TWELFTH AMENDMENT TO LEASE

         THIS TWELFTH  AMENDMENT TO LEASE (the  "Amendment") is made and entered
into this 12th day of May,  1999,  by and between  LAFP-SF,  Inc.,  successor in
interest to The Prudential Insurance Company of America ("Landlord"),  having an
office c/o Lowe Enterprises  Colorado,  Inc., 1515 Arapahoe  Street,  Tower III,
Suite 900, Denver,  Colorado 80202, and Data  Transmission  Network  Corporation
("Tenant"), having an office at 9110 West Dodge Road, Suite 200, Omaha, Nebraska
68114.

                                    RECITALS

A.       The  Prudential  Insurance  Company of America and Tenant  entered into
         that  certain  Lease  dated  as  of  May  2,  1995.  Subsequently,  The
         Prudential  Insurance  Company of America  and Tenant  executed a First
         Amendment to Lease dated  September  29,  1995,  a Second  Amendment to
         Lease  dated  January 5, 1996,  and a Third  Amendment  to Lease  dated
         January 5, 1996, and thereafter,  Landlord and Tenant executed a Fourth
         Amendment to Lease dated December 23, 1996, a Fifth  Amendment to Lease
         dated July 7, 1997,  a Sixth  Amendment  to Lease dated July 7, 1997, a
         Seventh  Amendment  to  Lease  dated  September  19,  1997,  an  Eighth
         Amendment to Lease dated September 19, 1997, a Ninth Amendment to Lease
         dated September 19, 1997, a Tenth amendment to Lease dated December 23,
         1997,  and an Eleventh  Amendment  to Lease dated March 31,  1998.  The
         lease and all  amendments  thereto are  hereinafter  referred to as the
         "Lease."

B.       Pursuant to the Lease, Tenant occupies Suites #175A, #175B, #100, #101,
         #110,  #130,  #200,  #300,  #301,  #310,  #315, #320, #325, #340, #350,
         #350A, #350B, #360, and #362, containing approximately 107,576 rentable
         square feet (the "Current  Premises") in the Building  known as Embassy
         Plaza, located at 9110 West Dodge Road, Omaha, Nebraska pursuant to the
         Lease.

C.       All  capitalized  terms not  defined  herein  shall  have the  meanings
         ascribed to them in the Lease.

         NOW,  THEREFORE,  in the  consideration  of the foregoing  promises and
other good and valuable considerations, the receipt and sufficiency of which are
hereby acknowledged, the parties hereto covenant and agree as follows:

         I.  EXTENSION OF LEASE FOR CURRENT PREMISES.

1. Term.  The term of the Lease with  respect to the Current  Premises is hereby
extended  for an  additional  term of five (5)  years  commencing  June 1,  2005
("Current  Premises  Extension  Commencement  Date") and  expiring  May 31, 2010
("Current Premises Extension Term").

                                     -279-
<PAGE>

2. Base Rent.  Tenant shall pay as Base Rent for the Current Premises  Extension
Term the sum of Two Million Two Hundred  Fifty-Nine  Thousand Ninety Six Dollars
($2,259,096.00)  per year,  payable in advance on the first day of each month in
the amount of  $188,258.00  per month,  in  accordance  with the  provisions  of
Paragraph  2 of  the  Lease.  In  addition,  Tenant  shall  continue  to  pay as
additional  Base Rent  $39,999.96 per year,  payable in monthly  installments of
$3,333.33, for the Additional Parking until May 31, 2005.

3. Base Expense Year and Base Tax Year.  The Base Expense Year and Base Tax Year
for the Current Premises Extension Term shall be the calendar year 2005.

4. Tenant Improvements. Landlord shall provide a tenant improvement allowance of
up to  $215,152.00 to be applied  toward the cost of Tenant's  desired  Premises
improvements.  All improvements  shall be performed in accordance with the terms
of the Tenant  Improvement  Work Schedule  attached hereto and by this reference
made a part hereof,  marked Exhibit "B". Such allowance  shall be made available
to Tenant beginning on the Expansion Premises Commencement Date.

