EXHIBIT 10.4.2

Confidential
August 5, 2002

MASTER AGREEMENT

     THIS AGREEMENT (“Agreement”) is entered into this 5th day of August, 2002,
by and between Level 3 Communications, Inc., a Delaware corporation and Level 3
Communications, LLC, a Delaware limited liability company (collectively, “Level
3”), on the one hand, and XO Communications, Inc., a Delaware corporation (“XO”)
and XO Intercity Holdings No. 2, LLC, a Delaware limited liability company
(“Grantee”), on the other hand.

     WHEREAS, Level 3 and Grantee have entered into various agreements which
they desire to amend as set forth in the Exhibits hereto.

     NOW, THEREFORE, in consideration of the premises, and the mutual conditions
and obligations set forth herein, the parties hereto hereby agree as follows
(capitalized terms used herein without definition having the meanings ascribed
to them in the Cost Sharing and IRU Agreement, dated as of July 18, 1998, as
amended (the “IRU Agreement”)):

  1. Transactions.

     Subject to the satisfaction or waiver of the conditions set forth below, at
the Closing each of Level 3, XO and Grantee shall execute and deliver to one
another the agreements in the forms attached hereto as Exhibits A, B, C and D.

  2. Closing.

     The closing of the transactions contemplated herein (the “Closing”) shall
occur no later than the third business day following satisfaction or waiver of
all of the conditions set forth in Section 4 (the “Closing Date”).

  3. Covenants.

     (a)  HSR Filing.

     The parties shall make the filings required under the Hart-Scott-Rodino
Antitrust Improvements Act of 1976, as amended (the “HSR Act”) with respect to
the transactions contemplated hereby and thereafter take all actions necessary
to resolve any investigation or other inquiry concerning such transactions
commenced by the Federal Trade Commission or the Antitrust Division of the
United States Department of Justice or any state attorneys general, in each case
to secure the required approvals without delay. XO and Grantee shall each pay
fifty percent (50%) of all filing fees associated with filing under the HSR Act.
Subject to the requirements of applicable law, each party will promptly furnish
to the other party such information and assistance as such other party may
reasonably request in connection with the preparation of such filings and
submissions, and will provide to the other party copies of all correspondence,
filings and written communications with the relevant governmental authorities in
connection with such filings and submissions.

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     (b)  Bankruptcy Court Approval

     Each of XO, Grantee and Level 3 agree to use good faith efforts to secure,
as soon as practicable after execution hereof, an order (the “Approval Order”),
in form and content satisfactory to XO and Level 3, of the bankruptcy court, in
the Chapter 11 bankruptcy proceeding now pending as Case No. 02-12947 (AJG) in
the United States Bankruptcy Court for the Southern District of New York, in
which XO is the debtor and debtor-in-possession, approving this Agreement and
the transactions contemplated hereby (including the Exhibits attached hereto),
after due and proper notice to all parties in interest and a hearing. In
soliciting the Approval Order, XO will provide testimony confirming the accuracy
of the representation contained in Section 5(e) hereof.

     (c)  Further Assurances

     Each of the parties shall execute such documents and other papers and take
such further actions as may be reasonably required or desirable to carry out the
provisions hereof and the transactions contemplated hereby. Each such party
shall use its commercially reasonable efforts to fulfill or obtain the
fulfillment of the conditions to the Closing as promptly as practicable
following the end of the period through which XO and Grantee have paid the
existing Recurring Charges under the IRU Agreement as provided in Section 6
below.

  4. Closing Conditions.

     The obligations of Level 3, XO and Grantee to complete the transactions
contemplated hereby on the Closing Date, as provided in Section 2 hereof, shall
be subject to the satisfaction, prior thereto or concurrently therewith, of the
following condition, subject to waiver by both parties in their sole discretion:

     (a)  Approval Order

     The Approval Order shall have been issued and remain in full force and
effect, and shall not be the subject to any pending appeal.

     (b)  Injunction, Violation of Law, etc.

     The consummation of the transactions contemplated by this Agreement shall
not be in violation of any law or regulation and not be subject to any
injunction or restraining order, and shall not be prohibited by any court order
or agreement binding upon XO, Grantee or Level 3.

     (c)  HSR Compliance

     Any applicable waiting period under the HSR Act shall have expired without
action taken to prevent the consummation of the transactions contemplated
herein.

     (d)  Compliance; No Breach or Misrepresentation

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     Each party shall have executed and delivered to the other party each
agreement attached as an Exhibit hereto and shall have complied with all the
provisions thereof to be performed upon such execution. There shall have been no
material breach of this Agreement or the occurrence of an event which, with the
passage of time or the giving of notice, would constitute a breach, and all of
the representations made by the parties in Section 5 shall be true and correct
as of the Closing Date in every material respect. Failure by XO and Grantee to
make each and every payment required by Section 6 of this Agreement on or before
the date such payment is due shall (without the giving of notice) constitute a
material breach of this Agreement by XO and Grantee.

  5. Representations.

     Each party represents and warrants to the other as follows:

     (a)  Each party has full power and authority to execute, deliver and
perform its obligations under this Agreement.

