<PAGE>

                                                                   Exhibit 10.21

                                OFFICE SUBLEASE
                                ---------------

     THIS OFFICE SUBLEASE, dated as of the 1st day of June 1998, is entered into
by and between Phase 1 Incubator, Inc., a Delaware Corporation ("Lessor"), and
                                                                 ------
SkyCache, Inc., a Delaware Corporation ("Lessee") provides:
                                         ------

1.   DESCRIPTION OF SUBLET PREMISES. Lessor hereby Subleases to Lessee, and
     ------------------------------
Lessee hereby hires from Lessor, those certain premises (the "Sublet Premises")
                                                              ---------------
situated in the City of Laurel, Maryland, and more particularly described on
Exhibit A (which is attached hereto and by this reference made a part hereof and
---------
incorporated herein) for a term (the "Term") commencing at 12:01 a.m. on June 1,
                                      ----
1998 (the "Commencement Date") and ending: (i) thirty days after written notice
           -----------------
from Lessee indicating plans to terminate the Subleasee or (ii) the expiration
or earlier termination of that certain Office Lease (the "Lease"), dated as of
                                                          -----
May 1, 1998, between TCP/IP, LLC as landlord and Lessor as tenant. Upon
expiration of the Term and provided Lessee shall not be in material default of
any terms or conditions of this Sublease, Lessee shall have the option to extend
this Sublease for an additional two years by giving Lessor written notice of its
intention to do so at least six months prior to the end of the Term, upon the
same terms and conditions as herein stated and at a rental rate that is
determined as the fair market value of the Sublet Premises at such time.
Notwithstanding the foregoing, in the event of a merger or consolidation of the
Lessee with or into another unaffiliated entity, or the acquisition by another
unaffiliated entity or person of all or substantially all of the Lessee's assets
or more than fifty (50%) of the Lessee's then outstanding voting stock, or the
liquidation, dissolution or winding up of the Lessee's business (other than in a
restructuring which results in the continuation of the Lessee's business by an
affiliated entity), then, the Term of this Sublease will extend for the greater
of the remainder of the Term or six (6) months.

2.   RENT.
     ----

2.1. Lessee shall pay to Lessor, without notice, demand, offset or reduction,
rent (the "Rent") in the amount of Eight Hundred Forty Dollars and Sixty Seven
           ----
Cents ($840.67), on the first (1st) day of each and every month of the Term. The
Rent shall be paid at the offices of Lessor or to such other person or entity or
at such other place or address as Lessor may hereafter direct in writing. If the
Commencement Date is not on the first (1st) day of a month, a prorated monthly
installment of the Rent shall be paid by Lessee to Lessor for the period of time
from the Commencement Date up to (but not including) the first (1st) day of the
next succeeding month, and, thereafter, the Rent shall be paid on the first
(1st) day of each and every month. If Lessee occupies the Sublet Premises before
the Commencement Date, the Rent shall commence, and all of the other terms,
provisions, covenants and conditions of this Sublease shall be in full force and
effect as of the date of occupancy by Lessee; provided, however, that Lessee
                                              --------  -------
shall neither occupy the Sublet Premises, nor place any property whatsoever in
or on the Sublet Premises, without the prior written consent of Lessor. If
Lessee is unable to occupy the Sublet Premises on the Commencement Date because
the Sublet Premises are not ready for occupancy at that time, or because a
previous tenant, sub-tenant or occupant of the Sublet Premises has held over
without Lessor's consent, or because of any cause or reason beyond the direct
control of Lessor, such delay shall not constitute a default on the part of
Lessor, nor shall such delay entitle Lessee to terminate or cancel this
Sublease, and Lessor shall not be liable for any damages Lessee may incur as a
result of its inability to occupy the Sublet Premises on the Commencement Date;

                                      1.
<PAGE>

provided, however, that in the event of such a delay, the Rent shall be abated
from the Commencement Date to the date on which the Sublet Premises are ready
for occupancy.

