SIXTH AMENDMENT TO LEASE



         THIS SIXTH  AMENDMENT  TO LEASE (the  "Amendment")  is made and entered
into this ______ day of June, 1997, 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., 1475 Lawrence  Street,  Suite 210,
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 Data  Transmission
         Network  Corporation entered into that certain Lease dated as of May 2,
         1995, for Suites #175A, #110, #200, #300, #301, #310, #320, #325, #340,
         #360,  and #362  containing  75,931  rentable  square feet (RSF) in the
         Building  known as Embassy  Plaza,  located  at 9110 West  Dodge  Road,
         Omaha, Nebraska ("the Premises").

B.       Subsequently,  The  Prudential  Insurance  Company Of America  and Data
         Transmission  Network  Corporation  executed a First Amendment To Lease
         dated  September 29, 1995, a Second  Amendment To Lease dated  November
         30,  1995,  a Third  Amendment  To Lease dated  January 5, 1996,  and a
         Fourth  Amendment  To Lease  between  LAFP-SF,  Inc.  and Tenant  dated
         December  23, 1996,  and a Fifth  Amendment To Lease dated June , 1997.
         The combined terms of the Lease and subsequent  Amendments shall herein
         be referred to as the "Lease". Under the Lease the Premises consists of
         a total of 88,136 RSF.

C.       Landlord and Tenant  acknowledge that the surface parking for the
         Building is overburdened,  in part as a result of Tenant's use thereof.

D.       Landlord  intends to construct  additional  parking  facilities to ser-
         vice Tenant and other tenants in the Building subject to the terms and
         conditions contained below.

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

         NOW,  THEREFORE,  in 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:

1.       Premises.  Effective  August 1, 1997,  the  Premises  shall be expanded
         to include  Suite 100,  measuring  4,544 RSF as shown on the floor plan
         attached hereto, marked Exhibit "A" (the "Expansion Space") and by this
         reference made a part hereof.  Notwithstanding  the above,  both Tenant
         and Landlord understand that the law offices of Young & White currently
         leases Suite 100 with such lease expiring  approximately July 31, 1997.
         Should  Young & White  holdover  and not  vacate  Suite 100 by July 31,
         1997,  Landlord  will  make  reasonable  efforts  to  pursue  its legal
         remedies to have Young & White removed from the space. If the Expansion
         Space is delivered  to Tenant after July 31, 1997,  Landlord and Tenant
         shall  execute a  Commencement  Date  Certificate  in the form attached
         hereto as Exhibit "C", confirming  Landlord's delivery of the Expansion
         Space and  commencement  of the Lease  with  respect  to the  Expansion
         Space.

2.       Term.  The term of the Lease with respect to the  Expansion  Space
         identified  in  Paragraph 1 above shall  commence  August 1, 1997,  and
         terminate upon termination of the Lease.

3.       Base Rent.  Tenant shall pay as Base Rent for the  Expansion  Space
         during the Term the sum of Six Hundred and Five Thousand,  Four Hundred
         Eighty-Eight Dollars and Sixteen Cents ($605,488.16) payable monthly as
         follows:

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                  August 1, 1997 - May 31, 2001     $6,248.00 / Month
                  June 1, 2001 - May 31, 2005       $6,626.67 / Month

4.       Adjustment  Rent.  Effective  upon  commencement  of the  Term  for the
         Expansion  Space,  Tenant shall pay Adjustment Rent with respect to the
         Expansion Space in accordance  with the terms and conditions  contained
         in Paragraph 2 of the Lease, except that the Base Expense Year and Base
         Tax Year  with  respect  to the  Expansion  Space  herein  shall be the
         calendar year 1997.

5.       Tenant  Improvements.  Landlord  shall  provide  a  tenant  improvement
         allowance of up to $45,440.00 to be applied toward the cost of Tenant's
         required building improvements.  All improvements shall be performed in
         accordance with the Tenant  Improvement Work Schedule  attached hereto,
         marked as Exhibit "B", and by this reference made a part hereof.

6.       Tenant's Proportionate Share. Tenant's Proportionate Share with respect
         to the  Expansion  Space shall be 3.45% (4,544 RSF / 131,740  RSF).  In
         addition,  as a part of this Amendment,  Tenant's  Proportionate  Share
         schedule for Tenant's  remaining  Premises  contained in paragraph 6 of
         the Fourth  Amendment  To Lease,  dated  December  23,  1996,  shall be
         replaced with the following schedule:

         January 1, 1997 - December 31, 1997   59.20% (77,990 RSF / 131,740 RSF)
         January 1, 1998 - May 31, 2005        66.90% (88,136 RSF / 131,740 RSF)


7.       Parking.  In order to avoid  future  problems  with  overburdened
         parking facilities at Embassy Plaza,  Tenant  acknowledges the building
         surface   parking  lot   currently   provides  a  parking  ratio  of
         approximately four (4) parking stalls per 1,000 rentable square feet of
         lease space and that this ratio may change from time to time. Effective
         January  1, 1998,  Tenant  agrees to cause its  employees,  independent
         contractors,  and other  invitees  to  conform to the  current  parking
         ratio, as it changes from time to time, and further agrees to only park
         up to the number of cars  permitted  using the  following  calculation:
         Tenant's total rentable  square feet / 1,000 times the current  parking
         ratio.  In  addition,   Tenant  agrees  not  to  allow  its  employees,
         independent  contractors,  and  other  invitees  to  park  cars  in the
         adjacent  Financial  Plaza parking lots.  Landlord  shall  maintain the
         right to use any means it determines,  in its reasonable discretion, to
         enforce Tenant's  parking space  limitations as described herein and/or
         in the Rules and  Regulations  including  towing,  and Tenant agrees to
         cooperate with Landlord in such efforts.