         II.      EXPANSION OF PREMISES

1. Premises.  Pursuant to the provisions of Paragraph 29 of the Lease, effective
the earlier of October 1, 2002 or the date Landlord  delivers  possession of the
Expansion  Premises to Tenant  ("Expansion  Premises  Commencement  Date"),  the
Premises  shall  be  expanded  to  include   Suites  120  and  160,   containing
approximately  23,563  rentable  square  feet as  depicted  on the  floor  plans
attached  hereto,  marked  Exhibit "A" and by this  reference made a part hereof
(the "Expansion Premises"), for a total in the Premises of approximately 131,139
rentable square feet.  Beginning on the Expansion  Premises  Commencement  Date,
the"Premises,"  as that term is defined  in the Lease,  shall be amended to mean
and include the Expansion  Premises and, subject to the terms of this Amendment,
the demise of the  Expansion  Premises  shall be subject to each and every term,
provision and condition set forth in the Lease as if the Expansion Premises were
originally   demised   thereunder.   Notwithstanding   the   foregoing,   Tenant
acknowledges  that Kirkham  Michael &  Associates,  Inc.  (the Current  Tenant")
currently leases the Expansion  Premises,  with such lease expiring on September
30,  2002.  Should the  Current  Tenant  holdover  and not vacate the  Expansion
Premises prior to the Expansion Premises Commencement Date, Landlord will not be
liable  to  Tenant  for any  damages  suffered  by  Tenant  as a result  of such
holdover. However, in such event, Landlord will use reasonable efforts to pursue
its legal  remedies to evict the Current  Tenant  from the  Expansion  Premises.
Alternatively,  in the event  that the  Current  Tenant  vacates  the  Expansion
Premises prior to the expiration of its lease  therefor,  Landlord shall deliver
possession of the Expansion Premises earlier than October 1, 2002. Tenant hereby
represents  and  warrants  to  Landlord  that  Tenant  shall  not  engage in any

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                                     -280-
<PAGE>

discussions with the Current Tenant  concerning early termination of the Current
Tenant's premises, or related issues,  without Landlord being present.  Landlord
and Tenant  agree that upon  delivery  by  Landlord  to Tenant of the  Expansion
Premises,  the parties will execute a Commencement  Date Certificate in the form
attached  hereto as Exhibit "C" confirming the Expansion  Premises  Commencement
Date.

2. Term.  The term of the Lease with respect to the Expansion  Premises shall be
for a period of ten years commencing on the Expansion Premises Commencement Date
and expiring ten years thereafter ("Expansion Premises Term").

3. Base Rent.  Tenant shall pay as Base Rent for the Expansion  Premises  during
the Expansion  Premises Term an amount equal to $20.00 per rentable  square foot
per year  increased by an amount equal to 3.5% per year for each  calendar  year
from the date of this  Amendment  until the calendar year in which the Expansion
Premises  Commencement Date occurs.  So, for example,  if the Expansion Premises
Commencement  Date  occurs in  calendar  year 2000,  the Base Rent per  rentable
square foot for the Expansion Premises would be $20.70 per rentable square foot.
Base Rent  shall be  payable  in monthly  installments  in  accordance  with the
provisions  of Paragraph 2 of the Lease.  In addition,  on the 61st month of the
Expansion  Premises Term, Base Rent shall be increased in an amount equal to the
monthly  installment  of Base Rent paid on the 60th  month,  as provided in this
Paragraph 3,  multiplied  by a number which is reached by dividing the "Consumer
Price Index for Wage  Earners and  Clerical  Workers in the City of  Denver--All
Items," as published by the Bureau of Labor Statistics, U.S. Department of Labor
(the "Denver  CPI")  figure for January of the  calendar  year in which the 61st
month of the Expansion Premises Term occurs by the Denver CPI figure for January
of the calendar month in which the Expansion Premises  Commencement Date occurs.
If the official  monthly Denver CPI is not available for use as a cost-of-living
index for the  months  provided  to be used as a basis for such  formula,  it is
agreed that the Denver CPI as issued and  published  for the earliest  preceding
months should be used in  determining  such formula.  If, at any time during the
term hereof,  the U.S. Bureau of Labor Statistics shall discontinue the issuance
of the Bureau of Labor  Statistics  Consumer Price Index,  the parties shall use
any other standard,  nationally recognized  cost-of-living index then issued and
available, which is published by the U.S. government.