2.2. If Lessor does not receive from Lessee each monthly rental payment within
ten (10) days of the due date, Lessor, at its option, may charge Lessee a late
charge equal to five percent (5%) of the monthly rental payment (together with
any additional rent as hereinafter provided) as additional rent, and such late
charge shall be due and payable by Lessee to Lessor immediately upon notice to
Lessee. In addition, if a check of Lessee's is returned to Lessor unpaid for any
reason, Lessor, at its option, may thereafter require that Lessee pay the Rent
and any other charges payable hereunder by a certified or cashier's check drawn
on a bank.

3.   USE OF SUBLET PREMISES. Lessee shall use the Sublet Premises only for
     ----------------------
office space and machine rooms. Lessee acknowledges and agrees that Lessor has
made no express or implied warranty, representation, undertaking or agreement
regarding the condition of the Sublet Premises or the ability of Lessee to use
the Sublet Premises in the manner or for the purposes contemplated by Lessee.
Lessee may not use the Sublet Premises for any purpose other than that stated in
this Section 3 without the prior written consent of Lessor.
     ---------

4.   ACCESS TO PREMISES. Lessor covenants and agrees that Lessee and its agents,
     ------------------
employees, contractors, invitees, sublessees and licensees shall have the right
of ingress and egress to the Sublet Premises through all common or public areas
of the building in which the Sublet Premises are located (including but not
limited to all public entrances, lobbies, elevators and corridors) during normal
business hours (which are hereby deemed to be daily from 8:00 a.m. to 6:00 p.m.,
and Saturdays from 8:00 a.m. to 1:00 p.m.) and at such other times as may be
established by Lessor in accordance with appropriate rules and regulations (or
otherwise as agreed upon by Lessor and Lessee). Lessee covenants and agrees that
Lessor, its agents, employees, contractors, invitees, sublessees and licensees
shall, without prior notice, have the right of reasonable access to the Sublet
Premises during normal business hours and at all times for emergency or security
purposes.

5.   MAINTENANCE AND REPAIRS; LESSEE'S OBLIGATION TO NOTIFY.
     ------------------------------------------------------

Lessee shall keep the Sublet Premises in reasonably good working order and
condition and shall make all repairs. Lessor shall make all replacements not
occasioned by the negligent or willful act of Lessee, its agents, employees,
contractors, invitees, sublessees or licensees. Lessee shall notify Lessor
immediately by telephone and as soon as possible in writing of any repairs or
replacements which, in the opinion of Lessee, need to be made to the Sublet
Premises and of any loss, damage or casualty to the Sublet Premises.

6.   TAXES.
     -----

6.1. Lessor shall pay all taxes assessed against the real estate (the "Real
                                                                       ----
Estate Taxes") on which the Sublet Premises are located on or before the date on
------------
which such taxes become due and payable.

6.2. Lessee shall pay all taxes, charges and levies assessed against the
personal property and fixtures owned by Lessee and placed, stored or used by
Lessee in conjunction with the Sublet Premises. Lessee shall pay, as additional
rent, any ad valorem, license or other tax imposed upon

                                       2.
<PAGE>

Lessor by reason of or with respect to this Sublease or the Rent, and such
additional rent shall be payable within thirty (30) days after Lessor notifies
Lessee that such amount is due and payable.

6.3. Either Lessor or Lessee may challenge the amount, validity or applicability
of any tax, fee, charge or obligation, and the non-challenging party shall
render all necessary and reasonable assistance to the challenging party in the
prosecution of such challenge. Notwithstanding such challenge, Lessor or Lessee,
as applicable, shall pay all taxes, fees, charges and obligations (including any
tax, fee, charge or obligation the amount, validity or applicability of which is
being challenged) on or before the date on which such taxes, fees, charges or
obligations become due and payable.

7.   ADJUSTMENTS FOR INCREASES IN REAL ESTATE TAXES AND BUILDING OPERATING
     ---------------------------------------------------------------------
     COSTS.
     -----

7.1. General.
     -------

(a)  For the purpose of this Section 7, the term "Base Year" shall mean the
                             ---------            ---------
     calendar year 1998.