         Notwithstanding  the above, should Tenant be in default of the Lease as
         a result of the parking provisions contained herein,  Tenant shall have
         up to sixty  (60) days to either  cure such  default  or reach  written
         agreement  with the  Landlord as to a solution  and schedule for curing
         such  default.  Tenant  will be  allowed,  as a method of  curing  such
         default,  to acquire from other tenants of the building,  if available,
         parking  spaces from that tenant's  parking stall  allocation not being
         required  by such  tenant.  Such  agreement  between  Tenant  and other
         building  tenant's  shall be documented by both parties and approved by
         Landlord,  with such  approval not to be  unreasonably  withheld.  Such
         documentation  shall  include  employee  counts for such  tenant and an
         agreement  for policing  such  action.  Such  agreement  shall end upon
         termination of such tenant's  rights for space within the Embassy Plaza
         building.

8.       Effect of  Agreement.  Except as  herein  specifically  provided,  the
         terms and conditions of the Lease shall continue in full force and
         effect.

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

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10.      The parties hereto hereby reaffirm and ratify all covenants,  represen-
         tations  and warranties in the Lease as amended by this Amendment.

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



Tenant:                                         Landlord:

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

                                                By:  Lowe Enterprises Investment
                                                     Management, Inc.
By:                                             Its:  Authorized Agent
   ----------------------------------


Its:                                            By:
    ---------------------------------              -----------------------------

                                                Its:
                                                    ----------------------------

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EXHIBIT "B" to be made a part of a Sixth  Amendment  To Lease  between  LAFP-SF,
INC.  (Landlord),  and DATA TRANSMISSION NETWORK CORPORATION  (Tenant),  dated ,
1997 (Page 1 of 2)



                        TENANT IMPROVEMENTS WORK SCHEDULE
                        ---------------------------------



                                    ARTICLE I
                       Landlord's Construction Obligations
                       -----------------------------------

     Landlord  shall have no  construction  obligations  under  this  Amendment.
Tenant accepts the Additional Premises in an "as is" condition,  with all faults
and  with  the  understanding  that  it  shall  be  responsible  for any and all
improvements required for its occupancy and use in accordance with Article II of
this Exhibit "B".



                                   ARTICLE II
                       Construction of Tenant Improvements
                       -----------------------------------

     Tenant  shall  have the right to place  partitions  and  fixtures  and make
improvements or other alterations in the Additional Space in accordance with the
provisions of Paragraph 9 of the Lease.  Landlord  shall provide Tenant a tenant
finish allowance of up to Forty Eight Thousand, Six Hundred Dollars and Nineteen
Cents  ($45,440.00)  to be applied  toward the cost of any such  tenant-provided
improvements as follows:

     1. The tenant finish 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 Additional  Space,
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 invoices are  accompanied  by lien waivers in the amount equal to
that of the  invoices.  The  tenant  finish  allowance  shall be  allocated  and
distributed subject to the provisions of this Exhibit "B" as follows:

              August 1, 1997 - July 31, 1998       Up To $45,440.00

     2. Upon the earlier of the end date  identified in the allocation  schedule
specified  in  Paragraph  1  above,  or  the  satisfaction  of  all  obligations
associated  with the  tenant  improvements  covered  under  this  Article II and
receipt of the  associated  lien waivers for the work,  the Tenant shall forfeit
any unused  portion of the allowance.  Any requests for payment  received by the
Landlord after the above specified end dates, will be returned to the Tenant and
will be the obligation and sole responsibility of the Tenant.

     3. 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)  execute  a set  of and  comply  with  all  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;

         (c)  maintain such insurance  (such as general  liability and workman's
              compensation)  and bonds (such as performance  and  completion) in
              force and effect as may be reasonably  requested by Landlord or as
              required by  applicable  law (but in any event said bonds shall be
              in amounts  equal to the full value or cost of the work being done
              by the Tenant contractor);

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EXHIBIT "B" to be made a part of a Sixth  Amendment  To Lease  between  LAFP-SF,
INC.  (Landlord),  and DATA TRANSMISSION NETWORK CORPORATION  (Tenant),  dated ,
1997. (Page 2 of 2)

         (d)  be  responsible  for reaching an agreement  with  Landlord 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  prompt-
              ly 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 ten (10) 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 Tenant
              shall  reimburse  Landlord  upon  demand for the amount so paid by
              Landlord, including Landlord's expenses and attorney's fees.

     4. 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 satisfy the requirements of this Article II.


     5. 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
attorneys' 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.


     6. 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 .


     7. Upon  completion  of the Tenant  Improvements,  Tenant shall  deliver to
Landlord  two (2)  copies of the "as  built"  plans and  specifications  for the
Tenant  Improvements  completed  under Article II of this Exhibit  within thirty
(30) days of completing the same.

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