4. Adjustment Rent.  Effective upon the Expansion  Premises  Commencement  Date,
Tenant  shall pay  Adjustment  Rent with  respect to the  Expansion  Premises in
accordance  with the  provisions  of Paragraph 2 of the Lease.  The Base Expense
Year and Base Tax Year for the  Expansion  Premises  Term shall be the  calendar
year in which the Expansion Premises Commencement Date occurs.

5. Tenant  Improvements.  Tenant is taking the Expansion  Premises in an "As Is"
condition,  and  Landlord  shall have no  responsibility  to provide  any tenant
improvements in such premises.  However,  Tenant shall have the right to use any
remaining tenant improvement allowance amounts which were granted by Landlord in
prior  Lease  amendments  for  other  premises  of  Tenant,   including  without
limitation  the  improvement  allowance  granted  to  Tenant  under  Article  1,
Paragraph 4 of this amendment,  subject to Landlord's  prior written approval of
such  improvements,  and provided  that all  improvements  shall be performed in
accordance  with the terms of the  Tenant  Improvement  Work  Schedule  attached

                                       3
                                     -281-
<PAGE>

hereto and by this  reference  made a part  hereof,  marked  Exhibit  "B".  Such
allowance shall be made available to Tenant beginning on the Expansion  Premises
Commencement Date.

6. Parking. Tenant shall have the right to use all of the exposed parking spaces
at the Building (but not at any other  building in the complex) at no additional
charge  during  the  Expansion  Premises  Lease  Term.  Landlord  shall  not  be
responsible for  unauthorized use of parking spaces,  and any such  unauthorized
use  shall not be a default  by  Landlord  under the  Lease.  Landlord  may,  at
Landlord's  sole  option,   relocate  Tenant's  parking  spaces  to  a  suitable
alternative  parking area during repair or  reconstruction of the parking areas.
If at any time  Landlord  fails to or is unable to provide all or any portion of
such  parking  spaces to Tenant,  such fact  shall not be a default by  Landlord
under the Lease, either in whole or in part, and Tenant shall not be entitled to
any claim or remedy.  Tenant and its  employees  shall have the right to use the
parking spaces  described  herein 24 hours a day, seven days per week.  Landlord
reserves the right to post and enforce reasonable rules governing  parking,  and
to take away the  parking  rights of any  individual  who fails to abide by such
rules.

7. Right of  Relocation.  The  Expansion  Premises  Term is expected to continue
beyond the expiration of the Current Premises Extension Term. As a result,  when
the Current  Premises  Extension  Term  Expires,  Tenant shall have the right to
choose what portion of the Premises,  as expanded  hereunder,  comprising 23,000
rentable  square  feet,  it will  continue  occupying  for the  duration  of the
Expansion  Premises Term (the  "Continuing  Premises"),  and what portion of the
Premises it surrenders to Landlord, subject to the following:

         (a)   The Continuing Premises must be physically contiguous, or located
               in the same pod of the  Building  on each of the first and second
               floors;

         (b)   Landlord  shall have no obligation to construct any  improvements
               in the Continuing Premises, and to the extent Tenant requires any
               consolidation finish work, such improvements shall be at Tenant's
               sole cost and expense; and

         (c)   Tenant shall be required to reimburse Landlord for the reasonable
               costs  incurred by Landlord  associated  with  construction  of a
               demising wall, and any other construction  necessary to segregate
               the Continuing  Premises from the remainder of the Premises being
               surrendered.