(b)  The term "Lessee's Proportionate Share" shall refer to the proportionate
               ----------------------------
     amount of increases in the real estate taxes to be paid by Lessee. Lessee's
     Proportionate Share shall be computed using a fraction, the numerator of
     which is the net rentable area of the Sublet Premises and the denominator
     of which is the total net rentable area of the building in which the Sublet
     Premises are located. Lessee's Proportionate Share is calculated on Exhibit
                                                                         -------
     B, which is attached hereto and by this reference made a part hereof.
     -

(c)  The obligations of Lessee set forth in this Section 7 shall survive any
                                                 ---------
     termination of this Sublease.

7.2. Real Estate Taxes.
     -----------------

(a)  Lessor shall pay the Real Estate Taxes payable during the Base Year as
provided in Section 6 of this Sublease. For each calendar year after the Base
            ---------
Year, Lessee shall pay to Lessor, as additional rent and within thirty (30) days
after notification by Lessor of the amount thereof, Lessee's Proportionate Share
of the amount by which the Real Estate Taxes assessed for such subsequent tax
year exceed the amount of the Real Estate Taxes for the Base Year.

(b)  If this Sublease terminates and Lessee surrenders the Sublet Premises
before the end of a calendar year, Lessee's Proportionate Share of the increase
in Real Estate Taxes for such year over the Base Year shall be prorated based
upon the portion of such year for which this Sublease is in effect and during
which Lessee occupied the Sublet Premises.

8.   LESSOR NOT LIABLE; INDEMNITY BY LESSEE. Lessor shall not be liable for any
     --------------------------------------
injury to persons (including death) or for any loss or damage to property
resulting from any cause other than the gross negligence or willful, wrongful
act of Lessor. Lessee shall indemnify and hold Lessor and Landlord (as defined
below) harmless for and from any and all suits, actions, damage, liability and
expense (including attorneys' fees) arising from or out of (A) any

                                       3.
<PAGE>

occurrence in or on the Sublet Premises or (B) the occupancy or use by Lessee of
the Sublet Premises.

9.   DESTRUCTION TO SUBLEASED PREMISES.
     ---------------------------------

9.1. If the Sublet Premises are damaged by fire, the elements, accident or any
other casualty but are not thereby rendered unteriantable in whole or in part,
Lessor shall promptly, at its sole cost and expense (unless such damage was
caused by Lessee or any agent, contractor, employee, invitee or licensee of
Lessee, in which event such damage shall be repaired at the sole cost and
expense of Lessee), cause such damage to be repaired, and the Rent shall not be
abated.

9.2. If the Sublet Premises are damaged by fire, the elements, accident or any
other casualty and are thereby rendered untenantable in part, Lessor shall
promptly, at its sole cost and expense (unless such damage was caused by Lessee
or any agent, contractor, employee, invitee or licensee of Lessee, in which
event such damage shall be repaired at the sole cost and expense of Lessee),
cause the damage to be repaired, and the Rent shall be abated proportionately in
accordance with the portion of the Sublet Premises rendered untenantable and
such abatement shall commence on the date Lessor is notified of the damage and
shall continue until the repairs have been completed.

9.3. If the Sublet Premises are damaged by fire, the elements, accident or
other casualty and are thereby rendered wholly untenantable, Lessor shall have
the option (to be exercised in its sole discretion) to (1) terminate this
Sublease immediately by giving written notice of such termination to Lessee, in
which event neither Lessor nor Lessee shall, after the date of such notice, have
any further liability to the other hereunder; or (2) cause such damage to be
repaired, in which event the Rent shall be abated in full, and such abatement
shall commence on the date Lessor is, notified of the damage and shall continue
until the repairs have been completed. If Lessor causes the Sublet Premises to
be repaired, such repairs shall be at the sole cost and expense of Lessor.

9.4. Lessor shall not be required to repair or replace, or to compensate Lessee
for, any property which Lessee is entitled to remove from the Sublet Premises.

