal, without notice to Tenant and without obligation to compensate
Tenant except as otherwise required by law.

16.      DEFAULTS AND REMEDIES.

         A.      DEFAULT.  The occurrence of any of the following shall
constitute a default (a "Default") by Tenant under this Lease:  (i) Tenant
fails to pay any Rent when due and such failure is not cured within five (5)
business days after Landlord delivers to Tenant written notice thereof,
although no legal or formal demand has been made therefor; (ii) Tenant fails to
perform any other provision of this Lease and such failure is not cured within
thirty (30) days (or immediately if the failure involves a hazardous condition)
after notice from Landlord; provided that no such notice shall be required if
Tenant has received two similar notices within three hundred sixty-five (365)
days prior to such failure; provided that if such failure will take longer than
this thirty (30)-day period to cure, Tenant shall have such longer period (not
to exceed sixty (60) days), as may be reasonably required to effectuate such
cure, as long as such cure is commenced within such thirty (30)-day period, and
such cure is prosecuted diligently to completion,; (iii) the leasehold interest
of Tenant is levied upon or attached under process of law; (iv) INTENTIONALLY
DELETED; or (v) any voluntary or involuntary proceedings are filed by or
against Tenant or any Guarantor of this Lease under any bankruptcy, insolvency
or similar laws and, in the case of any involuntary proceedings, are not
dismissed within thirty (30) days after filing.

         B.      RIGHT OF RE-ENTRY.  Upon the occurrence of a Default, Landlord
may elect to terminate this Lease or, without terminating this Lease, terminate
Tenant's right to possession of the Premises.  Upon any such termination,
Tenant shall immediately surrender and vacate the Premises and deliver
possession thereof to Landlord.  Tenant grants to Landlord the right to enter
and repossess the Premises and to expel Tenant and any others who may be
occupying the Premises and to remove any and all property therefrom without
being deemed in any manner guilty of trespass and without relinquishing
Landlord's rights to Rent or





                                       24
<PAGE>   29
any other right given to Landlord hereunder or by operation of law.

         C.      TERMINATION OF RIGHT TO POSSESSION.  If Landlord terminates
Tenant's right to possession of the Premises without terminating this Lease,
Landlord shall use commercially reasonable efforts to relet the Premises to
another tenant (a "Substitute Tenant") on then market terms which Landlord is
then quoting for comparable space in other buildings owned by Landlord in
Worldgate Office Park or which Landlord would quote if comparable space is not
then available in Worldgate Office Park; provided, however, (i) Landlord may
first lease Landlord's other available space and shall not be required to
accept any tenant offered by Tenant or to observe any instructions given by
Tenant about such reletting, (ii) Landlord shall have no obligation to solicit
or entertain negotiations with any other prospective tenants for the Premises
until Landlord obtains full and complete possession of the Premises; (iii)
Landlord shall not be obligated to lease the Premises to a Substitute Tenant
for a rental less than the current fair market rental then prevailing for
similar office uses in comparable buildings in the same market area as the
Building, nor shall Landlord be obligated to enter into a new lease under other
terms and conditions that are unacceptable to Landlord under Landlord's then
current leasing policies for comparable space in the Building; (iv) Landlord
shall not be  obligated to enter into a lease with any proposed tenant whose
use would:   (1) violate any restriction, covenant or requirement contained in
the lease of another tenant of the Building; (2) adversely affect the
reputation of the Building; or (3) be incompatible with the operation of the
Building as a first class building; and (v) Landlord shall not be obligated to
enter into a lease with any proposed Substitute Tenant which does not have, in
Landlord's reasonable opinion, sufficient financial resources or operating
experience to operate the Premises in a first class manner.  Tenant shall
reimburse Landlord for the actual costs and expenses (or reasonable estimate
thereof) of reletting the Premises, including, but not limited to, all
brokerage, advertising, legal, alteration, redecorating, repairing and other
expenses reasonably incurred to secure a new tenant for the Premises or portion
thereof.  In addition, if the consideration collected by Landlord upon any such
reletting or sale, after payment of the expenses of reletting, the Premises or
sale of the Building which have not been reimbursed by Tenant, is insufficient
to pay monthly the full amount of the Rent, Tenant shall pay to Landlord the
amount of each monthly deficiency as it becomes due.  If such consideration is
greater than the amount necessary to pay the full amount of the Rent and pay
Landlord for the expenses of reletting, then the full amount of such excess
shall be retained by Landlord and shall in no event be payable to Tenant.

         D.      TERMINATION OF LEASE.  If Landlord terminates this Lease,
Landlord may recover from Tenant and Tenant shall pay to Landlord, on demand,
as and for liquidated and final damages, an accelerated lump sum amount equal
to the amount by which Landlord's estimate of the aggregate amount of Rent
owing, from the date of such termination through the Expiration Date plus
Landlord's estimate of the aggregate expenses (incurred in good faith) of
reletting the Premises, exceeds Landlord's estimate of the fair rental value of
the Premises for the same period (after deducting from such fair rental value
the time needed to relet the Premises and the amount of concessions which would
normally be given to a new tenant) both discounted to present value at the rate
of six percent (6%) per annum.

         E.      OTHER REMEDIES.  Upon a default by Tenant under this Lease,
Landlord may, but shall not be obligated to, perform any obligation of Tenant
under this Lease, and, if Landlord so elects, all reasonable costs and expenses
paid by Landlord in performing such obligation, together with interest at the
Default Rate, shall be reimbursed by Tenant to Landlord on demand.  Any and all
remedies set forth in this Lease: (i) shall be in addition to any and all other
remedies Landlord may have at law or in equity; (ii) shall be cumulative; and
(iii) may be pursued successively or concurrently as Landlord may elect.  The
exercise of any remedy by Landlord shall not be deemed an election of remedies
or preclude Landlord from exercising any other remedies in the future.  Tenant
hereby expressly waives any and all rights of redemption, reentry or
repossession granted by or under any present or future laws or in equity and
any notice to quit or notice of Landlord's intention to re-enter the Premises.

         F.      BANKRUPTCY.  If Tenant becomes bankrupt, the bankruptcy
trustee shall not have the right to assume or assign this Lease unless the
trustee complies with all requirements of the United States Bankruptcy Code,
and Landlord expressly reserves all of its rights, claims and remedies
thereunder.

G.      WAIVER OF TRIAL BY JURY.  LANDLORD AND TENANT WAIVE TRIAL BY JURY IN
THE





                                       25
<PAGE>   30
EVENT OF ANY ACTION, PROCEEDING OR COUNTERCLAIM IN BROUGHT BY EITHER LANDLORD
OR TENANT AGAINST THE OTHER IN CONNECTION WITH THIS LEASE AND EACH PARTY
ACKNOWLEDGES THAT IT HAS HAD AN OPPORTUNITY TO CONSULT THEIR COUNSEL IN
CONNECTION WITH THIS WAIVER.

         H.      VENUE.  If either Landlord or Tenant, desires to bring an
action against the other in connection with this Lease, such action shall be
brought in the federal courts located in the Commonwealth of Virginia, or state
or local courts located in Fairfax County, Virginia.  Landlord and Tenant
consent to the jurisdiction of such courts and waive any right to have such
action transferred from such courts on the grounds of improper venue or
inconvenient forum.

17.      HOLDING OVER.  If Tenant retains possession of the Premises after the
expiration or termination of the Term or Tenant's right to possession of the
Premises, Tenant shall be deemed a tenant by the month.  Tenant shall pay Rent
during such holding over at one hundred fifty percent (150%) of the Rent in
effect immediately preceding such holding over computed on a monthly basis for
each month or partial month that Tenant remains in possession.  Except as
otherwise provided above with respect to the payment of Rent, Tenant shall, as
a monthly tenant, be subject to all of the terms, conditions, covenants and
agreements of this Lease.  Tenant shall give Landlord at least thirty (30)
days' written notice of any intention to quit the Premises, and Tenant shall be
entitled to thirty (30) days' written notice to quit the Premises; provided,
however, that if Tenant is in default hereunder, Tenant shall not be entitled
to any notice to quit, the usual thirty (30) days' notice to quit being hereby
expressly waived.  Tenant shall also pay, indemnify and defend Landlord from
and against all claims and damages, consequential as well as direct, sustained
by reason of Tenant's holding over. Notwithstanding the foregoing provisions of
this Section, in the event that Tenant shall hold over as set forth in the
first sentence of this Section and if Landlord shall desire to regain
possession of the Premises, then at any time prior to Landlord's acceptance of
Rent from Tenant as a monthly tenant hereunder, Landlord, at its option, may
forthwith reenter and take possession of the Premises by any legal process in
force in the Commonwealth of Virginia.

18.      SECURITY DEPOSIT.

         A.      AMOUNT.  Simultaneously with the execution of this Lease,
Tenant shall deposit with Landlord the amount set forth in ITEM 8 of the
Schedule, as a security deposit (the "Security Deposit").

         B.      SECURITY.  Such Security Deposit shall be considered as
security for the payment and performance by Tenant of all of Tenant's
obligations, covenants, conditions and agreements under this Lease except as
hereinafter provided.

         C.      FORM.  Such Security Deposit may, at Tenant's option, be
deposited by Tenant with Landlord in the form of cash or Tenant shall deliver
to Landlord one or two (2) irrevocable letter(s) of credit (collectively, the
"Letter of Credit"), in the aggregate amount set forth in ITEM 8 of the
Schedule.  If Tenant elects to provide the Letter of Credit as such Security
Deposit, Tenant shall maintain the Letter of Credit in full force and effect
throughout the entire term of this Lease and until thirty (30) days after the
end of the Term, and shall cause the Letter of Credit to be renewed or replaced
not less than sixty (60) days prior to its expiry date.  The Letter of Credit
shall (i) be unconditional, irrevocable, transferable, payable to Landlord on
sight at a metropolitan Washington, D.C. area financial institution, in partial
or full draws, (ii) be substantially in the form attached hereto and
incorporated herein as EXHIBIT G, and otherwise be in form and content
acceptable to Landlord, (iii) shall be issued by a financial institution
reasonably acceptable to Tenant and Landlord, and (iv) contain an "evergreen"
provision which provides that it is automatically renewed on an annual basis
unless the issuer delivers sixty (60) days' prior written notice of
cancellation to Landlord and Tenant.  Any and all fees or costs charged by the
issuer in connection with the Letter of Credit shall be paid by Tenant.  In the
event that the Security Deposit is made in the form of cash, then Landlord
agrees that such amount shall be held by Landlord in an interest bearing
account in a financial institution whose deposits are insured by the Federal
Deposit Insurance Corporation as security for Tenant's performance of its
obligations, covenants, conditions and agreements under this Lease.  Landlord
may commingle the Security Deposit with Landlord's other funds. The interest
rate payable on such Security Deposit shall be two and one-quarter percent
(2.25%)





                                       26
<PAGE>   31
per annum compounded.

         D.      RIGHT TO DRAW.

                 (i)      If the Security Deposit is in the form of cash, in
         the event of any default by Tenant hereunder, Landlord shall have the
         right, but shall not be obligated, to apply all or any portion of the
         Security Deposit to compensate Landlord (whether in whole or in part)
         for such default, in which event, within fifteen (15) days thereafter,
         Tenant shall be obligated to deposit with Landlord the amount
         necessary to restore the balance of the Security Deposit to its
         original amount; provided, however, neither the application of the
         Security Deposit as set forth above nor the payment by Tenant to
         restore such Security Deposit shall operate to cure such default or to
         estop Landlord from pursuing any remedy to which Landlord would
         otherwise be entitled.

                 (ii)     If the Security Deposit is in the form of a Letter of
         Credit, Landlord shall have the right to draw upon the Letter of
         Credit in whole or in part and apply the proceeds thereof as may be
         necessary to compensate Landlord for any default under this Lease on
         the part of Tenant, and Tenant, within fifteen (15) days after
         Landlord delivers written demand therefor to Tenant, shall forthwith
         restore the Letter of Credit to its original amount; provided,
         however, neither the application of the Security Deposit as set forth
         above nor the restoration by Tenant of such Security Deposit shall
         operate to cure such default or to estop Landlord from pursuing any
         remedy to which Landlord would otherwise be entitled unless the amount
         of the Security Deposit applied by Landlord is sufficient in
         Landlord's judgment to completely cure the default and Tenant has
         restored the Security Deposit to its original amount within five (5)
         days of the date of the draw down by Landlord of the Letter of Credit,
         then Landlord shall not be entitled to pursue any additional remedies
         with respect to the particular default which Landlord has determined
         was fully cured by the application of the Security Deposit.  Should
         Landlord elect to draw the full amount of the Letter of Credit upon a
         default by Tenant, Tenant expressly waives any right it might
         otherwise have to prevent Landlord from drawing on the Letter of
         Credit and agrees that an action for damages and not injunctive or
         other equitable relief shall be Tenant's sole remedy in the event
         Tenant disputes Landlord's claim to any such amounts.