                                       4
                                     -282-
<PAGE>

         III.     GENERAL PROVISIONS

1. Notices.  Landlord's notice address as set forth in Paragraph 24 of the Lease
shall be revised to reflect the following changes:

                  If to Landlord:   LAFP-SF, Inc.
                                    c/o Lowe Enterprises Colorado, Inc.
                                    1515 Arapahoe Street, Tower III, Suite 900
                                    Denver, Colorado 80202
                                    Attn:  Senior Vice President - Operations
                  with a copy to:   Pacific Realty Group, Inc.
                                    1905 Harney Street, Suite 403
                                    Omaha, Nebraska 68102
                                    Attn:  Senior Vice President - Management
                                           Operations

2.  Density Cap.  Tenant shall not allow an occupancy  density in excess of five
(5) people per 1,000  rentable  square feet. If Tenant fails to comply with such
occupancy  limit, in addition to Landlord's  other rights and remedies under the
Lease,  Landlord  shall have the right and option to  terminate  this Lease upon
written notice to Tenant effective on the date set forth in such notice.

3.  Removal  of  Equipment.  Paragraph  15 of the  Lease is  hereby  amended  by
inclusion of the following language at the end of said paragraph:

                  "The  application of the foregoing  provisions  shall include,
         without limitation, any and all telecommunications  equipment installed
         in the Premises or elsewhere in the Building by or on behalf of Tenant,
         including   wiring,   or  other   facilities   for   telecommunications
         transmittal."

4. Brokerage Disclosure.  Tenant represents and warrants to Landlord that it has
dealt only with Pacific  Realty  Group,  acting as  Landlord's  exclusive  agent
("Broker") in the negotiation of this Amendment.  Landlord shall make payment of
the  brokerage  fee  due to the  Broker  pursuant  to and in  accordance  with a
separate  agreement  with the Broker.  Landlord and Tenant each hereby agrees to
indemnify and hold  harmless the other of and from any and all damages,  losses,
costs  or  expenses  (including  without  limitation,  all  attorneys'  fees and
disbursements)  by reason of any claim of or  liability  to any other  broker or
other  person  claiming  through  the  other  party  and  arising  out  of or in
connection with the negotiation, execution and delivery of this Amendment.

                                       5
                                     -283-
<PAGE>

5. Reaffirmation of Lease. Except as herein specifically provided, the terms and
conditions  of the Lease shall  continue  in full force and effect.  The parties
hereto hereby reaffirm and ratify all covenants,  representations and warranties
in the Lease as amended by this Amendment.

6. Binding Effect. This Amendment shall be binding upon and inure to the benefit
of the parties hereto, their successors and assigns.

         IN WITNESS WHEREOF, Landlord and Tenant have executed this Amendment as
of the day and year first above written.

Tenant:                                              Landlord:

Data Transmission Network                   LAFP-SF, Inc.
Corporation, a Delaware
corporation

By:/s/ Greg T. Sloma                        By:  Lowe Enterprises Investment
   ------------------------                 Management, Inc.
   Greg T. Sloma                            Its: Authorized Agent

Its:President and COO                       By:/s/ Tom Maul
                                               -------------------------
                                                Tom Maul

                                            Its:Vice President

                                       6
                                     -284-
<PAGE>



EXHIBIT "B" to be made a part of a Twelfth  amendment To Lease between  LAFP-SF,
INC.  (Landlord),  and DATA  TRANSMISSION  NETWORK  CORPORATION  (Tenant)  dated
______, 2000.

                        TENANT IMPROVEMENTS WORK SCHEDULE

                                    ARTICLE 1

                       Landlord's Construction Obligations

Tenant  accepts  the  Expansion  Premises in an "As Is"  condition  and shall be
responsible for any and all  improvements  required for its occupancy and use in
the Premises in accordance  with Article II of this Exhibit "B". If requested by
Tenant,  Landlord's representative will provide construction management services
for Tenant's work in the space.  Landlord's  fee for such services will be 5% of
the total construction cost, including any associated professional fees.