9.5. Lessor shall be obligated to make no payments for damages, compensation or
claims for inconvenience, loss of business or annoyance arising from any damage
to or repair of the Sublet Premises or the building in which the Sublet Premises
are located.

10.  EMINENT DOMAIN. If all or any part of the Sublet Premises, or all means of
     --------------
access thereto, are taken or condemned pursuant to the power of eminent domain,
or by purchase in lieu thereof, this Sublease shall terminate and Lessee shall
have no claim against Lessor or to any portion of the award or purchase price
for the value of any unexpired term of this Sublease, but the foregoing shall
not limit Lessee's right to compensation from the condemning or purchasing
authority for the value of any of Lessee's property taken (other than Lessee's
leasehold interest in the Sublet Premises). In the event of a temporary taking
pursuant to the power of eminent domain, this Sublease shall not terminate but
the term hereof shall be extended by the period of the taking and the Rent shall
abate in proportion to the area for the period of such taking.

                                       4.
<PAGE>

11.   SERVICES PROVIDED BY LESSOR.
      ---------------------------

11.1. Lessor is responsible for furnishing the following services.

(a)   Heating and Air Conditioning. Lessor shall provide heating and air
      ----------------------------
conditioning during the appropriate seasons, and shall attempt to maintain an
average temperature, subject to governmental regulations, of 68 degrees in the
winter season and 74 degrees in the summer season.

(b)   Electricity. Lessor shall provide electricity for lighting purposes and
      -----------
for the operation of all equipment. As soon as practicable, Lessor will provide
generator backup and Universal Power Supplies to support Lessee's equipment.

(c)   Janitorial Service. Janitorial service and supplies shall be provided in
      ------------------
and about the Sublet Premises by the Lessor.

(d)   General Repair and Maintenance. Lessor shall make the necessary
      ------------------------------
arrangements for all structural repairs, including but not limited to structural
columns and floors (excluding floor coverings such as carpet and floor tile) of
the Building, the roof of the Building, and the exterior walls of the Building
(excluding glass within the Building) provided Lessee gives Lessor notice
specifying the need for and nature of such repairs; provided, however, if Lessor
is required to make any repairs to such portions of the Sublet Premises by
reason, in whole or in part, of the negligent act or failure to act by Lessee or
Lessee's contractors or subcontractors or its or their agents or employees, or
by reason of any unusual use of the Sublet Premises by Lessee (whether or not
such use is a permitted use hereunder), Lessor may collect the cost of such
repairs, upon demand.

11.2. Lessor shall not be liable for the interruption of any of the above-
described services caused by repairs, replacements, improvements, alterations,
strikes, lock-outs, accidents, the inability of Lessor to secure such services
or to obtain fuel or supplies or other cause or causes beyond the reasonable
control of Lessor. Lessor and Lessee hereby covenant and agree that an
interruption of any of the above-described services shall not be deemed to be an
eviction or disturbance of Lessee's use, occupancy or possession of the Sublet
Premises or any part thereof, nor shall any such interruption render Lessor
liable in damages, nor relieve Lessee from its obligation to perform in
accordance with the terms, provisions and conditions of this Sublease including
the payment of the Rent.

12.   RIGHTS OF LESSEE SUBORDINATE. This Sublease, and the rights of Lessee
      ----------------------------
hereunder, are subject and subordinate to all ground or underlying Subleases and
to all mortgages or deeds of trust which may now or hereafter affect this
Sublease, the Sublet Premises, the building in which the Sublet Premises are
located or the land on which such building is constructed. The foregoing
subordination provision shall be self-operative and no further instrument of
subordination shall be required; provided, however, that in confirmation of such
subordination, Lessee hereby agrees, upon the request of Lessor or the Landlord,
to execute and deliver, in recordable form, any instrument of subordination or
confirmation of subordination required by Lessor.

Notwithstanding the foregoing, in the event of a foreclosure under any mortgage
or deed of trust affecting the Sublet Premises or the building in which the
Sublet Premises are located, or in the

                                       5.
<PAGE>

event of the termination of Lessor's interest or the eviction of Lessor under
any ground or other underlying Sublease, the holder of the note secured by the
mortgage or deed of trust, or the purchaser at such foreclosure sale, or the
landlord under such ground or other underlying Sublease, shall have the option
to recognize this Sublease, in which event this Sublease shall continue in full
force and effect and Lessee shall attorn to the new landlord hereunder.