                 (iii)    Landlord shall have the right to draw upon the Letter
         of Credit in any of the following circumstances: (i), if the total
         assets of the issuer of the Letter of Credit are at anytime less than
         Three Billion Dollars ($3,000,000,000.00), or such issuer has a
         Standard & Poor's commercial paper rating of less than A-1 (provided
         if at anytime the current Standard & Poor's commercial paper rating
         system is no longer in existence, a comparable rating of a comparable
         commercial paper rating system from a comparable company shall be
         selected by Landlord, in its reasonable discretion, for purposes of
         this Section 18) and Tenant fails to deliver to Landlord a replacement
         Letter of Credit complying with the terms of this Lease within thirty
         (30) days of request therefor from Landlord, (ii) if the credit rating
         of the issuer of the Letter of Credit is downgraded from the credit
         rating of such issuer at the time of the issuance of the Letter of
         Credit, the issuer of the Letter of Credit shall enter into any
         supervisory agreement with any governmental authority, or the issuer
         of the Letter of Credit shall fail to meet any capital requirements
         imposed by applicable law, and Tenant fails to deliver to Landlord a
         replacement Letter of Credit complying with the terms of this Lease
         within thirty (30) days of request therefor from Landlord, and (ii) if
         Tenant fails to provide Landlord with any renewal or replacement
         Letter of Credit complying with the terms of this Lease at least
         thirty (30) days prior to expiration of the then-current Letter of
         Credit, where the issuer of such Letter of Credit has advised Landlord
         of its intention not to renew the Letter of Credit.  In the event the
         Letter of Credit is drawn upon due solely to the circumstances
         described in the foregoing clauses (i), (ii) or (iii), the amount
         drawn shall be held by Landlord without interest as a Security Deposit
         to be otherwise retained, expended or disbursed by Landlord for any
         amounts or sums due under this Lease to which the proceeds of the
         Letter of Credit could have been applied pursuant to this Lease, and
         Tenant shall be liable to Landlord for restoration, in cash or Letter
         of Credit complying with the terms of this Lease, of any amount so
         expended to the same extent as set forth in this Section 18.





                                       27
<PAGE>   32
         E.      RIGHT TO PLEDGE OR ASSIGN.  Landlord shall have the right to
pledge or assign its interest in the Security Deposit and proceeds thereof to
any lender holding a security interest in the Premises.  In the event of any
sale or transfer of Landlord's interest in the Building, Landlord shall
transfer the Security Deposit to such purchaser or transferee, in which event
Tenant shall look solely to the new landlord for the return of the Security
Deposit and Landlord shall thereupon be released from all liability to Tenant
for the return of such Security Deposit.  No mortgagee or other purchaser of
any or all of the Building at any foreclosure proceeding brought under the
provisions of any mortgage shall (regardless of whether the Lease is at the
time in question subordinated to the lien of any mortgage) be liable to Tenant
or any other person for any or all of such sums or the return of any Security
Deposit (or any other or additional Security Deposit or other payment made by
Tenant under the provisions of this Lease), unless Landlord has actually
delivered the Security Deposit, or proceeds thereof, to such mortgagee or
purchaser.  If the Security Deposit is in the form of a Letter of Credit and if
requested by any such mortgagee or other purchaser, Tenant shall obtain an
amendment to the Letter of Credit which names such mortgagee or other purchaser
as the beneficiary thereof in lieu of Landlord.  This Security Deposit shall
not be transferable by Tenant to any assignee or subtenant, but shall be held
and returned directly to Tenant.

         F.      RESERVATION OF RIGHTS.  No right or remedy available to
Landlord as provided in this Section 18 shall preclude or extinguish any other
right to which Landlord may be entitled.  In furtherance of the foregoing, it
is understood that in the event Tenant fails to perform its obligations under
this Lease, any amounts recovered from the Security Deposit shall not be deemed
liquidated damages.  Landlord may apply such sums to reduce Landlord's damages
and such application of funds shall not in any way limit or impair Landlord's
right to seek or enforce any and all other remedies available to Landlord to
the extent allowed hereunder, at law or in equity.  Landlord shall not be
required to keep, and Tenant hereby waives the benefit of any provision of law
requiring Landlord to keep, the Security Deposit separate from its general
funds, on deposit in the Commonwealth of Virginia or in escrow or otherwise in
trust for Tenant. In no event shall the Security Deposit be considered an
advanced payment of Rent, and in no event shall Tenant be entitled to use the
Security Deposit for the payment of Rent.

         G.      REDUCTION OF SECURITY DEPOSIT.  As of the date that Tenant
pays to Landlord the first full month of Base Rent due under this Lease (i.e.,
in the amount of $395,629.03), if no default hereunder has occurred that
remains uncured, then the security deposit then required hereunder to be
maintained by Tenant with Landlord shall be reduced to an amount equal to Three
Hundred Ninety-five Thousand Six Hundred Twenty-nine and Three/100 Dollars
($395,629.03).  If the security deposit is in the form of cash, the foregoing
reduction shall be effectuated by Landlord applying to the monthly installment
of Base Rent coming due after the effective date of such reduction the amount
by which the security deposit is reduced pursuant to the foregoing provisions.
If the security deposit is in the form of a Letter of Credit, then the
foregoing reduction shall be effectuated by Tenant replacing the Letter of
Credit then being held by Landlord with a new Letter of Credit (complying with
the terms of this Lease) delivered to Landlord in the amount of Three Hundred
Ninety-five Thousand Six Hundred Twenty-nine and Three/100 Dollars
($395,629.03); provided, however, if Tenant provides to Landlord the Letter of
Credit in two (2) letters of credit, one of which being in the amount of Three
Hundred Ninety-five Thousand Six Hundred Twenty-nine and Three/100 Dollars
($395,629.03), then the reduction shall be effectuated by Landlord retaining
the Letter of Credit in the amount of Three Hundred Ninety-five Thousand Six
Hundred Twenty-nine and Three/100 Dollars ($395,629.03) and returning to Tenant
the other Letter of Credit.

         H.      RETURN OF SECURITY DEPOSIT.  Within thirty (30) days after the
expiration of the Term and vacation of the Premises by Tenant, Landlord shall
(provided that Tenant is not in default under the terms hereof) return and pay
back any Security Deposit to Tenant, less such portion thereof as Landlord
shall have retained to make good any default by Tenant with respect to any of
Tenant's aforesaid obligations, covenants, conditions or agreements, and if the
Security Deposit is in the form of cash, less the costs of maintaining the
Security Deposit in an interest bearing account charged by the financial
institution or depository holding such Security Deposit.

19.      INTENTIONALLY OMITTED.





                                       28
<PAGE>   33
20.      ESTOPPEL CERTIFICATE.  Tenant agrees that, from time to time upon not
less than ten (10) business days' prior request by Landlord, Tenant shall
execute and deliver to Landlord an estoppel certificate in the form attached
hereto as EXHIBIT D or such other certificate as reasonably requested by
Landlord, it being agreed that such certificate may be relied upon by any
prospective purchaser, mortgagee or other person having or acquiring an
interest in the Building.  If Tenant fails to execute and deliver any such
certificate within ten (10) business days after request, Tenant shall be deemed
to have irrevocably appointed Landlord and Landlord's beneficiaries as Tenant's
attorneys-in-fact, coupled with an interest, solely to execute and deliver such
certificate in Tenant's name.

21.      FINANCING.

         A.      SUBORDINATION.  This Lease is and shall be expressly subject
and subordinate at all times to (i) any ground or underlying lease of the
Building, now or hereafter existing, and all amendments, renewals and
modifications to any such lease, and (ii) the lien of any mortgage or deed of
trust now or hereafter encumbering fee title to the Building or the leasehold
estate, or both, under any such lease, and all amendments, renewals and
modifications to any such mortgage or deed of trust, unless such ground lease
or ground lessor, or mortgage or mortgagee, expressly provides or elects that
the Lease shall be superior to such lease or mortgage.  If any such mortgage or
trust deed is foreclosed, or if any such lease is terminated, upon request of
the mortgagee, holder or lessor, as the case may be, Tenant will attorn to the
purchaser at the foreclosure sale or to the lessor under such lease, as the
case may be, and at the request of such purchaser enter into a new lease with
such purchaser or lessor with the identical terms and conditions of this Lease.
The foregoing provisions are declared to be self-operative and no further
instruments shall be required to effect such subordination or attornment, or
both; provided, however, that Tenant agrees upon request by any such mortgagee,
holder, lessor or purchaser at foreclosure, to execute and deliver such
subordination or attornment, or both, instruments as may be required by such
person to confirm such subordination or attornment, or both, or any other
documents required to evidence superiority of the ground lease or mortgage,
should ground lessor or mortgagee elect such superiority.  If Tenant fails to
execute and deliver any such instrument or document within ten (10) business
days after request, Tenant shall be deemed to have irrevocably appointed
Landlord and Landlord's beneficiaries as Tenant's attorneys-in-fact, coupled
with an interest, to execute and deliver such instrument or document in
Tenant's name.  Landlord agrees that within fifteen (15) days of execution of
this Lease, Landlord shall obtain a subordination, non-disturbance and
attornment agreement for Tenant from NationsBank in its customary form.
Landlord hereby represents and warrants that as of the date of this Lease,
NationsBank is the only mortgagee that currently has a security interest in the
Land and Building and that there is no ground lease currently affecting the
Land and Building.  Notwithstanding the foregoing provisions of this Section
21, Tenant's subordination and attornment to any mortgage or ground lease, as
set forth in this Section 21, shall be conditioned upon Landlord obtaining a
subordination, non-disturbance and attornment agreement for Tenant from the
holder of such mortgage ("mortgagee") or the ground lessor under such ground
lease; provided that (i) if such mortgagee or ground lessor is a Qualified
Lender (hereinafter defined), then the form of such subordination, attornment
and non-disturbance agreement shall be in such mortgagee's or such ground
lessor's customary form, and (ii) if such mortgagee or ground lessor is not a
Qualified Lender, then the form of such subordination, attornment and
non-disturbance agreement shall either be in a form that such mortgagee or
ground lessor has in the past provided to a tenant leasing comparably sized
office space, or in a form otherwise commercially reasonable.  For purposes
hereof, the term "Qualified Lender" shall mean any entity that is in the
business of, or regularly engages in, underwriting or originating commercial
real estate loans, including without limitation, any life insurance company,
bank, financial institution, savings and loan institution, pension fund, real
estate investment trust, conduit, correspondent loan originator, investment
banking company or real estate investment trust.  Tenant shall reimburse
Landlord for any fee paid by Landlord to any mortgagee or ground lessor for
providing such agreement

         B.      MORTGAGEE REQUIREMENTS.  Tenant agrees to give any beneficiary
of a mortgage or lessor under a ground lease, by certified mail, return receipt
requested, a copy of any notice of default served upon Landlord, provided that
prior to such notice Tenant has been notified in writing by Landlord (or such
beneficiary or lessor) of the address of such beneficiary or lessor.  Tenant
further agrees that if Landlord shall have failed to cure such default within
the time provided for in this Lease, then such mortgagees and/or





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<PAGE>   34
lessor shall have a reasonable period of time thereafter (which shall be
limited to 120 days if such default materially adversely affects Tenant's
occupancy or use of the Premises) to commence and diligently pursue the
remedies necessary to cure such default (including but not limited to
commencement of foreclosure proceedings, if necessary, to effect such cure), in
which event Tenant shall not take any action to remedy such default, abate any
Rent and or to terminate this Lease (in each such case only to the extent it
has a right to do so) while such remedies are being so diligently pursued.  In
the event of the sale of the Building, by foreclosure or deed in lieu thereof,
or the termination of any ground lease, the purchaser at such sale or lessor
shall only be responsible for the return of any Security Deposit to the extent
that such purchaser or lessor actually receives such Security Deposit.  Tenant
further agrees that any such purchaser or lessor shall not be bound by (i) any
payment of Rent for more than one (1) month in advance, (ii) any amendment or
modification of this Lease made without the consent of Landlord's mortgagee or
such purchaser or lessor or (iii) any acts, omissions, events or conditions
arising prior to the time any such successor becomes the record owner of the
Building except for those acts, omissions, events, conditions which (a)
continue after the time such purchaser or lessor becomes the record owner of
the Land and Building; (b) which materially and adversely affect Tenant's use
and occupancy of the Premises and (c) are of a nature which are reasonably
capable of cure by such purchaser or lessor.

22.      QUIET ENJOYMENT.  As long as no Default exists, Tenant shall
peacefully and quietly have and enjoy the Premises for the Term, free from
interference by Landlord or any person claiming by through or under Landlord,
subject, however, to the provisions of this Lease.  The loss or reduction of
Tenant's light, air or view will not be deemed a disturbance of Tenant's
occupancy of the Premises nor will it affect Tenant's obligations under this
Lease or create any liability of Landlord to Tenant.  Additionally, no
disturbance of Tenant's occupancy of the Premises by other occupants or tenants
of the Building, including any noise or odors, will affect Tenant's obligations
under this Lease or create any liability of Landlord to Tenant, although
Landlord agrees to use its reasonable efforts to alleviate such disturbance
upon notice from Tenant to Landlord thereof.