As a part of this Amendment,  Landlord and Tenant have agreed to upgrade certain
parts of the Building's  electrical and mechanical  systems.  Landlord agrees to
pay one-half of the cost to replace the main electrical  switchgear and feeders,
at an estimated cost of  approximately  $58,000.  The remaining  one-half of the
cost  therefor  ("Tenant's  Share of  Switchgear")  shall be paid by  Tenant  as
Additional  Rent  amortized  over the remaining  term of the Lease in accordance
with the provisions  contained in the next paragraph below. Tenant agrees to pay
for the entire  remaining cost of electrical  system  upgrades upon receipt from
Landlord of invoices  therefor.  Landlord shall retain a qualified MEP engineer,
which shall bid the  specifications  for the electrical and mechanical  upgrades
following  approval  thereof by  Landlord  and Tenant as  provided in Article II
below.  No portion of Tenant's  costs  associated  with such  electrical  system
upgrade  shall be amortized  over the Lease term,  except for Tenant's  Share of
Switchgear.

Landlord  agrees to loan to Tenant the initial  costs of the  mechanical  system
upgrade plus an  additional  $100,000  for other  incidental  costs  incurred by
Tenant  provided  that  Tenant's  pro  rata  share  of the  total  costs  of the
mechanical  system  upgrade paid by Landlord  (based upon a useful life for such
upgrades of 17.5 years),  together  with said  $100,000  and  Tenant's  Share of
Switchgear,  shall be repaid to Landlord as Additional  Rent  amortized over the
remaining  portion of the current term of the Lease (which expires May 31, 2005)
for the Current Premises, at a rate of 12% per annum, noncompounded.

                                   ARTICLE II

                       Construction of Tenant Improvements

Tenant  shall  have  the  right  to  place  partitions  and  fixtures  and  make
improvements  or  other  alterations  in the  Premises  in  accordance  with the
provisions of Paragraph 9 of the Lease.  Tenant shall have the right to apply up
to $84,320.19,  which amount  constitutes the amount  remaining from improvement

                                       7
                                     -285-
<PAGE>

allowances  previously  provided  by  Landlord to Tenant for Suites 315 and 350A
("Carry Over Improvement Allowance"),  and, in addition,  Landlord shall provide
to Tenant an  additional  improvement  allowance  of up to two  hundred  fifteen
thousand,  one hundred fifty-two  dollars  ($215,152)  ("Additional  Improvement
Allowance"),  all  of  which  may  be  applied  toward  the  cost  of  any  such
tenant-provided  improvements as follows (the Carry Over  Improvement  Allowance
and the Additional  Improvement  Allowance are hereinafter sometimes referred to
together as the "Improvement Allowance"):

1. At least  three  (3) weeks  prior to the  commencement  of any  construction,
Tenant  shall  furnish  Landlord  with  two  (2)  complete  sets  of  plans  and
specifications  for Tenant's proposed  improvements for review and assessment as
to the  compatibility of said  improvements  with the Building's  systems.  Such
review  shall be a  Tenant's  sole  cost and  expense  and  shall  not be unduly
delayed.  Tenant agrees to make such modifications as are reasonably required by
Landlord.

2. The Improvement  Allowance shall be paid in periodic  installments,  not more
frequently  than  once per  month,  equal to the  total of the  contractor's  or
consultant's  invoice amounts for improvements  made to the Premises,  excluding
any furnishings or business  equipment  (such as computers,  satellite/microwave
dish, office equipment, etc.), as submitted by Tenant and verified to Landlord's
reasonable satisfaction; provided, however, that such payments will be made only
if Tenant is not then in default  under the terms of this Lease and the invoices
are accompanied by lien waivers in the amount equal to that of the invoices, and
provided  further that the Final  Installment,  as defined in paragraph 8 below,
shall be paid only upon  compliance  with the  provisions  of said  paragraph 8.
Landlord  shall make such  periodic  payments  within  thirty  days of  complete
submittal by Tenant in accordance  with the provisions  hereof.  The Improvement
Allowance shall be allocated and  distributed  subject to the provisions of this
Exhibit "B" within the following time frame, subject to Excusable Delays:

         Additional  Improvement  Allowance  - Between  the  Expansion  Premises
Commencement Date and the later of 12 months thereafter or June 30, 2003.