13.  TERMS OF TENANT'S LEASE. This sublease is subject to all of the terms and
     -----------------------
conditions of that certain lease agreement (the "Lease Agreement") dated May 1,
1998, by and between TCP/IP, LLC (the "Landlord"), as landlord, and Lessor, as
tenant, together with all exhibits and attachments thereto, a copy of which
Lease Agreement is attached hereto as Exhibit C and made a part hereof for all
purposes, except as specifically excepted or addressed herein. Lessee shall
assume and perform the obligations of Lessor as tenant in the Lease Agreement,
to the extent said terms and conditions are applicable to the Sublet Premises.
Lessee shall not commit or permit to be committed on the Sublet Premises any act
or omission which shall violate any term or condition of the Lease Agreement. In
the event of a termination of the Lease Agreement, the Sublease shall terminate
coincidentally therewith with any liability of Lessor to Lessee. In the event of
any conflict or dispute arising out of this Sublease and under the terms of the
Lease Agreement, the terms and provisions of this Sublease shall control.

14.  RULES AND REGULATIONS. Lessee, on behalf of itself and its agents,
     ---------------------
employees, contractors, invitees and licensees, hereby agrees to observe and
strictly comply with certain rules and regulations (the "Rules and
                                                         ---------
Regulations"), hereafter adopted by Lessor or the Landlord. Lessor shall give
-----------
Lessee written notice of the Rules and Regulations as soon as practicable after
they have been modified or supplemented. Lessor and the Landlord shall not be
liable for the non-observance or violation by Lessee, or any agent, employee,
contractor, invitee or licensee of Lessee, of any of the Rules and Regulations.

15.  NO ASSIGNMENT OR SUBLETTING. Lessee covenants and agrees that it will not
     ---------------------------
assign this Sublease, or sublet the Sublet Premises, without the prior written
consent of Lessor (which consent shall not be unreasonably withheld, conditioned
or delayed) and, if required by the Lease Agreement, the Landlord.
Notwithstanding the foregoing, Lessee may assign this Sublease without the
consent of the Lessor in the event of a merger or consolidation in which the
Lessee is not the surviving entity or in the event of a transfer of this
Sublease (i) to a subsidiary of Lessee or (ii) pursuant to a sale of all or
substantially all of the assets of the Lessee.

16.  QUIET ENJOYMENT. Lessor agrees that, subject to and upon compliance with
     ---------------
the terms, provisions, covenants and conditions of this Sublease, Lessee shall
and may peaceably and quietly have, hold and enjoy the Sublet Premises for the
Term of this Sublease and any renewal or extension of the Term.

17.  PERSONAL PROPERTY AT RISK OF LESSEE. All personal property of every kind
     -----------------------------------
and description which may at any time be placed in or on the Sublet Premises by
Lessee (including but not limited to the equipment and inventory of Lessee)
shall be at Lessee's sole risk.

18.  COVENANTS BY LESSEE. Lessee covenants and agrees to comply with all
     -------------------
federal, state and local laws, statutes, ordinances, rules, regulations, orders
and requirements relative to

                                       6.
<PAGE>

Lessee's occupancy and use of the Sublet Premises. Lessee further covenants and
agrees to permit nothing to be done in, on or concerning the Sublet Premises
which would invalidate, conflict with or increase the. premiums for the fire,
casualty and liability insurance covering the Sublet Premises or the building in
which the Sublet Premises are located. Lessee shall not bring or keep or permit
to be brought or kept in or on the Sublet Premises any flammable, explosive or
other dangerous materials.