23.      BROKER.  Tenant represents to Landlord that Tenant has dealt only with
the broker(s) set forth in ITEM 9 of the Schedule (collectively the "Broker")
in connection with this Lease and that, insofar as Tenant knows, no other
broker negotiated this Lease or is entitled to any commission in connection
herewith.  Tenant agrees to indemnify, defend and hold Landlord and Landlord's
beneficiaries and agents harmless from and against any claims for a fee or
commission made by any broker, other than the Broker, claiming to have acted by
or on behalf of Tenant in connection with this Lease.  Landlord agrees to
indemnify, defend and hold Tenant harmless from and against any claims for a
fee or commission made by any broker claiming to have acted by or on behalf of
Landlord in connection with this Lease.  Landlord agrees to pay the Broker, or
to cause Landlord's Broker to pay Tenant's Broker, a commission in accordance
with separate agreements to which it or its Broker is a party.

24.      NOTICES.  All notices and demands to be given by one party to the
other party under this Lease shall be given in writing, mailed or delivered to
Landlord or Tenant, as the case may be, at the following addresses:

         IF TO LANDLORD:          Monument One LLC
                                    c/o The John Akridge Company
                                    601 Thirteenth Street, N.W., Suite 300 North
                                    Washington, D.C. 20005
                                    Attention:  Mr. Michael J. Darby

         WITH COURTESY COPY TO:   Holland & Knight, LLP
                                    2100 Pennsylvania Avenue, N.W.
                                    Suite 400
                                    Washington, D.C. 20037
                                    Attention:  David C. Silver, Esquire



                                       30
<PAGE>   35


         IF TO TENANT:                     e.spire Communications, Inc.
                                           133 National Business Parkway, 
                                           Suite 200
                                           Annapolis Junction, Maryland 21701
                                           Attention:  Ms. Riley Murphy, Esquire


         WITH COURTESY COPY TO: e.spire Communications, Inc.
                                           133 National Business Parkway,
                                           Suite 200
                                           Annapolis Junction, Maryland 21701
                                           Attention:  Real Estate Department

or at such other address as either party may hereafter designate in accordance
with this Section.  Notices shall be delivered by hand or by United States
certified or registered mail, postage prepaid, return receipt requested, or by
a nationally recognized overnight air courier service that provides receipts.
Notices shall be considered to have been given upon the earliest to occur of
actual receipt, three (3) business days after posting in the United States mail
in the manner specified above or one (1) business day following delivery to
such air courier service or on the date of delivery if delivered by hand and a
receipt for such delivery signed by the recipient or the recipient's authorized
agent has been obtained.  In the event that Tenant requests that more than one
(1) person or address receive notices on its behalf hereunder, Landlord shall
use commercially reasonable efforts to send notices to all requested parties;
however, it shall not be a condition to the effectiveness of any notice that
more than one (1) person or address receive such notices.

25.      PARKING.  Tenant shall have the right to use 3.7 unreserved parking
spaces per 1,000 rentable square feet in the Premises made available to it by
Landlord and/or the parking operator (as Landlord may determine), at no charge
to Tenant.  Such parking spaces may be in the parking areas under or adjacent
to the Building, as surface parking, in a parking garage under the Building or
in a parking garage under an adjacent building, as Landlord shall determine.
Tenant shall have access to the parking area servicing the Building twenty-four
(24) hours per day, seven (7) days per week.   Tenant agrees that it and its
employees shall observe reasonable safety precautions in the use of the parking
areas to which the parking spaces apply, and shall at all times abide by all
rules and regulations promulgated by Landlord, the owner of the adjacent
building or the applicable parking garage operator governing the use of the
parking area to which the parking spaces apply.  It is understood and agreed
that Landlord has no responsibility to provide security to the parking area and
shall not be responsible for any theft of any property.  It is also understood
and agreed that Landlord does not assume any responsibility for any damage or
loss to any automobiles parked in any such parking areas or to any personal
property located therein, or for any injury sustained by any person in or about
such parking areas.


26.      MISCELLANEOUS.

         A.      SUCCESSORS AND ASSIGNS.  Subject to Section 14 of this Lease,
each provision of this Lease shall extend to, bind and inure to the benefit of
Landlord and Tenant and their respective legal representatives, successors and
assigns, and all references herein to Landlord and Tenant shall be deemed to
include all such parties.

         B.      ENTIRE AGREEMENT.  This Lease, and the riders and exhibits, if
any, attached hereto which are hereby made a part of this Lease including those
described in ITEM 10 of the Schedule, represent the complete agreement between
Landlord and Tenant, and Landlord has made no representations or warranties
except as expressly set forth in this Lease.  No modification or amendment of
or waiver under this Lease shall be binding upon Landlord or Tenant unless in
writing signed by Landlord and Tenant.

         C.      TIME OF ESSENCE.  Time is of the essence of this Lease and each
and all of its provision.

         D.      EXECUTION, DELIVERY AND AUTHORITY.  Submission of this
instrument for examination or signature by Tenant does not constitute a
reservation of space or an option for lease, and it is not effective until
execution and delivery by both Landlord and Tenant.  Execution and delivery of
this Lease by Tenant to Landlord shall constitute an irrevocable offer by
Tenant to lease the Premises on the terms and conditions





                                       31
<PAGE>   36
set forth herein which offer may not be revoked for ten (10) days after such
delivery.  Tenant hereby represents and warrants that the individual signing
this Lease is duly authorized to execute and deliver this Lease on behalf of
Tenant and that Tenant is a duly organized entity under the laws of the state
of its formation, is qualified to do business in the Commonwealth of Virginia,
is in good standing under the laws of the state of its formation and the laws
of the Commonwealth of Virginia, and has the power and authority to enter into
this Lease, and that all action requisite to authorize Tenant to enter into
this Lease has been duly taken.

         E.      SEVERABILITY.  The invalidity or unenforceability of any
provision of this Lease shall not affect or impair any other provisions.

         F.      GOVERNING LAW.  This Lease shall be governed by and construed
in accordance with the law of the Commonwealth of Virginia.

         G.      ATTORNEYS' FEES.  In the event suit shall be brought by either
party hereto against the other to enforce any of the provisions of this Lease,
the prevailing party in any such action shall be entitled to recover from the
other party all of its expenses incurred in connection with such action,
including reasonable attorneys' fees, disbursements and actual costs.

         H.      JOINT AND SEVERAL LIABILITY.  If Tenant is comprised  of  more
than  one  party, each such party shall be jointly and severally liable for
Tenant's obligations under this Lease.

         I.      FORCE MAJEURE.  Landlord shall not be in default hereunder and
Tenant shall not be excused from performing any of its obligations hereunder if
Landlord is prevented from performing any of its obligations hereunder due to
any accident, breakage, strike, shortage of materials, acts of God or other
causes beyond Landlord's reasonable control.

         J.      CAPTIONS.  The headings and titles in this Lease are for
convenience only, and shall have no effect upon the construction o
interpretation of this Lease.

         K.      NO WAIVER.  No receipt of money by Landlord from Tenant after
termination of this Lease or after the service of any notice or after the
commencing of any suit or after final judgment for possession of the Premises
shall renew, reinstate, continue or extend the Term or affect any such notice
or suit.  No waiver of any default of Tenant shall be implied from any omission
by Landlord to take any action on account of such default if such default
persists or is repeated, and no express waiver shall affect any default other
than the default specified in the express waiver and then only for the time and
to the extent therein stated.

         L.      LIMITATION OF LIABILITY; EFFECT OF SALE.  Any liability of
Landlord in the event of any claim against Landlord arising out of or in
connection with this Lease, the relationship of Landlord and Tenant or Tenant's
use of the Premises shall be limited solely to its interest in the Building;
provided, however, if Tenant has delivered to Landlord written notice of a
claim against Landlord under this Lease and the Building is subsequently sold
and Tenant files a law suit for such claim with a court with jurisdiction over
such claim within ninety (90) days after the sale of the Building and
diligently prosecutes such claim, then Tenant shall also be entitled to look to
the net proceeds of the sale (i.e., after all commissions, closing costs and
other costs associated with the sale have been satisfied) for the satisfaction
of such claim. In no event shall any personal liability be asserted against
Landlord in connection with any such claim nor shall any recourse be had to any
other property or assets of Landlord.  No property owned by any member or
partner of Landlord, or any of their employees, officers, directors,
shareholders, members or partners, shall be subject to levy, execution or other
enforcement procedures for the satisfaction of any judgment (or other judicial
process) or for the satisfaction of any other remedy of Tenant in connection
with any such claim.  Landlord and any successor will be relieved of its
obligations accruing after any sale of the Building or Premises and Tenant will
look solely to the successor for performance of those obligations, provided
such purchaser assumes or is deemed to have assumed all of the obligations of
Landlord under this Lease. Notwithstanding anything contained in this Lease to
the contrary, in no event shall Landlord be liable to Tenant on account of any
claims for lost business or profits or any indirect or consequential losses or
damages or any punitive





                                       32
<PAGE>   37
damages. Except in connection with a breach of any provision of Sections 17, 20
or 21 hereof, in no event shall Tenant be liable to Landlord on account of any
claims for any indirect or consequential losses or damages or any punitive
damages.

         M.      NO PARTNERSHIP.  Nothing contained in this Lease shall be
deemed or construed to create a partnership or joint venture of or between
Landlord and Tenant or to create any other relationship between the parties
hereto other than that of Landlord and Tenant.

         N.      TENANT'S DUTY TO MITIGATE.  Tenant agrees to use commercially
reasonable efforts to mitigate any damages that Tenant may suffer as a result
of any default by Landlord hereunder.

         O.      COMPLIANCE WITH LAWS AS OF COMMENCEMENT DATE.    If (i) as of
the date the Commencement Date, the common areas of the Building are in
violation of any applicable federal or state law, rule or regulation, the
violation of which would have a material adverse affect on Tenant's use thereof
or expose Tenant to any unreasonable health or safety risk, and (ii) an order
(after all final appeals have been exhausted) of any court or governmental
entity requires that such violation be cured, then Landlord shall promptly cure
such violation, and Landlord shall be responsible for the cost of curing such
violation (without including such expenditure as an "Expense").
Notwithstanding the foregoing, if the requirement that is violated results from
Tenant's business or particular use of the Premises or any alteration made by
Tenant in the Premises or Tenant or any of its agents or employees otherwise
caused such violation or was responsible for maintaining the item in violation
pursuant to the terms hereof, then Tenant shall pay for or reimburse Landlord
for the cost to cure such violation.

         P.      ENVIRONMENTAL REPRESENTATION.  Landlord represents and
warrants that, to the best of Landlord's actual knowledge (without inquiry or
investigation), as of the date hereof and as of the Commencement Date, no
hazardous materials are or will be located in the Premises or on the Land or
the Building in violation of any federal or applicable state law or any
regulations promulgated thereunder, the violation of which would have a
material adverse affect on Tenant's use thereof or expose Tenant to any health
or safety risk.  Tenant acknowledges that it has received a copy of Landlord's
Phase I Environmental Assessment, dated June 27, 1997, prepared by SCS
Engineers (the "Environmental Report") and that it is familiar therewith.
Tenant further acknowledges that Landlord makes no representation or warranty
as to the accuracy or completeness of such Environmental Report.

27.      ACCESS RIGHTS TO ADDITIONAL MONUMENT BUILDINGS.  As of the date hereof
Monument Fund, L.L.C. (the "Monument Fund") owns a controlling interest and is
the managing member of Landlord.  Monument Fund also owns a controlling
interest and is the managing member of each of the entities that own the
properties commonly known as Monument II, Monument III and Monument IV, located
in Worldgate, Fairfax County, Virginia, on which may be built additional office
buildings (each such additional office building, so long as the owning entity
is controlled by the Monument Fund, shall be referred to herein as an
"Additional Monument Building").  If any tenant of an Additional Monument
Building requests in writing that Landlord permit e.spire Communications, Inc.
or its permitted successors and assigns to provide to such tenant
telecommunications access services through optic fiber wiring, then, Landlord
shall cause such entity owning such Additional Monument Building to provide
e.spire Communications, Inc. or its permitted successors and assigns with
non-exclusive access to such Additional Monument Building to install such optic
fiber wiring through the telephone closets therein as may be necessary to
provide such services to such tenant (subject to such rules and regulations as
may be promulgated, and in such exact locations as may be determined, by the
owner of such Additional Monument Building); provided, however, that the
material terms and conditions upon which such access to such Additional
Monument Building shall be granted by such owner ("Owner") to Tenant shall be
as follows: (i) in the event that Owner has then granted to any other
telecommunications service provider ("TSP"), other than to any Major Tenant TSP
(hereinafter defined), access to such Additional Monument Building for a
purpose similar to the purpose for which Tenant is granted access to such
Additional Monument Building, then the material terms and conditions upon which
such access shall be granted by Owner to Tenant shall be no less favorable than
the most favorable material terms and conditions (including price, if any) upon
which Owner has then granted to said TSP access to such Additional Monument
Building for a purpose similar to the purpose for which Tenant is granted
access to such 






                                       33
<PAGE>   38
Additional Monument Building, (ii) in the event that Owner has
granted to a Major Tenant TSP, access to such Additional Monument Building for
a purpose similar to the purpose for which Tenant is granted access to such
Additional Monument Building, then the material terms and conditions upon which
such access shall be granted by Owner to Tenant shall be reasonably comparable
to the  material terms and conditions (including price, if any) upon which Owner
has then granted to said Major Tenant TSP access to such Additional Monument
Building for a purpose similar to the purpose for which Tenant is granted
access to such Additional Monument Building, or (iii) in the event there has
not then been granted to any other TSP access to such Additional Monument
Building for a purpose similar to the purpose for which Tenant is granted
access to such Additional Monument Building, then the material terms and
conditions upon which such access shall be granted by Owner to Tenant shall be
reasonably comparable to the material terms and conditions then generally being
granted by owners of first-class office buildings in Herndon and Reston,
Virginia to other TSPs that are provided access to such buildings by such
owners for purposes similar to the purpose for which Tenant is granted access
to such Additional Monument Building.  For purposes hereof, the term "Major
Tenant TSP" shall mean any telecommunications service provider providing its
own telecommunications service that has entered into a lease for at least
twenty-five thousand (25,000) square feet of space in a single Additional
Monument Building.  The rights set forth in this Section 27 are personal to
e.spire Communications, Inc. and shall not be transferable to any other third
party, tenant, subtenant or assignee other than to an entity that is an
assignee of this Lease and succeeds to all or substantially all of the business
and assets of e.spire Communications, Inc.  For purposes of this Section 27 and
Section 28, "control" shall be deemed to be ownership of more than fifty
percent (50%) of the legal and equitable interest of the controlled corporation
or other business entity.