         Carry Over  Improvement  Allowance - Between the date of this Amendment
and June 30, 2000

("Tenant  Improvement  Periods").   For  purposes  of  the  foregoing  sentence,
"Excusable Delays" shall mean any delays due to acts of God, strike,  riot, war,
weather,  failure to obtain labor and materials,  or any other reason beyond the
reasonable  control of Tenant;  provided,  however,  that for  purposes  of this
definition,  Tenant's or its  contractors' or agents' lack of funds shall not be
deemed to be a cause  beyond  the  reasonable  control  of  Tenant.  The  Tenant
Improvement  Periods shall be extended on a day-to-day  basis for each day of an
Excusable Delay,  provided that Tenant shall immediately  notify Landlord in the
event of an Excusable Delay.

                                       8
                                     -286-
<PAGE>

3.  Tenant  will not be entitled to any payment or credit for any portion of the
improvement  allowance which is not actually invoiced and submitted by Tenant on
or before  sixty  days  following  the end of each  Tenant  Improvement  Period,
subject to an Excusable  Delay, in accordance with the provisions of paragraph 2
above.

4. In addition to the provisions set forth in Paragraph 9 of the Lease, Tenant's
contractor shall (and its contract shall so provide):

         (a)   conduct  its  work in  such a  manner  so as not to  unreasonably
               interfere   with  other   tenants  in  the   Building,   Building
               operations,  or any  other  construction  occurring  on or in the
               Building or the Premises;

         (b)   comply with all applicable rules and regulations  relating to the
               construction   activities  in  or  on  the  Building  as  may  be
               reasonably  promulgated  from  time to time  by  Landlord  or its
               agents, upon receipt of same from Landlord;

         (c)   maintain such insurance (such as general  liability and workman's
               compensation) in force and effect as may be reasonably  requested
               by Landlord or as customarily  required for similar  construction
               activities;

         (d)   be responsible  for reaching an agreement with Landlord (to which
               Landlord  shall not  unreasonably  withhold  its consent) and its
               agents as to the terms and conditions  for all  contractor  items
               relating to the conducting of its work, including but not limited
               to, those matters relating to hoisting, systems interfacing,  use
               of  temporary  utilities,  storage  of  materials,  placement  of
               dumpsters,  access  to the  Premises  and the  Building,  and the
               purchase and return of Building standard materials;

         (e)   upon completion of any tenant improvements, Tenant shall promptly
               furnish Landlord with sworn owner's and  contractors'  statements
               and full  and  final  waivers  of lien  covering  all  labor  and
               materials included in such improvements.  Tenant shall not permit
               any mechanic's lien to be filed against the Building, or any part
               thereof,  arising out of any improvement performed, or alleged to
               have been performed,  by or on behalf of Tenant. If any such lien
               is filed,  Tenant shall within thirty (30) days  thereafter  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

                                       9
                                     -287-
<PAGE>

               Tenant  shall  reimburse  Landlord  upon demand for the amount so
               paid by Landlord,  including  Landlord's  expenses and attorney's
               fees.

5. Landlord shall have the right to approve all subcontractors to be used by the
Tenant's contractor,  which approval shall not be unreasonably  withheld as long
as such subcontractors comply with the requirements of paragraph 4 above.