19.  NO ALTERATIONS OR IMPROVEMENTS. Lessee shall not alter or improve, nor
     ------------------------------
cause any alterations or to be made to, the Sublet Premises without Lessor's
prior written consent (which consent shall not be unreasonably withheld,
conditioned or delayed). Upon such consent of Lessor, any such alterations or
improvements shall be made or constructed at Lessee's sole cost and expense, by
one or more contractors approved in writing by Lessor and using materials
approved in writing by Lessor. In addition, all such alterations and
improvements, and all such contractors, shall comply with all applicable
federal, state and local laws, statutes, ordinances, rules, regulations, orders
and requirements. Lessee covenants and agrees to indemnify and hold Lessor
harmless for and from any and all claims, costs, demands and expenses (including
attorneys' fees) relative to or resulting from such alterations and improvements
and the work necessary therefor. Lessor shall have the right, to be exercised at
Lessor's option and in its sole discretion, to require Lessee to remove any or
all of such alterations improvements, decorations and furnishings and to repair,
at Lessee's sole cost and expense, any damage to the Sublet Premises resulting
from such removal.

20.  COVENANT TO CARE FOR SUBLET PREMISES.  Lessee covenants and agrees to
     ------------------------------------
commit no waste and to take good care of the Sublet Premises. Lessee shall, at
Lessee's sole cost and expense and to the complete satisfaction of Lessor,
repair any and all damage or injury to the Sublet Premises, and the building in
which the Sublet Premises are located, that is caused by Lessee or any agent,
employee, contractor, invitee or licensee of Lessee. If Lessee fails to make
such repairs, Lessor may, after ten (10) days prior written notice to Lessee,
make such repairs and the cost of such repairs shall be deemed to be additional
rent hereunder and shall be paid by Lessee to Lessor within ten (10) days after
demand is made therefor upon Lessee. Lessee covenants and agrees not to change
any locks or lock systems in or on the Sublet Premises without the prior written
consent of Lessor, and Lessee further covenants and agrees, upon the expiration
of the Term of this Sublease, to return all keys to locks installed in or on the
Sublet Premises or the building in which the Sublet Premises are located to
Lessor and to quit and surrender the Sublet Premises in clean and good
condition, reasonable wear and tear excepted.

21.  CERTAIN LOADING PROHIBITED. Lessee shall not place a load upon the floor of
     --------------------------
the Sublet Premises that exceeds the design or lawful floor load per square
foot. Lessor reserves the right to establish and regulate the weight and
positioning of all safes, computers and any other heavy equipment or equipment
constituting a "live" load in order to provide for the proper distribution of
weight, and Lessee agrees to bear the entire cost and expense, if any, necessary
to determine such weight and position.

                                       7.
<PAGE>

22.   DEFAULT OF LESSEE.
      -----------------

22.1. Each of the following shall, if not cured within the time periods
      prescribed in Section 22.2. hereof, constitute an event of default (each,
                            ----
      an "Event of Default") under this Sublease:
          ----------------

          (a)  The Rent or any additional rent is not paid within five (5) days
          of the due date.

          (b)  Lessee fails or is unable to pay its debts generally as they
          become due.

          (c)  Lessee transfers property in fraud of creditors.

          (d)  Lessee makes an assignment for the benefit of creditors.

          (e)  A receiver or trustee is appointed for any of Lessee's assets and
          such appointment is not vacated within thirty (30) days.

          (f)  Lessee fails to comply with any term, provision, covenant or
          condition of this Sublease.

22.2. Lessor shall give Lessee notice of each and every Event of Default as it
or they occur and Lessee shall have ten (10) days from the date of such notice
to cure any and all Events of Default described in Section 22.1(a) hereof and
                                                   ----------------
thirty (30) days from the date of such notice to cure (or commence and prosecute
a good faith effort to cure, if an Event of Default cannot reasonably be cured
within such thirty-day period) any and all Events of Default described in
Section 22.1(b)-(h) hereof.
-------------------