28.      AVAILABLE SPACE IN ADDITIONAL MONUMENT BUILDINGS.  Landlord shall
deliver to Tenant a space availability report showing the space then available
in each of the Additional Monument Buildings.  Such space availability report
shall be delivered by Landlord to Tenant for each Additional Monument Building
during the month after the month in which the Commencement Date occurs and
during each month thereafter until at least ninety percent (90%) of the
rentable space in such Additional Monument Building has been initially leased
(the "Initial Lease-up Period").  After the Initial Lease-up Period for any
Additional Monument Building, Landlord shall only be required to deliver to
Tenant a space availability report for such Additional Monument Building only
during any month after which there has been (or there anticipates to
immediately be) a material change in the status of available space in such
Additional Monument Building.  During the Term of this Lease, the terms of any
lease of any space by Tenant in any Additional Monument Buildings shall be
negotiated on the then market terms.

29.      ANTENNA RIGHTS.

         A.      SCOPE OF RIGHTS.  Subject to the terms and conditions of this
Lease (including without limitations, the terms of this Section 29 and provided
Tenant is not in Default under this Lease, Landlord hereby grants Tenant the
non-exclusive right to install, maintain and operate during the Term the radio
antennae, satellite dish and related equipment ("Antenna Equipment") on a
portion of the roof of the Building in the location (i.e., the two (2) areas of
approximately 500 square feet each) indicated on EXHIBIT H (such location being
referred to as (the "Site"), provided, the Antenna Equipment (i) does not
affect the structure of the Building, the roof of the Building, the warranty
for the roof of the Building or the safety of the Building; (ii) does not
affect the electrical, mechanical or any other system of the Building or the
functioning thereof; (iii) does not interfere with the operation of the
Building or the provision of services or utilities to other tenants in the
Building, and (iv) is otherwise approved by Landlord in writing (which approval
shall not be unreasonably, withheld, conditioned or delayed).

         B.      COMPLIANCE WITH LAW.  Tenant shall install, maintain and
operate the Antenna Equipment in compliance with all present and future rules
and regulations of any local, State or Federal authority having jurisdiction
with respect thereto, including, without limitation, the rules and regulations
of the Federal Communications Commission ("FCC"), the Federal Aviation
Administration ("FAA") the Antenna Equipment being permitted under the laws,
rules and regulations of Fairfax County, Virginia, the City of Herndon,
Virginia and any other governmental and quasi-governmental authorities having
appropriate jurisdiction over the Building or Tenant's use of the Antenna
Equipment.  Tenant shall deliver to Landlord written proof of





                                       34
<PAGE>   39
compliance within twenty (20) days of Landlord's written request. Tenant shall
obtain all permits, licenses, variances, authorizations and approvals that may
be required in order to install the Antenna Equipment, subject to Section 11.B
hereof.

         C.      INSTALLATION PROCEDURES.

                 (i)      Prior to installation of the Antenna Equipment and
         any modifications or changes thereto, Tenant shall submit in writing
         all plans for Landlord's approval to Landlord, Attention:  Vice
         President, Network and Facilities, at 10 Woodbridge Center Drive,
         Woodbridge, New Jersey 07095 and shall commence work only after having
         obtained Landlord's written approval. The style, color, materials,
         exact location and method of installation of the Antenna Equipment are
         subject to the prior written approved of Landlord.

                 (ii)     All of such installations, modifications or changes
         shall conform to Landlord's reasonable technical requirements,
         including but not limited to, design and installation specifications,
         interference control devices and weight and windload requirements.

                 (iii)    The Antenna Equipment shall be clearly marked to show
         Tenant's name, address, telephone number, the name of the person to
         contact in case of emergency, FCC call sign, frequency and location;
         the transmissions lines shall be identified at the bottom and top of
         each line.

                 (iv)     In the event Tenant requires an electric power supply
         and/or usage different from that currently provided by Landlord and
         included within the Base Fee, Tenant shall, at its sole cost and
         expense, obtain such power supply.  Any work performed in connection
         therewith shall comply with the provisions of subsections 5(a) and
         5(b).  Any power lines installed by Tenant shall run within Landlord's
         current easements.  Any deviation from such easement rights shall be
         corrected at Tenant's expense, payable as an additional fee hereunder
         within ten (10) days of Tenant's receipt of an invoice therefor.

                 (v)      In the event a zoning variance is required in
         connection with the installation or modification of the Antenna
         Equipment, Landlord shall have the right, at its discretion, to allow
         Tenant to obtain such variance at Tenant's sole cost and expense,
         provided however, that such variance and/or the conditions under which
         such variance would be granted shall in no way impair or affect the
         zoning otherwise affecting the Land and/or the Building.  Landlord
         shall, at Tenant's request and expense, reasonably cooperate with
         Tenant in obtaining such variance.

                 (vi)     In all matters where Landlord's approval is required
         and where Landlord makes a reasonable determination that interference
         or other disruption with the business of Landlord or other existing
         licensees is likely to result from Tenant's contemplated action,
         Landlord shall have the right to withhold such approval by written
         notice to Tenant, setting forth in reasonable detail the basis for
         Landlord's disapproval.

                 (vii)    All work performed at the Building in connection with
         the installation and modification of the Antenna Equipment shall be
         performed at Tenant's expense by Tenant's employees or by contractors
         approved by Landlord in its good faith judgement; provided, however,
         at Landlord's sole election, Landlord shall be entitled to designate a
         contractor (which contractor has been selected by Tenant from a list
         of at least three contractors prepared by Landlord) to perform any
         such installation or modification that in Landlord's judgement may (i)
         affect the structure of the Building, the roof of the Building, the
         warranty for the roof of the Building or the safety of the Building;
         (ii) affect the electrical, mechanical or any other system of the
         Building or the functioning thereof; or (iii) interfere with the
         operation of the Building or the provision of services or utilities to
         Tenant or other tenants in the Building.

                 (viii)   Landlord shall provide to Tenant sufficient access to
         the Site during specified business hours (Monday through Friday 8:30
         a.m. - 5:00 p.m.) for the maintenance and operation of





                                       34
<PAGE>   40
         the Antenna Equipment.  Access to the Site at other times will be
         available only on an emergency basis, by telephoning the property
         manager for the Building (currently Bonnie Howard of The John Akridge
         Companies) at (202) 638-3000.  Tenant shall reimburse Landlord for all
         out-of-pocket costs and expenses reasonably incurred by Landlord as a
         result of such emergency access.  All access to the Site shall be
         subject to the continuing control of, and reasonable security and
         safety procedures established by, Landlord.

                 (ix)     Tenant shall pay Landlord (within 30 days after
         receipt of an invoice therefor) an amount equal to all cost incurred
         by Landlord to have an engineer review the plans and specifications
         for the Antenna Equipment and specifications and method for attaching
         the Antenna Equipment to the Building, such payment not to exceed
         $5,000.

                 (x)      Tenant shall install any screen or other covering for
         the Antenna Equipment that Landlord in its reasonable discretion may
         require (the size, type and style of which shall be subject to
         Landlord's prior written approval) in order to camouflage or conceal
         the Antenna Equipment.

                 (xi)     The Antenna Equipment shall not be more than
         thirty-six inches (36") in height (without Landlord's written
         approval, which shall not be unreasonably withheld, conditioned or
         delayed) and not more than the weight that Landlord shall determine is
         appropriate for the roof (which Landlord shall specify to Tenant upon
         Tenant's written request).

         D.      INTERFERENCE.

                 (i)      The installation, maintenance and operation of the
         Antenna Equipment shall not interfere electronically or otherwise,
         with (i) the equipment, facilities, site use and marketability or
         operations of Landlord, or (ii) the equipment, facilities or
         operations of Landlord's present licensees or tenants at the Building.
         If any interference is caused by the installation, maintenance and
         operation of the Antenna Equipment, Tenant shall, upon written or oral
         request, suspend its operations until such time as the interference
         has been eliminated, except for intermittent testing after performing
         such repair, modification, replacement or other action for the purpose
         of correcting the interference.  If Tenant is unable to rectify the
         interference, then upon Landlord's request, Tenant shall (at Tenant's
         cost) remove the Antenna Equipment from the Building (and restore the
         Site and the Building area affected to the condition existing prior to
         installation of the Antenna Equipment) and comply with the provisions
         of Section 29.E hereof governing removal of the Antenna Equipment.
         All transmitters and/or repeater systems at the Site shall be equipped
         with, at a minimum, a single stage isolator and a bandpass filter or
         bandpass/reject type duplexer.  No notch type duplexers will be
         allowed.  Complete technical characteristics for required equipment
         (including response curves) shall be furnished to Landlord and
         approved for use prior to Tenant's installation of the Antenna
         Equipment.

                 (ii)     Tenant waives any and all claims against Landlord for
         any interference caused to or with Tenant's Antenna Equipment by the
         present or future equipment or facilities of Landlord or any of its
         tenants or licensees.

         E.      MAINTENANCE AND REMOVAL OF THE ANTENNA EQUIPMENT.

                 (i)      Tenant shall, at its sole cost and expense, be
         responsible for the maintenance of the Antenna Equipment in accordance
         with all applicable laws and regulations and this Lease.  All
         maintenance work shall be performed by Tenant's employees or by
         certified contractors, previously approved in writing by Landlord,
         such approval not to be unreasonably withheld, conditioned or delayed,
         provided, however that Landlord's approval shall only be required for
         work which affects the roof structure or the method or mechanics of
         the attachment of the Antenna Equipment to the roof and to or through
         the Building.

                 (ii)     At the expiration or earlier termination of this
         Lease, Tenant shall remove the





                                       36
<PAGE>   41
         Antenna Equipment from the Building (and restore the Site and the
         Building area affected to the condition existing prior to installation
         of the Antenna Equipment) at Tenant's sole cost and expense.  The
         removal shall be performed by a certified contractor previously
         approved in writing by Landlord (such approval not to be unreasonably
         withheld, conditioned or delayed), in a workmanlike manner in
         accordance with a previously approved removal plan (such approval not
         to be unreasonably withheld, conditioned or delayed) and without
         causing any damage or material and continuous interference to the
         structures, equipment, or operations of Landlord or any of its
         licensees or tenants at the Building.  Should any interference, damage
         or destruction occur, remedy thereof shall be immediately commenced
         and diligently pursued by Tenant at Tenant's sole cost and expense.
         IfTenant fails to eliminate any such interference or to make any such
         repair within seven (7) days of receiving written or oral notice of
         the occurrence of interference or damage, Landlord may perform the
         necessary work at Tenant's cost and expense and such amount shall be
         paid by Tenant, as Additional Rent hereunder, within thirty (30) days
         of Tenant's receipt of an invoice therefor.

         F.      INDEMNIFICATION.

                 (i)      Tenant shall indemnify and save Landlord harmless
         from and against any and all loss, costs, liabilities, damages,
         judgements, and expenses, including reasonable attorney's fees, in
         connection with claims resulting from bodily injury or death of any
         person, or from damage to any property sustained by any person,
         including Tenant, arising from the installation, removal and
         maintenance of the Antenna Equipment, except to the extent resulting
         from Landlord's negligence or willful misconduct.