6. Tenant shall indemnify and hold harmless  Landlord,  its agents,  contractors
(including Building Contractor), and any mortgagee of Landlord, from and against
any and all  losses,  damages,  costs  (including  costs of suit and  attorney's
fees),  liabilities,  or causes of action for injury to or death of any  person,
for damage to any property, and for mechanic's,  materialmen's or other liens or
claims  arising  out of or in  connection  with  the work  done by the  Tenant's
contractor (and Tenant's  contractor's  subcontractors  and  sub-subcontractors)
under its contract with Tenant.

7. The failure by Tenant,  after receiving  written notice, to materially comply
with any of the  provisions  of Article II of this  Exhibit  shall  constitute a
Default  by Tenant  under the terms of the  Lease and  Landlord  shall  have the
benefit of all remedies  provided for in the Lease,  except  Tenant shall have a
thirty (30) day right to cure Default upon receipt of written notice.

8. Within thirty (30) days of the  completion of Tenant's  improvements,  Tenant
shall deliver to Landlord two (2) copies of the final drawings showing the plans
and specifications of all improvements completed by Tenant or on Tenant's behalf
under  Article II.  Landlord  shall not be  obligated to pay to Tenant the Final
Installment  of the  Improvement  Allowance  until  Tenant  has  delivered  such
drawings.  For purposes hereof, the "Final  Installment" shall mean with respect
to each phase of work,  an amount  equal to 10% of the full cost of such work as
retainage.

9. With respect to the  electrical  upgrades which will be made at Tenant's sole
cost,  Landlord  shall submit to Tenant  invoices for the cost of all such work.
Tenant shall reimburse  Landlord for all such amounts within thirty (30) days of
receipt of each  invoice.  All such  amounts owed to Landlord  shall  constitute
Additional  Rent,  and  Tenant's  failure to so reimburse  Landlord  within said
thirty-day period shall constitute a material monetary default under the Lease.

                                       10
                                     -288-
<PAGE>



EXHIBIT "C" to be made a part of a Twelfth  Amendment To Lease between  LAFP-SP,
INC.  (Landlord),  and DATA  TRANSMISSION  NETWORK  CORPORATION  (Tenant)  dated
______, 1999.

                           COMMENCEMENT DATE AGREEMENT

         This Commencement Date Agreement is entered into by Landlord and Tenant
pursuant to Article II, Paragraph 1 of this Amendment.

1. DEFINITIONS.  In this Agreement,  the following terms have the meanings given
to them:

         (a)   Landlord: LAFP-SP, Inc.

         (b)   Tenant: Data Transmission Network Corporation

         (c)   Lease: Lease between Landlord and Tenant,  dated May 2, 1995, and
               amended by First  Amendment to Lease dated  September 29, 1995, a
               Second  Amendment  to  Lease  dated  January  5,  1996,  a  Third
               Amendment to Lease dated  January 5, 1996, a Fourth  Amendment to
               Lease dated  December 23, 1996, a Fifth  Amendment to Lease dated
               July 7, 1997,  a Sixth  Amendment  to Lease dated July 7, 1997, a
               Seventh  Amendment to Lease dated  September  19, 1997, an Eighth
               Amendment to Lease dated September 19, 1997, a Ninth Amendment to
               Lease dated  September 19, 1997, a Tenth Amendment to Lease dated
               December 23, 1997, and an Eleventh Amendment to Lease dated March
               31, 1998.

2.  CONFIRMATION OF THE COMMENCEMENT DATE WITH REGARD TO THE OCCUPANCY BY TENANT
OF SUITE.  Landlord and Tenant confirm that the  Commencement  Date of the Lease
with regard to the Expansion Premises is ___________, 20___.

     Landlord and Tenant have executed this  Commencement  Date  Agreement as of
the date set below.

Tenant:                                              Landlord:
Data Transmission Network                   LAFP-SP, Inc.
Corporation, a Delaware corporation

By:/s/ Greg T. Sloma                        By:Lowe Enterprises Investment
   ---------------------------                 Management, Inc.
   Greg T. Sloma
Its:President and COO                       Its:  Authorized Agent

Date:5/4/99                                 By:____________________________

                                            Its:____________________________

                                            Date:___________________________




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