22.3. Upon notice to Lessee by Lessor of the occurrence of an Event of Default
and the failure of Lessee to cure such Event of Default within the time periods
stated above, Lessor shall have the right and option (1) to terminate this
Sublease by written notice to Lessee (in which event Lessee shall immediately
surrender the Sublet Premises to Lessor) and retain all monies (including a
security deposit, if applicable) received from Lessee (but without prejudice to
Lessor's rights to recover from Lessee any amounts remaining to be paid under
the Sublease, including the Rent not yet due and payable), or (2) to enter the
Sublet Premises and remove Lessee and Lessee's property therefrom with or
without force and without being liable to Lessee in any manner whatsoever for
any damage, and to attempt to relet the Sublet Premises for Lessee's account on
such terms as Lessor alone shall determine, or (3) to continue this Sublease and
sue for Lessee's performance hereunder (including payment of the Rent or any
additional rent as it becomes due). In all events, Lessor shall be entitled to
recover from Lessee all costs and expenses incurred by Lessor as a result of an
Event of Default, including reasonable attorneys' fees. The proceeds of any
reletting during the term of this Sublease shall be applied first to all
expenses incurred as a result of Lessee's default and of such reletting
(including, without limitation, reasonable attorneys' fees, leasing commissions
and the cost of any alterations and redecorating of the Sublet Premises that
Lessor deems to be desirable) and second to payment of the Rent and any
additional rent due hereunder. Lessee shall be liable to Lessor for any
deficiency (including all costs of collection and reasonable attorneys' fees)
but shall not be entitled to any surplus that may arise.

                                       8.
<PAGE>

22.4. The remedies provided Lessor above are in addition to, and not in lieu
of, any other rights and remedies Lessor may have under this Sublease, at law or
in equity. No delay by Lessor in the enforcement of the provisions of this
Sublease shall be deemed to constitute a waiver of any default of Lessee, and
the pursuit by Lessor of one or more remedies shall not be deemed to constitute
an election of remedies to the exclusion of any other remedy. Notwithstanding
any other provision of this Sublease, Lessor shall be under no obligation to
relet the Sublet Premises if Lessee, for any reason whatsoever, vacates the
Sublet Premises before the end of the Term.

23.   NOTICES.  Any notice, request or demand required or permitted to be given
      -------
pursuant to this Sublease shall be in writing and delivered by messenger or sent
by United States mail, certified, postage prepaid, return receipt requested, to
the following persons at the indicated addresses:

      To Lessor:     Phase 1 Incubator, Inc.
                     312 Laurel Avenue
                     Laurel, MD 20707
                     Attn: Lisa Losito, Vice President

      To Lessee:     SkyCache, Inc.
                     312 Laurel Avenue
                     Laurel, MD 20707
                     Attn: Robert M. Dunham, Treasurer

      Any such notice, request or demand, if delivered or mailed (as the case
may be) in the manner aforesaid, shall be deemed given on the date hand-
delivered at the specified address (whether or not any person is there to
receive it), or on the day of deposit in the United States mail, as the case may
be. Either party may, at any time, designate by written notice to the other
party (in accordance with the provisions of this Section 23) a change in the
                                                 ----------
above address or addresses, but such change shall be binding upon the person to
whom it is sent only from and after the date of receipt by such person.

24.   HOLDOVER TENANCY.  Any holding over by Lessee with the consent of Lessor
      ----------------
after the expiration of the Term of this Sublease (or any renewal or extension
thereof) shall be construed to be a tenancy from month to month and shall be at
the Rent (and in accordance with) all of the other terms, provisions, covenants
and conditions contained in this Sublease.

25.   MECHANICS' AND OTHER LIENS. Lessee will not permit any mechanic's,
      --------------------------
laborer's or materialman's lien to stand against the Sublet Premises for any
labor or material furnished to Lessee or claimed to have been furnished to
Lessee in connection with work of any character performed or claimed to have
been performed on the Sublet Premises by or at the direction or sufferance of
Lessee. Lessee shall have the right to contest the validity or amount of -any
such lien or claimed lien if Lessee shall have such lien bonded off and released
of record.

26.   SUCCESSORS AND ASSIGNS.  Subject to the provisions of Section 15 of this
      ----------------------                                ----------
Sublease, this Sublease and all of the terms, provisions, covenants and
conditions contained herein shall inure to the benefit of, and be binding upon,
Lessor and Lessee and their respective heirs, devisees, personal
representatives, successors and assigns.