                 (ii)     Tenant covenants and agrees that the installation,
         operation and removal of the Antenna will be at its sole risk.
         Landlord shall not be liable to Tenant for any loss, damage or expense
         arising from any damage to the Antenna Equipment that may be
         occasioned by, through, or in connection with any acts or omissions of
         Landlord or Landlord's agents or employees except for Landlord's or
         Landlord's agents' or employees' gross negligence or wilful
         misconduct. Tenant agrees to indemnify and hold Landlord and all other
         persons or entities having facilities located at the Site harmless,
         from all costs of any damage done to Landlord's or other persons
         facilities or equipment located at the Building, that occur as a
         result of the installation, operation or maintenance of Tenant's
         Antenna Equipment.  Tenant hereby assumes the risk of the inability to
         operate Tenant's Antenna Equipment as a result of any structural or
         power failure at the Building or failure of Tenant's Antenna Equipment
         for any reason whatsoever other than the Landlord's gross negligence
         or wilful misconduct and agrees to indemnify and hold Landlord
         harmless from all damages and costs of defending any claim or suit for
         damages of any kind including business interruption (and attorney's'
         fees) asserted against Landlord by reason of such failure.

         G.      TAXES.  Tenant hereby acknowledges that the existence of the
Antenna Equipment and Tenant's other improvements may result in an increase in
the assessed valuation of the Building.  Upon receipt of documentation showing
that the Antenna Equipment or Tenant's other improvements caused such increase
Tenant shall reimburse Landlord for any increase in the real estate taxes
payable by Landlord as a consequence of the increase in assessed valuations,
subject to Tenant's right to obtain an exemption therefor on Landlord's behalf.
Landlord shall, at Tenant's sole cost and expense, cooperate with Tenant to
obtain an abatement of any such increase assessment.  In the event any sales,
use or other tax shall be payable by Landlord in connection with this Lease,
Tenant shall reimburse Landlord on demand for such payments or shall furnish
necessary documentation to the appropriate government authorities to show that
fee payments hereunder are exempt from such sales, use or other tax.

         H.      MISCELLANEOUS.  The rights set forth in this Section 29 are
personal to e.spire Communications, Inc. and shall be for the benefit (subject
to the terms and conditions hereof) of any assignee of this Lease or subtenant
of the Premises, but shall not be transferable to any other third party.
Landlord shall be entitled to grant to other persons or entities the right to
use certain portions of the roof; provided, however, that (i) Landlord shall
not grant to another user the right to erect an antenna (a) in front of
Tenant's signage on the side of the Building facing the Dulles Toll Road in a
manner that obstructs the





                                       37
<PAGE>   42
visibility of such sign from the Dulles Toll Road, (b) in the two (2)
restricted area identified on EXHIBIT H (the area of each being approximately
500 square feet), (c) on the parapet of the Building or in a manner that the
antenna extends away from the Building over the edge of the parapet, or (d)
within three (3) feet of the outside edge of the Building, (ii) Landlord shall
use reasonable efforts to locate any antenna of any other user of the roof in a
location that in Landlord's reasonable judgement would not be likely to
materially interfere with the operation of any Antenna of Tenant then on the
roof of the Building, and (iii) if any other user of the roof of the Building
places an antenna in an area on the roof of the Building that materially
interferes with the operation of an Antenna of Tenant then on the roof,
Landlord shall cause such other user to move its antenna to a location that in
Landlord's reasonable judgement would not materially interfere with the
operation of any Antenna of Tenant then on the roof of the Building.

30.      ACCESS RIGHTS FOR CABLING COMMUNICATIONS EQUIPMENT.  Landlord shall
grant to Tenant non-exclusive access to the risers in the Building to install
such optic fiber wiring (the "Telecom Cabling") therein as may be necessary for
Tenant to provide telecommunications access services to the Premises (subject
to such rules and regulations as may be promulgated by Landlord from time to
time), provided that such Telecom Cabling (i) does not affect the structure or
safety of the Building; (ii) does not affect the electrical, mechanical or any
other system of the Building or the functioning thereof; and (iii) does not
interfere with the operation of the Building or the provision of services or
utilities to Tenant or any other tenant of the Building.  Tenant shall install
and maintain the Telecom Cabling in compliance with all present and future
rules and regulations of any local, State or Federal authority having
jurisdiction with respect thereto, including, without limitation, the laws,
rules and regulations of the FCC, Fairfax County, Virginia, the City of
Herndon, Virginia and any other governmental and quasi-governmental authorities
having appropriate jurisdiction over the Building or Tenant's use of the
Telecom Cabling.  Tenant shall obtain all permits, licenses, variances,
authorizations and approvals that may be required in order to install and
maintain such Telecom Cabling.  Tenant shall, at its sole cost and expense, be
responsible for the insurance and maintenance of the Telecom Cabling and its
compliance with all applicable laws, rules and regulations.  Tenant shall
indemnify and save Landlord harmless from and against any and all loss, costs,
liabilities, damages, judgements, and expenses (including reasonable attorney's
fees) arising in connection with the installation, removal and maintenance of
the Telecom Cabling.

         IN WITNESS WHEREOF, the parties hereto have executed this deed of
Lease under seal in manner sufficient to bind them as of the day and year first
above written.

LANDLORD:

MONUMENT ONE LLC, a Delaware limited liability company

By:      The Monument Fund, LLC, its Sole Member

         By:     Monument Akridge, L.L.C., its Managing Member

                 By:      JACo Manager, L.L.C., its Managing Member

                          By:     JACo Manager, Inc.,  its     WITNESS:
                                  Managing Member




                                  By:               seal
                                         _______________   _____________________





                                         Name:
                                         Title:
TENANT:

E.SPIRE COMMUNICATIONS, INC.




                                       38
<PAGE>   43

By:                                 seal
   -------------------------------------
   Name:
   Its:                                       WITNESS/ATTEST:




                                              ---------------------------------
                                              [corporate seal]





                                       39
<PAGE>   44
                                   EXHIBIT A

                            FLOOR PLAN - FIRST FLOOR



<PAGE>   45
                            FLOOR PLAN- SECOND FLOOR


<PAGE>   46

                            FLOOR PLAN - THIRD FLOOR
<PAGE>   47




                            FLOOR PLAN- FOURTH FLOOR

<PAGE>   48


                            FLOOR PLAN - FIFTH FLOOR




<PAGE>   49


                            FLOOR PLAN - SIXTH FLOOR
<PAGE>   50


                           FLOOR PLAN - SEVENTH FLOOR
<PAGE>   51


                                   EXHIBIT A-1
                               DESCRIPTION OF LAND
<PAGE>   52

<PAGE>   53
                                    EXHIBIT B

                                 WORK AGREEMENT

       This Work Agreement is attached to and made a part of that certain Lease
       Agreement dated _______________ 1998 (the "Lease"), between Monument One
       LLC ("Landlord"), and e.spire Communications, Inc. ("Tenant"). The terms
       used in this Exhibit that are defined in the Lease shall have the same
       meanings as provided in the Lease.


1.     GENERAL.

       1.1.   PURPOSE. This Agreement sets forth the terms and conditions
              governing the design, permitting and construction of the Tenant
              Work (hereinafter defined) to be installed in the Premises. As set
              forth in Section 1.A of the Lease, the Tenant Work (as hereinafter
              defined) to be constructed in the Premises may be constructed in
              two (2) phases: Phase I (as hereinafter defined) and Phase II (as
              hereinafter defined). References herein to "any phase" shall refer
              to either Phase I or Phase II, and references herein to "each
              phase" shall refer to Phase I and Phase II.

       1.2.   TENANT'S REPRESENTATIVE. Tenant acknowledges that Tenant has
              appointed Michael Knouse as its authorized representative
              ("Tenant's Representative") with full power and authority
              to bind Tenant for all actions taken with regard to the Tenant
              Work. Landlord shall not be obligated to respond to
              or act upon any plan, drawing, change order or approval or other
              matter relating to the Tenant Work until it has been executed by
              Tenant's Representative. Neither Tenant nor Tenant's
              Representative shall be authorized to direct Landlord's general
              contractor with respect to the Tenant Work. Tenant shall cause
              Tenant's Representative to attend all of Landlord's weekly
              progress meetings in order to expedite and facilitate any and all
              approval necessary in connection with the construction process. In
              the event that the Landlord's general contractor performs any such
              work under the direction of Tenant or Tenant's Representative,
              then Landlord shall have no liability for the cost of such work,
              the cost of corrective work required as a result of such work, any
              delay that may result from such work, or any other problem in
              connection with such work.

2.     WORK AND MATERIALS.

       2.1.   STANDARD SHELL WORK. The Standard Shell Work (hereinafter defined)
              will be performed by Landlord at Landlord's sole cost and expense.
              The term "Standard Shell Work" means and refers to the following
              elements of the Premises: concrete floor (without floor covering);
              unfinished perimeter walls; unfinished ceilings (without
              acoustical ceilings, ceiling tiles, suspension system, insulation
              or light fixtures); closets for telephone and electrical systems
              (but not the systems themselves); building systems within the
              building core only as follows: mechanical (including heating,
              ventilating and air conditioning systems, but excluding any
              supplemental HVAC required by the Tenant Plans), electrical and
              plumbing systems, the design of which building systems shall be
              consistent with the specification set forth on EXHIBIT B-1 hereof
              and primary fire sprinkler distribution loop connected to core and
              secondary branch distribution to the Premises with upturned
              sprinkler heads in a ratio of one (1) sprinkler head per 130
              rentable square feet in the Premises.

       2.2.   TENANT IMPROVEMENTS; IMPROVEMENT ALLOWANCE. Landlord, at Tenant's
              expense, shall construct the Premises in accordance with the
              Tenant Plans. Tenant Plans shall be conclusive as to the entire
              scope of work to be performed by Landlord ("Tenant Work").
              Landlord agrees to provide Tenant with a maximum contribution not
              to exceed Twenty-six Dollars ($26.00) per rentable square foot
              which is equal to a total of Four Million Three Hundred Forty-nine
              Thousand Four Hundred Ten Dollars ($4,349,410.00) for the Tenant

<PAGE>   54

              Work ("Improvement Allowance"). The Improvement Allowance may be
              applied only against work, services and materials which are
              incurred in connection with Landlord's construction of the Tenant
              Work (including Landlord's Construction Management Fee,
              hereinafter defined). Tenant shall pay an administrative fee
              ("Landlord's Construction Management Fee") to compensate Landlord
              for reviewing the Tenant Plans and managing the Tenant Work in an
              amount equal to One Hundred Thousand Dollars ($100,000.00).
              Landlord's Construction Management Fee shall first be deducted
              from the Improvement Allowance and Landlord shall pay the
              remaining Improvement Allowance directly to the contractor(s)
              performing the Tenant Work. To the extent the full Improvement
              Allowance is not used by the Tenant for the purposes set forth
              herein, then, provided that Tenant is not then in default under
              the Lease (including without limitation, any provision of this
              Work Agreement), the balance thereof shall be paid to Tenant upon
              the occurrence of the Phase II Commencement Date and Tenant
              taking possession of the entire Premises.

3.     ARCHITECT AND ENGINEERS FOR TENANT WORK. Tenant shall employ an architect
       reasonably acceptable to Landlord (the "Leasehold Architect") to prepare
       all plans for the Tenant Work. Tenant shall employ the engineers (the
       "Leasehold Engineers") designated by Landlord to prepare the base
       building engineering drawings relating to the Tenant Work.

4.     PLANS FOR THE TENANT WORK. Tenant shall be entitled to deliver to
       Landlord the Tenant Plans (including the programming and space plans,
       design development drawings and construction documents) for the Phase I
       Tenant Work separately from Tenant's delivery of such plans for the Phase
       II Tenant Work; in which case, the approval process, and pricing and
       construction process and time requirements of this Work Agreement shall
       apply to each phase separately.

       4.1.   Tenant shall promptly cause the Leasehold Architect to prepare
              programming and space plans for each phase (i.e., Phase I and
              Phase II) of the Tenant Work, in form approved by Tenant, for
              submissions to Landlord. If Landlord has any comments with respect
              to the programming and space plans, Landlord shall make such
              comments known to Tenant in writing within five (5) business days
              following submission of the programming and space plans to
              Landlord. If Landlord desires that the programming and space plans
              be modified in any reasonable manner, then Tenant shall promptly
              revise the programming and space plans to address Landlord's
              concerns.

       4.2.   Tenant shall promptly cause the Leasehold Architect to prepare
              design development drawings for each phase of the Tenant Work, in
              form approved by Tenant, for submission to Landlord, which
              drawings shall be based upon the programming and space plans
              approved by Landlord. If Landlord has any comments with respect to
              the design development drawings, Landlord shall make such comment
              known to Tenant in writing within five (5) business days following
              submission of the design development drawings to Landlord. If
              Landlord desires that the design development drawings be modified
              in any reasonable manner, then Tenant shall revise such drawing to
              address Landlord's concerns.

       4.3.   Tenant shall cause the Leasehold Architect and the Leasehold
              Engineers to prepare final construction documents (including
              without limitation, all mechanical, electrical, telephone,
              finishes selections and fire/life safety plans and specifications)
              for each phase of the Tenant Work (the "construction documents"),
              in form approved by Tenant, for submission to Landlord no later
              than the date that is forty-five (45) days after Landlord approves
              the design development drawings for such phase of the Tenant Work,
              which construction documents shall be based upon the programming
              and space plans and the design development drawings for such phase
              approved by Landlord. If Landlord has any comment with respect to
              the construction documents, Landlord shall make such comments
              known to Tenant in writing within ten (10) business days following
              submission of the construction documents to Landlord, or if the
              nature of the required revisions is such that they cannot be
              revised within

<PAGE>   55

              such ten (10) day period, Tenant shall have such longer period as
              may be reasonably required to make such revisions, as long as
              Tenant promptly commences such revisions and diligently pursues
              completion thereof. If Landlord desires that the construction
              documents be modified in any reasonable manner, then Tenant
              shall, within three (3) business days after Landlord indicates
              the revisions required to the construction documents, make such
              revisions and resubmit the construction documents to Landlord for
              its approval. The construction documents for the Tenant Work that
              have been submitted by Tenant and approved by Landlord shall be
              referred to herein as the "Tenant Plans."