                                       9.
<PAGE>

27.   RELATIONSHIP OF PARTIES. Nothing contained in this Sublease shall be
      -----------------------
deemed or construed by the parties hereto or by any third person as creating the
relationship of principal and agent or a partnership or joint venture between
the parties hereto, it being expressly understood and agreed that no provision
contained herein nor any act of the parties hereto shall be deemed to create any
relationship between the parties hereto other than the relationship of landlord
and tenant.

28.   SEVERABILITY. If any provision of this Sublease or the application thereof
      ------------
to any person or circumstance shall, for any reason or to any extent, be held or
determined to be invalid or unenforceable, the remainder of this Sublease and
the application of such provision to other persons or circumstances shall not be
affected thereby, but rather shall be enforced to the greatest extent permitted
by law.

29.   WAIVER. No waiver of any condition or legal right or remedy shall be
      ------
implied by the failure of either party to declare a forfeiture, or for any other
reason, and no waiver of any condition or covenant shall be valid unless it be
contained in a writing signed by both parties, nor shall the waiver of a breach
of any condition be claimed or pleaded to excuse the future breach of the same
condition or covenant.

30.   APPLICABLE LAW. This Sublease shall be governed by and construed in
      --------------
accordance with the laws of the State of Maryland.

31.   TITLES. The titles contained in this Sublease are inserted only for
      ------
convenience and are not to be construed as a part of this Sublease or as a
limitation upon the scope of the particular provisions to which they refer.

32.   ENTIRE AGREEMENT; INCORPORATION OF EXHIBITS.
      -------------------------------------------

32.1. This Sublease (together with the Exhibits referred to in Section 32.2
                                                                       ----
hereof which are hereby incorporated herein by reference) contains the entire
agreement between Lessor and Lessee relative to the Sublet Premises, and
supersedes all prior and contemporaneous negotiations, understandings and
agreements, written or oral, between the parties. This Sublease shall not be
amended or modified, and no waiver of any provision hereof shall be effective,
unless and until set forth in a written instrument authorized and executed with
the same formality as this Sublease.

32.2. The following Exhibits are attached hereto and by this reference made a
part hereof and incorporated herein:

(a)   Exhibit A - Description of Sublet Premises
      ----------

(b)   Exhibit B - Calculation of Lessee's Proportionate Share
      ---------

(c)   Exhibit C - Office lease
      ---------



                           [SIGNATURE PAGE FOLLOWS]

                                      10.
<PAGE>

     IN WITNESS WHEREOF, the parties have executed this Office sublease as of
the date first written above.

                                    LESSOR:

                                    Phase Incubator, Inc.

                                    /s/ Lisa M. Losito
                                    ------------------
                                    Name:  Lisa M. Losito
                                    Title: Vice President


                                    LESSEE:

                                    /s/ Robert M. Dunham
                                    --------------------
                                    Name:  Robert M. Dunham
                                    Title: Treasurer
<PAGE>

                                   EXHIBIT A
                                   ---------

                        Description Of Sublet Premises
                        ------------------------------



The offices noted on the building plans (a copy of which is attached as exhibit
A1) as Work Area I and Office 8 comprising a total of 636 square feet. Rent is
based upon an annual rent of $13.00 per square foot, including a 22% core
factor. Total rentable square footage is 776 square feet.
<PAGE>

                                   EXHIBIT B
                                   ---------
                  Calculation Of Lessee's Proportionate Share
                  -------------------------------------------

Net Rentable Area of Sublet Premises: 776 square feet

Net Rentable Area of Building in which Sublet Premises are Located:  14,000
          square feet Lessee's Proportionate Share = 5.5%
<PAGE>

                                   EXHIBIT C
                                   ---------

                                 Office Lease
                                 ------------


     Office Lease between TCP/IP, LLC and Phase 1 Incubator, Inc. dated as of
May 1, 1998, is attached.