       4.4.   All Tenant Plans for the Tenant Work shall be subject to
              Landlord's prior written approval, which shall not be unreasonably
              withheld, conditioned or delayed, except that Landlord shall have
              complete discretion with regard to granting or withholding
              approval of Tenant Plans to the extent they impact the Building's
              structure or systems; provided, however, in determining whether to
              consent or condition its consent to any particular item of Tenant
              Work, Landlord shall be able to take into consideration, among
              other things, the appearance of any item of Tenant Work visible
              from the exterior of the Building and Landlord's interest in the
              uniformity of the window coverings and maintaining an appearance
              consistent with the first-class nature of the Building.
              Notwithstanding the forgoing, Tenant shall be entitled to cover
              the windows (from the inside) on the first and second floors of
              the Building, subject to Landlord's reasonable approval of all
              window coverings to be used as set forth above. Any changes,
              additions or modifications that Tenant desires to make to the
              Tenant Plans also shall be subject to Landlord's prior written
              approval, which shall not be unreasonably withheld, conditioned or
              delayed except as provided above for Building structures or
              systems. Notwithstanding the foregoing, Tenant shall be solely
              responsible for the content of the Tenant Plans and coordination
              of the Tenant Plans with base building design. In addition,
              Landlord's approval of the Tenant Plans shall not constitute a
              warranty, covenant or assurance by Landlord that (i) any equipment
              or system shown thereon will have the features or perform the
              functions for which such equipment or system was designed, (ii)
              the Tenant Plans satisfy applicable code requirements, (iii) the
              Tenant Plans are sufficient to enable the Landlord's contractor to
              obtain a building permit for the Tenant Work, or (iv) the Tenant
              Work described thereon will not interfere with, and/or otherwise
              adversely affect, base Building systems. Tenant shall be solely
              responsible for the Tenant Plans' compliance with all applicable
              laws, rules and regulations of any governmental entity having
              jurisdiction over the Building and the Premises.

       4.5.   Promptly after the construction documents for each phase have been
              approved by Landlord, Tenant shall provide to Landlord a CAD
              diskette of such construction documents and, upon any revisions to
              such construction documents pursuant to this Work Agreement,
              provide to Landlord a CAD diskette of the revised construction
              documents.

       4.6.   If Landlord requests any additional information or clarifications
              from Tenant regarding the Tenant Work or the Tenant Plans, Tenant
              shall provide such information or respond to such inquiries, as
              requested, within three (3) business days after such request.

       4.7.   Once the Lease has been fully executed, Landlord and Tenant intend
              for each deadline expressed in this Work Agreement to bind the
              parties even if any such deadline is before the date the Lease is
              executed.

       4.8.   Tenant shall pay the cost of preparing the Tenant Plans, subject
              to application of the Planning Allowance (hereinafter defined).
              Landlord shall provide the Tenant with an allowance of Two Dollars
              ($2.00) per rentable square foot of the Premises to provide space
              programming and planning, and design and construction document
              preparation for the Premises (the "Planning Allowance"). Such
              Planning Allowance shall be paid to third parties by Landlord at
              the direction of the Tenant within thirty(30) days of receipt by
              Landlord of

<PAGE>   56

       invoices substantiating the costs incurred by Tenant and lien waivers (if
       required by Landlord), in a form reasonably acceptable to Landlord. To
       the extent the full Planning Allowance is not used by the Tenant for the
       purposes set forth herein, then, provided that Tenant is not then in
       default under the Lease (including without limitation, any provision of
       this Work Agreement), the balance thereof shall be paid to Tenant by good
       check upon the occurrence of the Phase II Commencement Date and Tenant
       taking possession of the entire Premises. In addition, Landlord shall
       reimburse Tenant for the actual out-of-pocket costs incurred by Tenant
       for a preliminary test fit of the Premises, a space plan and three (3)
       revisions thereto. Tenant shall pay all additional costs incurred if
       Tenant requires more than three (3) revision to the space plan or if
       Tenant makes any revision to the space plan after it has been approved by
       Landlord.

5.     PRICING AND CONSTRUCTION.

       5.1.   COST ESTIMATE. Within ten (10) business days after approval of the
              construction documents for any phase by Landlord, Landlord shall
              submit (or cause to be submitted) to Tenant a written estimate of
              the total cost of the Tenant Work (the "Cost Estimate") for such
              phase, which Cost Estimate shall be based upon fixed price bids
              solicited from at least three (3) general contractors (two (2) of
              which shall be selected by Tenant from among Landlord's list of
              approved contractors to be delivered to Tenant, and at least one
              of which shall be selected by Landlord). The Cost Estimate shall
              include all amounts charged by Landlord's contractor for
              performing all work and providing all materials in connection with
              the Tenant Work (including the Landlord's general contractor's
              general conditions, overhead and profit, as well as permit fees,
              and Landlord's Construction Management Fee). Landlord shall enter
              into a guaranteed maximum price contract with the general
              contractor based upon the Cost Estimate accepted by Tenant
              pursuant to Section 5.2 below. The amount, if any, by which the
              Cost Estimate for any phase exceeds the Improvement Allowance (as
              hereinafter defined) applicable to such phase shall be referred to
              herein as "Excess Cost" for such phase. For purposes of
              determining Excess Cost for each phase, the portion of the
              Improvement Allowance applicable to each phase shall be determined
              on a pro rata basis, based upon the relative rentable square feet
              contained in each phase.

       5.2.   ACCEPTANCE OF COST ESTIMATE; VALUE ENGINEERING. Tenant, within ten
              (10) days after receipt of the Cost Estimate for each phase, shall
              notify Landlord as to whether Tenant (i) accepts such Cost
              Estimate and wishes Landlord to commence the Tenant Work for such
              phase; or (ii) wishes to redesign such phase of the Premises so as
              to reduce such Excess Cost, if any. In the event Tenant wishes to
              redesign such phase of the Premises so as to reduce such Excess
              Cost:

              (a)    said redesign shall be preformed at Tenant's sole expense,
                     subject to application of any available Improvement
                     Allowance;

              (b)    said redesign (including revisions to the Tenant Plans)
                     shall be completed and submitted to Landlord for approval
                     within ten (10) days after Tenant's notice to Landlord that
                     it wishes to redesign such phase of the Premises so as to
                     reduce such Excess Cost; and

              (c)    said redesign shall be conclusive and binding on Tenant.

       In the event Tenant elects not to redesign such phase of the Premises or
       to partially redesign such phase of the Premises, any remaining Excess
       Cost relating to such phase shall be paid for in full by Tenant within
       five (5) business days after Tenant's receipt of the Cost Estimate (or
       within 5 business days after Tenant's receipt of a revised Cost Estimate,
       if Tenant redesigns the Premises) for such phase. Landlord shall not be
       obligated to commence any work until such Excess Cost is paid, and any
       delay in such payment shall be deemed a Tenant Delay. The term "Award
       Date," with respect to any phase, shall mean the date on which all of the
       following have occurred (i) Tenant has notify

<PAGE>   57

       Landlord of Tenant's accepts of the Cost Estimate (after any redesign has
       occurred pursuant to Section 5.2 hereof) for such phase and of Tenant's
       wish that Landlord commence the Tenant Work for such phase, and (ii)
       Tenant has paid to Landlord any Excess Cost for such phase pursuant to
       this EXHIBIT B.

       5.3.   CONSTRUCTION. After approval of the construction documents and the
              Cost Estimate for any phase, Landlord shall administer the
              construction of the Tenant Work for such phase in a good and
              workmanlike manner in substantial compliance with the approved
              construction documents and any change orders approved by Landlord
              for such phase. All Tenant Work shall be constructed by Landlord's
              general contractor with the exception of those items constructed
              by Tenant's contractor or vendor which shall be limited to
              telephone equipment, computer equipment, specialized office
              equipment, wiring, systems furniture, audio/visual equipment,
              Tenant's Network Operation Center Equipment and Tenant's Cyber
              Cafe equipment (collectively, "Tenant Special Equipment").
              Landlord's prior written approval shall be required of any Tenant
              Special Equipment and any other equipment installed by Tenant or
              any vendor or contractor of Tenant and any equipment required to
              be installed by Landlord pursuant to the Tenant Plans (other than
              normal and customary office equipment), which approval shall not
              be unreasonably withheld, conditioned or delayed unless they
              affect the Building's structure or systems. Tenant shall be
              solely responsible for the work of its contractors and vendors.

6.     CHANGE ORDERS. If Tenant request any change or addition to the work or
       materials to be provided by Landlord with respect to any phase pursuant
       to this Exhibit after Landlord's approval of the construction documents
       for such phase, Tenant shall submit with such request revised
       construction documents for such phase for Landlord's approval, which
       approval shall not be unreasonably withheld, conditioned or delayed
       unless the proposed change or addition affects the Building's structure
       or systems. Landlord shall respond to Tenant's request for consent no
       event later than five (5) business days after it being made. If Landlord
       approves such a request, Landlord shall as soon as practicable after such
       approval notify Tenant of the cost of such change order and the delay in
       substantial completion of the Tenant Work in the Premises, if any, due to
       the change order which would be Tenant's sole responsibility. All
       additional expenses attributable to any change order requested by Tenant
       and approved by Landlord, whether or not such change order relates to
       improvements paid for under the Tenant Work, shall be payable by Tenant,
       along with an additional construction management fee equal to five
       percent (5%) of the costs relating to such change order, upon approval by
       Tenant of the change order cost and/or delay, if any. Any delay in
       substantial completion of the Tenant Work in the Premises resulting from
       such change order shall be deemed a Tenant Delay.

7.     SUBSTANTIAL COMPLETION.

       7.1.   SUBSTANTIAL COMPLETION. For purposes of this Lease (including all
              provisions of this Work Agreement), the Premises (or any
              particular floor thereof), and the Tenant Work therein, shall
              conclusively be deemed to be "substantially complete" as soon as
              (i)  the Tenant Work (specifically excluding any of Tenant's
              telephone equipment, special office equipment, computer
              equipment, audio/visual equipment, cabling and wiring,
              supplemental HVAC equipment, systems furniture, other furniture
              and personal property, Tenant Special Equipment and any items to
              be installed or constructed by Tenant or Tenant's contractor or
              vendor) to be installed in the Premises (or such particular floor
              thereof) by Landlord pursuant to this Work Agreement has been
              constructed in accordance with the Tenant Plans approved by
              Landlord and any change orders approved by Landlord, as certified
              by Landlord's construction manager, subject to Landlord's
              completion of any punch list items of work which do not
              materially interfere with Tenant's permitted and intended use of
              the Premises (or such particular floor thereof), (ii) the
              Building's parking area is available for Tenant's use in
              accordance with the Lease, (iii) reasonable access to the
              Premises (or such


<PAGE>   58

              particular floor thereof) is available for Tenant, (iv) the base
              Building systems necessary to provide the services required
              hereunder to be provided by Landlord to the Premises (or such
              particular floor thereof) are functional and (v) a temporary or
              permanent certificate of occupancy has been obtained for the
              Premises (or such particular floor thereof); provided, however
              that no such certificate of occupancy shall be required for the
              Premises (or such particular floor thereof) to be substantially
              complete if Landlord is unable to obtain such certificate of
              occupancy as a result of any act or omission of Tenant or
              Tenant's contractors or vendors, including without limitation, as
              a result of any systems furniture, Tenant Special Equipment or
              any work to be installed or constructed by Tenant or Tenant's
              contractors or vendors not being complete at the time that the
              Premises would otherwise be substantially complete. Tenant shall
              cooperate with Landlord in connection with Landlord obtaining
              certificate(s) of occupancy for the Premises. Notwithstanding the
              above, (a) the Premises (or such particular floor thereof) shall
              be considered substantially complete even though there remain to
              be completed in the Premises punch list items, including but not
              limited to minor or insubstantial details of construction,
              decoration or mechanical adjustment, the lack of completion of
              which will not materially interfere with Tenant's permitted and
              intended use of the Premises (or such particular floor thereof).
              In addition, notwithstanding the foregoing, upon the occupancy by
              Tenant of any portion of any floor of the Premises for the
              purpose of conducting business therein (as opposed to preparing
              the Premises for Tenant's use) such floor shall be deemed to be
              substantially complete, whether or not any of the other
              conditions to substantial completion have then occurred.

       7.2.   TENANT DELAYS. Any delay resulting from any of the following items
              shall be referred to individually as a "Tenant Delay":

              (a)    Tenant failure to comply with any of the deadlines 
                     specified in this Work Agreement; or

              (b)    Any changes or additions to the Tenant Work or to the
                     Tenant Plans (including without limitation, any changes or
                     additions requested or required by Tenant or by any
                     governmental authority or required in order to comply with
                     any legal or base Building requirements or otherwise) with
                     respect to any phase subsequent to the date of Landlord's
                     approval of the construction documents for such phase; or

              (c)    Tenant's failure to pay when due any amounts required
                     pursuant to this Work Agreement; or

              (d)    The performance of any work by any person or firm employed
                     or retained by Tenant; or

              (e)    Tenant's request for materials, finishes or installations
                     which are not available as needed to meet the general
                     contractor's schedule for substantial completion ("Long
                     Leadtime Items"); or

              (f)    Tenant's or Tenant's agent, including Tenant's contractors,
                     vendors, and Tenant's Representative's interference with
                     the general contractor's schedule;

              (g)    Any default under the Lease (including without limitation
                     any provisions of this Work Agreement); or 

              (h)    Any other Tenant-caused delay in completing construction of
                     the Tenant Work.

              On or before the date that Landlord submits to Tenant written
              estimates of the cost of the Tenant Work for any phase pursuant to
              Section 5.1 of this Work Agreement, Landlord shall identify the
              Long Leadtime Items for such phase shown on the construction
              documents of which Landlord is then aware. If Landlord's general
              contractor provides Landlord with notice (written or oral) of a
              Tenant Delay, Landlord shall provide Tenant notice thereof within
              five (5) days of Landlord's receipt of such written notice from
              Landlord's general contractor.

       7.3.   LANDLORD DELAY. The following items shall be referred to
              individually as a "Landlord Delay":
<PAGE>   59

              (i)    Landlord's failure to substantially complete the Tenant
                     Work to be built by Landlord in the Premises (or any
                     particular floor thereof, in the case of a Landlord Delay
                     applicable to such particular floor) pursuant to this
                     EXHIBIT B within the applicable Allocated Construction
                     Period (hereinafter defined); and

              (ii)   Landlord's failure to comply with any of the deadlines
                     specified in Sections 4.1, 4.2 and 4.3 (or of the deadlines
                     applicable to the plans for any particular floor thereof,
                     in the case of a Landlord Delay applicable to any such
                     particular floor) of this Work Agreement.

              The "Allocated Construction Period" applicable to the Premises (or
              any floor thereof) shall mean the one hundred sixty-eight (168)
              calendar day period after the Award Date for the Premises (or in
              the case of any particular floor, the Award Date applicable to the
              phase in which such floor is located), which period shall be
              extended one (1) day for each day of delay caused by an event of
              Tenant Delay or event of Construction Force Majeure. The term
              "Construction Force Majeure" shall mean acts of God, strikes,
              sabotage, accidents, acts of war, fire and casualty, moratoriums
              affecting construction, insurance reimbursement problems or
              delays, emergencies, energy shortage, the failure of the
              applicable governmental authority to issue building permits for
              the Tenant Work within twenty-one (21) days after Landlord has
              submitted to the appropriate authority an application for such
              building permit with the required documentation, the failure of
              the applicable governmental authority to conduct inspections
              required in connection with the Tenant Work (and issue its
              approval or disapproval thereof) within three (3) business days
              after Landlord has submitted to the appropriate authority a
              request for such inspection or the failure of the applicable
              governmental authority to issue a certificate of occupancy for the
              Premises within five (5) business days after Landlord has
              submitted to the appropriate authority an application (which
              application shall have been submitted after receipt of the
              approvals from all inspections that are required as a condition
              precedent to submitting such application) for such certificate of
              occupancy. Landlord shall provide to Tenant notice of the
              occurrence of an event of Construction Force Majeure within five
              (5) business days of Landlord becoming aware that such event of
              Construction Force Majeure has occurred.

       7.4.   PUNCH LIST. Prior to delivery of possession of the each phase of
              the Premises to Tenant, Landlord, Landlord's construction manager,
              the Leasehold Engineer and the Leasehold Architect shall prepare a
              preliminary punch list in writing for Landlord and Tenant's review
              and Landlord and Tenant shall examine the applicable phase of the
              Premises and shall agree on a final "punch list" for such phase
              which shall specify the items of work that require correction,
              repair or replacement. Tenant shall approve such punch list in
              writing within two (2) working days of the walk-through.

8.     MOVING ALLOWANCE. Landlord shall provide the Tenant with an allowance of
       Two Dollars ($2.00) per rentable square foot of the Premises to cover the
       cost of the Tenant's relocation to the Premises, including, without
       limitation, expenses for movers, move consultants, furniture and
       equipment take-down and set-up (the "Moving Allowance"). Such Moving
       Allowance will be paid to third parties by Landlord at the direction of
       the Tenant within thirty (30) days of receipt by Landlord of invoices (in
       a form reasonably acceptable to Landlord) substantiating the costs
       thereof incurred by Tenant. To the extent the full Moving Allowance is
       not used by the Tenant for the purposes set forth herein, then, provided
       that Tenant is not then in default under the Lease (including without
       limitation, any provision of this Work Agreement), the balance thereof
       shall be paid to Tenant by good check upon the occurrence of the Phase II
       Commencement Date and Tenant taking possession of the entire Premises.

9.     POSSESSION BY TENANT. The taking of possession of each phase of the
       Premises by Tenant shall constitute an acknowledgment by Tenant that such
       phase of the Premises is in good condition and that all work and
       materials provided by Landlord with respect to such phase are
       satisfactory and
<PAGE>   60

       Landlord has satisfactorily performed all work with respect to such phase
       to be performed by it pursuant hereto, subject to certain punch list
       items described in Paragraph 7.3, above and subject to Landlord's
       obligation to correct any Latent Defects (hereinafter defined) in such
       phase of the Premises as hereinafter provided. Landlord shall correct any
       Latent Defects in any phase of the Premises which materially impair
       Tenant's use of such phase of the Premises, provided that Tenant delivers
       to Landlord written notice thereof within twelve (12) months after the
       applicable commencement date for such phase. Landlord agrees to use
       reasonable efforts to complete all items of Tenant Work on the punch list
       for each phase within thirty (30) days after such punchlist is prepared,
       or as soon thereafter as is reasonably practicable. As used herein, the
       term "Latent Defect," with respect to any phase, shall mean a defect in
       the physical condition of the such phase of the Premises existing as of
       the date such phase of the Premises is delivered to Tenant that could not
       have reasonably been discovered prior to Tenant taking possession of such
       phase of the Premises by a commercially reasonable visual inspection of
       such phase of the Premises; provided, however, that it is understood that
       a Latent Defect shall not include a defect in the design of the Tenant
       Work.


<PAGE>   61
                                   EXHIBIT B-1

                            MODIFIED SHELL DEFINITION

STRUCTURE:              Post-Tensioned Reinforced Concrete Frame, 100 lbs/sq.
                        ft. minimum load capacity. (Meets GSA requirements)

EXTERIOR:               Precast concrete frames with reflective insulated
                        windows and curtainwall.

COLUMN SPACING:         28' X 42'

ROOF:                   Ballasted IRMA roofing with a 10 year warranty.

FINISHED CEILING
HEIGHT:       9' 6" on first floor, 9' on floors 2 through 7.

HVAC SYSTEM:Central plant on rooftop penthouse with centrifugal chillers and
                        cooling towers. Two air handling units on each floor
                        serving all VAV zones. Trunk ducts, all VAV boxes and 
                        perimeter slot diffusers will be installed with the 
                        base building.  Interior diffusers and flex duct will be
                        paid from the Improvement Allowance.  

                        Building HVAC design criteria shall be:

                                 Summer:     76 degrees F db +- 2 degrees and
                                             50% +- 5%.
                                             Up to 95 degrees F db outside air
                                             temperatures.

                                 Winter:     70 degrees F db +- 2 degrees and
                                             30% +- 5%.
                                             Down to 0 degrees F db outside air
                                             temperatures.

                        Outside air for ventilation will be provided at the rate
                        of 20 CFM per person consistent with current ASHRAE 
                        Guidelines for acceptable indoor air quality.

                        Mechanical system will permit two zones of operation
                        per floor. Medium and low pressure duct work will be
                        installed from each floor's air handling unit to the
                        VAV's. 18 exterior VAV boxes with electric resistance
                        heaters averaging 1200 CFM in size and 10 interior VAV
                        boxes averaging 1400 CFM in size will be installed with
                        thermostats and 20' of cable with the base building.

                        Each floor will be equipped with 87 tons of HVAC 
                        capacity, or the equivalent ratio of one ton of HVAC for
                        each 266 square feet of floor area.

                        A supplemental cooling tower with condenser water riser
                        serving each floor will be installed with the base
                        building use by the tenants for 24 hour operation of 
                        tenant supplied air handling units.

ELECTRICAL SYSTEM:      5.2 watts/sq. ft. on Tenant's floor and 2.5 watts/sq.
                        ft. available for lighting on tenant floor.  Single
                        electrical service entrance with single-pad mounted
                        transformer outside of the Building.  480V plug-in bus 
                        riser with step-down dry type transformers with K factor
                        for 120/208V for Tenant's power distribution at each 
                        floor.

                        Location for additional emergency generator and conduit
                        to main electrical room will be installed with base 
                        building.

LIFE SAFETY:            Fire standpipe and base building fire alarm system will 
                        be installed per building

<PAGE>   62

                        code.  Vertical sprinkler riser distribution to each 
                        floor and sprinkler loop with upturned heads at the
                        ratio of one head per 130 sq. ft. will be installed with
                        the base building.

WET COLUMNS:            4 wet columns per floor for use by Tenant.

WINDOW COVERINGS:       1" slat Venetian blinds, to be paid for from Tenant's
                        Improvement Allowance.

ENERGY MANAGEMENT:      Automated base building energy management system with
                        remote monitoring.

ELEVATORS:              4 traction passenger elevators, 350 feet per minute, 
                        with 3,500 lb. capacity.

SECURITY SYSTEM:        Building perimeter and elevators.

REST ROOMS:             1 set of women's and men's restrooms will be fully 
                        finished on each floor with base building.

TYPICAL FLOOR LOBBIES:  Typical floor lobby finishes and finishes to corridors
                        connecting stairs will be paid for from Tenant's
                        Improvement Allowance.

FIBER OPTICS:           Vertical risers to accommodate T-1 lines for connection
                        to the Internet are provided to each floor of the
                        Building.

PARKING:                Underground parking with finished garage elevator lobby.

ADDITIONAL ITEMS:       -      Perimeter diffusers and bulkheads on each floor 
                               as per base building drawings.

                        -      Sprinkler heads are a ratio of one head per 130 
                               sq. ft. as installed within the Building (as set
                               forth above).

                        -      An allowance of $5,000.00 per floor to provide 
                               construction of lobby areas on each floor.  This
                               is in addition to Landlord provided restrooms on
                               each floor.

                        -      Two (2) sets of 4" conduits will be made 
                               available for the sole use of Tenant to install
                               their own fiber optic service from street to the
                               Building telephone riser.
<PAGE>   63
                                    EXHIBIT C

                              RULES AND REGULATIONS

Definitions of terms are set forth in the Lease to which these Rules and
Regulations are attached by reference.

       The following rules and regulations have been formulated for the safety
and well-being of all tenants of the Building and to ensure compliance with all
municipal and other requirements. Strict adherence to these rules and
regulations is necessary to guarantee that each and every tenant will enjoy a
safe and unannoyed occupancy in the Building in accordance with the Lease. Any
violation of these rules and regulations by Tenant that continues ten (10) days
or more after notice thereof from Landlord, shall be deemed to be a Default
under the Lease.

       Landlord may, upon request by any tenant, waive the compliance by such
tenant to any of these rules and regulations, provided that (i) no waiver shall
be effective unless signed by Landlord or Landlord's authorized agent, (ii) any
such waiver shall not relieve such tenant from the obligation to comply with
such rule and regulation in the future unless expressly consented to by
Landlord, (iii) no waiver granted to any tenant shall relieve any other tenant
from the obligation of complying with the rules and regulations unless such
other tenant has received a similar waiver in writing from the Landlord, and
(iv) any such waiver by Landlord shall not relieve Tenant from any obligation or
liability of Tenant to Landlord pursuant to the Lease for any loss or damage
occasioned as a result of Tenant's failure to comply with any such rule or
regulation.

1.     The sidewalks, entrances, passages, courts, elevators, vestibules,
       stairways, corridors, halls or other parts of the Building not occupied
       by any tenant shall not be obstructed or encumbered by any tenant or used
       for any purpose other than ingress and egress to and from the Premises.
       Landlord shall have the right to control and operate the public portions
       of the Building and the facilities furnished for common use of the
       tenants in such manner as Landlord deems best for the benefit of the
       tenants generally. No tenant shall permit the visit to the Premises of
       persons in such numbers or under such conditions as to interfere with the
       use and enjoyment by other tenants of the entrances, corridors, elevators
       and other public portions or facilities of the Building.

2.     No awnings or other projections shall be attached to the exterior of any
       wall of the Building or visible from outside the Building without the
       prior written consent of Landlord. No drapes, blinds, shades or screens
       visible from outside the Premises shall be attached to or hung in, or
       used in connection with, any window or door of the Premises, without the
       prior written consent of Landlord. Such awnings, projections, curtains,
       blinds, shades, screens or other fixtures must be of a quality, type,
       design and color, and attached in the manner, reasonably approved by
       Landlord.

3.     No showcases or other articles shall be put in front of or affixed to any
       part of the exterior of the Building, nor placed in the halls, corridors
       or vestibules without the prior written consent of Landlord.

4.     The water and wash closets and other plumbing fixtures shall not be used
       for any purposes other than those for which they were constructed, and no
       sweepings, rubbish, rags, chemicals, paints, cleaning fluids or other
       substances shall be thrown therein. All damages resulting from any misuse
       of the fixtures shall be borne by the tenant who, or whose servants,
       employees, agents, visitors or licensees, shall have caused the same.

5.     Except as otherwise provided herein, there shall be no marking, painting,
       drilling into or in any way defacing the Building or any part of the
       Premises visible from public areas of the Building. Tenant shall not
       construct, maintain, use or operate within the Premises any electrical
       device, wiring or apparatus in connection with a loud speaker system or
       other sound system, except as reasonably required for its communication
       system and approved prior to the installation thereof by Landlord. No
<PAGE>   64

       such loudspeaker or sound system shall be constructed, maintained, used
       or operated outside of the Premises.

6.     No vehicles or animals, birds or pets of any kind (other than seeing-eye
       dogs assisting disabled persons) shall be brought into or kept in or
       about the Premises, and no cooking (except for hot-plate or microwave
       cooking by Tenant's employees for their own consumption, the equipment
       for and location of which are first approved by Landlord) shall be done
       or permitted by any tenant on the Premises. No tenant shall cause or
       permit any unusual or objectionable odors to be produced upon or permeate
       from the Premises.

7.     Except as specifically set forth in the Lease (and then only to the
       extent permitted under all applicable laws and regulations, including all
       zoning laws and regulations), No space in the Building shall be used for
       manufacturing of goods for sale in the ordinary course of business, for
       the storage of merchandise for sale in the ordinary course of business,
       or for the sale at auction of merchandise, goods or property of any kind.
       Furthermore, the use of the Premises by each tenant was approved by
       Landlord prior to execution of the Lease and such use may not be changed
       without the prior approval of Landlord.

8.     No tenant shall make any unseemly or disturbing noises or disturb or
       interfere with occupants of the Building or neighboring buildings or
       Premises or those having business with them whether by the use of any
       musical instrument, radio, talking machine, unmusical noise, whistling,
       singing or in any other way. No tenant shall throw anything out of the
       doors or windows or down the corridors or stairs.

9.     No flammable, combustible or explosive fluid, chemical, asbestos or other
       hazardous substance or any other material harmful to tenants of the
       Building shall be brought, installed in or kept upon the Premises; except
       that Tenant shall be permitted to use and keep in the Premises such
       cleaning, copier and other supplies as are reasonable and customary for
       office use, provided that Tenant uses, stores and disposes of same in
       accordance with all applicable laws. No space heaters, fans or individual
       air conditioning units may be used in the Premises, without Landlord's
       prior written consent in accordance with EXHIBIT B of the Lease. Any
       electrical or extension cords deemed to be a fire hazard by Landlord in
       Landlord's sole discretion shall be removed.

10.    No additional locks or bolts of any kind shall be placed upon any of the
       doors or windows by any tenant, nor shall any changes be made in existing
       locks or the mechanism thereof. The doors leading to the corridors or
       main halls shall be kept closed during business hours except as they may
       be used for ingress or egress. Each tenant shall, upon the termination of
       his tenancy, restore to the Landlord all keys of stores, offices, storage
       and toilet rooms either furnished to, or otherwise procured by, such
       tenant, and in the event of the loss of any keys so furnished, such
       tenant shall pay to Landlord the cost thereof.

11.    Landlord reserves the right to make reasonable inspections of all freight
       to be brought into the Building and to exclude from the Building all
       freight which violates any of these rules and regulations or the Lease.

12.    No tenant shall pay any employees on the Premises, except those actually
       working for such tenant on the Premises.

13.    Landlord reserves the right to exclude from the Building at all times any
       person who is not known or does not properly identify himself to the
       Building management, security guard on duty or security system monitor.
       Landlord may, at its option, require all persons admitted to or leaving
       the Building between the hours of 6:00 p.m. and 8:00 a.m., Monday through
       Friday, and at any hour, Saturdays, Sundays and legal holidays, to
       register. Each tenant shall be responsible for all persons from whom he
       authorizes entry into or exit out of the Building, and shall be liable to
       Landlord for all acts or

<PAGE>   65

       omissions of such persons.

14.    The Premises shall not, at any time, be used for lodging or sleeping or
       for any immoral or illegal purpose.

15.    Each tenant, before closing and leaving the Premises at any time, shall
       see that all windows are closed.

16.    Landlord's employees shall not perform any work or do anything outside of
       their regular duties, unless under special instruction from the
       management of the Building. The requirements of tenants will be attended
       to only upon application to Landlord and any such special requirements
       shall be billed to Tenant (and paid with the next installment of Rent
       due) at the schedule of charges maintained by Landlord from time to time
       or at such charge as is agreed upon in advance by Landlord and Tenant.

17.    Canvassing, soliciting and peddling in the Building is prohibited and
       each tenant shall cooperate to prevent the same.

18.    There shall not be used in any space, or in the public halls of the
       Building, either by any tenant or by jobbers or others, in the delivery
       or receipt of merchandise, any hand trucks except those equipped with
       rubber tires and side guards, and Tenant shall be responsible to Landlord
       for any loss or damage resulting from any deliveries of Tenant to the
       Building.

19.    Mats, trash or other objects shall not be placed in the public corridors.

20.    Landlord does not maintain finishes within the Premises which are
       non-standard, such as kitchens, bathrooms, wallpaper, special lights,
       etc. However, should the need for repairs of items not maintained by
       Landlord arise, Landlord will arrange for the work to be done at Tenant's
       expense.

21.    Drapes installed by Landlord for the use of Tenant or drapes installed by
       Tenant, which are visible from the exterior of the Building, must be
       cleaned by Tenant at least once a year, without notice, at Tenant's own
       expense.

22.    No minors shall be allowed to congregate or play in the common areas of
       the Building. It shall be the responsibility of all tenants to see that
       the minor children of their employees who visit the Building (whether
       during the normal hours of operation of the Building, or after-hours, on
       Saturdays, Sunday or legal holidays) are adequately supervised by an
       adult and do not assemble or play in the common areas of the Building.

<PAGE>   66


                                    EXHIBIT D

                              ESTOPPEL CERTIFICATE

                                     [Date]
------------------------
------------------------
------------------------
------------------------

                Re: ______________________________________

To Whom It May Concern:

       It is our understanding that you have placed or committed to place a
mortgage upon the subject premises and have required this certification by the
undersigned.

       The undersigned, as Tenant, under that certain Agreement of Lease dated
________ __, 199_ ("Lease") made with ____________________ ("Landlord"), hereby
ratifies the Lease and certifies as follows:

1.     The undersigned has entered into occupancy of the "Premises" described in
       the Lease on ______________ __, 199_;

2.     [The undersigned is presently preparing the Premises for use by the
       undersigned][[The undersigned is presently open and conducting business
       with the public in the Premises]];

3.     The operation and use of the Premises do not involve the generation,
       treatment, storage, disposal or release of hazardous substance or a solid
       waste into the environment and that the Premises are being operated in
       accordance with all applicable environmental laws, zoning ordinances and
       building codes;

4.     Base Rent in the annual amount of $___________ is payable from the
       Commencement Date, i.e., ____________________________;

5.     The Lease is in full force and effect and has not been assigned,
       modified, supplemented or amended;

6.     The Lease represents the entire agreement between the parties as to the
       leasing of the Premises;

7.     The Term of the Lease expires on ____________ __, ____;

8.     To the best of the undersigned's knowledge (after due investigation and
       inquiry), all conditions under the Lease to be performed by the Landlord
       [other than payment of the allowance set forth in the Work letter, being
       EXHIBIT B to the Lease] have been satisfied, including but without
       limitation, all co-tenancy requirements thereunder, except as followed
       __________________;

9.     [Intentionally omitted][[All required contributions by Landlord to Tenant
       on account of Tenant's improvements have been received]];

10.    To the best of Tenant's knowledge, on this date, there are no existing
       defenses or offsets which the undersigned has against the enforcement of
       the Lease by the Landlord.

11.    No rental has been paid in advance, except that the first month's rent in
       the amount of $__________ and the Security Deposit in the amount of
       $___________ have been deposited with Landlord.

                                                        ------------------------

                                                        By: ____________________
                                                        Name: __________________

<PAGE>   67

                                                       Its: ____________________
<PAGE>   68

                                    EXHIBIT E

                              INTENTIONALLY OMITTED
<PAGE>   69


                                    EXHIBIT F

                               HVAC SPECIFICATIONS

A.     SUMMER:

       Room conditions not in excess of 76 degrees F dry bulb and 55% relative
       humidity when outside conditions do not exceed 95 degrees F dry bulb and 
       78 degrees F wet bulb, provided that Tenant complies with the following
       conditions:

       1.     Light-colored blinds, fully drawn, with slats at 45 degrees angle
              coincident with peak sun load;

       2.     Electric load does not exceed 5 watts per square foot; and

       3.     People load does not exceed an average one person per 100 square
              feet for office space.

B.     WINTER:

       Room conditions of not less than 68 degrees F and 30% relative humidity
       when the outside dry bulb temperature is not less than 0 degrees F.

NOTE:         The foregoing HVAC Specifications shall not apply to any kitchen,
              conference room, computer room, copy room or other areas of the 
              Premises in which excessive heat is produced, which areas may
              require supplemental air-conditioning in order to comply with the
              foregoing specification.
<PAGE>   70


                                    EXHIBIT G

                            FORM OF LETTER OF CREDIT

[Lending Institution Name]
[Address of Lending Institution]                        Date: __________, 199__

              IRREVOCABLE STANDBY LETTER OF CREDIT NO. ____________

Account Party
[Account Party's Name]
[Account Party Address]

In favor of: Beneficiary

[Beneficiary Name], its successors and assigns
[Beneficiary Address]

       AMOUNT                                        EXPIRY DATE:
USD  _____________                                           [Expiry Date]
[Dollar Amount] U.S. Dollars Only

Gentlemen:

We hereby open our irrevocable letter of credit in your favor for an amount of
USD [Numeric Dollar Amount] ([Alphabetic Dollar Amount] U.S. Dollars Only)
available by your draft at sight drawn on the [Lending Institution Name, Lending
Institution Address], bearing the clause "Drawn under [Lending Institution Name]
Letter of Credit No. _______ dated ________, 199__," and accompanied by the
following document:

       Beneficiary's signed statement stating that: "[Beneficiary Name] is
       entitled to draw upon this Letter of Credit pursuant to the terms of that
       Lease dated [Lease Date] for premises at [Premises Address] between
       [Account Party Name] and [Beneficiary Name]. [Beneficiary Name] hereby
       makes demand for the payment of the entire amount of the Letter of
       Credit" Such statement shall be conclusive as to such matters.

This Letter of Credit is transferable to a transferee of Beneficiary's entire
interest as landlord in that Lease dated [Lease Date] for premises at [Premises
Address] between [Account Party Name] and [Beneficiary Name], as the same may be
amended or modified from time to time.

This Letter of Credit sets forth in full the terms of our undertaking and such
undertaking shall not in any way be modified, amended, or amplified by reference
to any document(s), instrument(s), contract(s), or agreement(s) referred to
herein or in which this Letter of Credit relates, and any such reference shall
not be deemed to incorporate herein by reference any document(s), instrument(s),
contract(s), or agreement(s).

It is a condition of this Letter of Credit that it shall be deemed automatically
extended without amendment for one year from the present or any future
expiration date of this Letter of Credit unless at least sixty (60) days prior
to the then current expiration date we notify the Beneficiary by registered
letter that we elect not to consider this Letter of Credit renewed for such
additional period. If such notice is given, then during such notice period
(i.e., the at least sixty (60) day period commencing on the date of such notice
and ending with the then applicable expiration date of this Letter of Credit),
this Letter of Credit shall remain in full force and effect and Beneficiary may
draw up to the full amount of the sum when accompanied by a statement described
above in the first paragraph of this Letter of Credit.

Partial draws shall be permitted under this Letter of Credit.

We hereby engage with you that drafts drawn and presented in compliance with the
terms of this credit will be immediately honored by us if presented at any of
our offices on or before [Expiry Date], as such date may be extended pursuant to
the terms hereof.

This Letter of Credit is subject to The Uniform Customs and Practice for
Documentary Credits (1993 Revision), International Chamber of Commerce
Publication No. 500, which is incorporated by reference herein.

Very truly yours,

------------------------------
Authorized Signature
<PAGE>   71


                                    EXHIBIT H

                     LOCATION OF TENANT'S ANTENNA EQUIPMENT