     (b)  The execution, delivery and performance of this Agreement by each
party has been duly and validly authorized by all necessary action on the part
of each party, which authorization shall include any affiliates of the parties
responsible for performing under this Agreement.

     (c)  This Agreement is a legal, valid and binding obligation of each party,
enforceable against each party in accordance with its terms.

     (d)  The execution and delivery of this Agreement by each party and the
performance by such party and its affiliates of the terms, covenants and
conditions contained herein will not violate the articles of incorporation or
by-laws of each party or affiliates, or any order of a court or arbitrator, and
will not conflict with and will not constitute a material breach of, or default
under, the provisions of any material contract by which either party and its
affiliates are bound.

     (e)  XO and Grantee hereby represent and warrant to Level 3 that the only
creditors of Grantee as of the date hereof are, and as of the Closing Date shall
be, Level 3, XO (and certain of its subsidiaries) and the lenders party to XO’s
secured credit facilities.

  6. Payments and Actions Pending Closing.

     Simultaneously with the execution hereof, XO and Grantee have paid to Level
3, by wire transfer of immediately available funds, twelve million six hundred
sixty one thousand dollars (U.S. $12,661,000.001), the receipt of which is
hereby acknowledged by Level 3. XO and Grantee further agree to pay Level 3 four
million three hundred thirty nine thousand dollars (U.S. $4,339,000.00), which
shall be due and payable (without demand) in accordance with the following
schedule:

          Date   Payment Due

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September 30, 2002
  $ 1,580,000.00  

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October 30, 2002
  $ 1,580,000.00  
November 30, 2002
  $ 1,179,000.00  

The foregoing payments: (a) shall be made by XO and Grantee in cash, and no
credits granted or otherwise available to XO or Grantee may be used to pay the
foregoing amounts; (b) shall be nonrefundable; and (c) shall not be affected by
or subject to any set off, counterclaim, defense or abatement whatsoever. In the
event that Closing occurs, the foregoing payments (if and only if full payment
is made on or before the dates set forth above) shall constitute payment in full
of the Recurring Charge, relocation expenses, unscheduled maintenance and repair
costs, Impositions, interest, penalties or other charges due and payable by
Grantee under the IRU Agreement (and by XO under and pursuant to that certain
Amendment of Guaranty Agreement executed on or about May 9, 2000) as of the date
hereof and through January 1, 2003. In the event that this Agreement is
terminated pursuant to Section 7 hereof prior to the Closing, the payments made
by XO and Grantee under this Section 6 shall be retained by Level 3 and applied
against amounts otherwise due and owing under the IRU Agreement. In addition,
each party shall immediately give effect to the credits and other payment
concessions and terms contained in Exhibits B and D attached hereto as if the
Closing had occurred (except that the credits granted to XO shall not be used to
pay amounts due from XO and Grantee as set forth above). In the event that this
Agreement is terminated pursuant to Section 7 hereof prior to the Closing, then
all payments that would have otherwise been due and owing from one party to the
other which were not made as a result of application of credits granted pursuant
to Exhibits B and D shall be made within 10 days following such termination (so
that the parties’ respective rights and obligations are governed in accordance
with various agreements executed prior to the execution of this Agreement). In
the event that such payment is not made, each party shall be entitled to offset
from its payment obligations hereunder any amounts due and owing from the other
party. In addition, prior to the Closing or termination of this Agreement, no
party shall take any action with respect to any default or other violation of
any agreement being amended in an Exhibit hereto if such default or other
violation would be cured, released or otherwise become discharged if the Closing
occurs.

  7. Termination.

     This Agreement may be terminated:

     (a)  by either party if the Approval Order has not been issued by October
31, 2002; or

     (b)  by either party if the Closing has not occurred on or prior to
February 28, 2003; or

     (c) by the non-defaulting party if the other party materially defaults in
the performance of its obligations hereunder, or if there is a material
misrepresentation with respect to the matters contained in Section 5 above, and
such defaulting party fails to cure such material default within (i) three (3)
days following written notice of default from Level 3 in the event of a breach
of XO’s and Grantee’s payment obligations under Section 6 hereof, or (ii) thirty
(30) days following written notice thereof by the other party for all other
breaches.

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In any such case if the Closing has not occurred as a result of a party’s
failure to perform its obligations under this Agreement, such party shall not
have the right to terminate this Agreement.

  8. Legal Matters.

     (a)  Neither party may assign this Agreement without the prior written
approval of the other party, it being understood that assumption of this
Agreement as part of the plan approved in XO’s bankruptcy proceeding shall not
be deemed an assignment thereof. After the Closing Date, this Agreement, and
each of the Exhibits attached hereto, shall be binding and enforceable on all
parties in interest, including without limitation any chapter 7 trustee in the
event of a conversion of XO’s Chapter 11 bankruptcy proceeding to a Chapter 7
bankruptcy proceeding.

     (b)  No waiver by either party of any right or remedy under this Agreement
shall be deemed to be a waiver of any other or subsequent right or remedy under
this Agreement. The consent by one party to any act by the other party requiring
such consent as provided by this Agreement shall not be deemed to render
unnecessary the obtaining of consent to any subsequent act for which consent is
required as provided by this Agreement, regardless of whether similar to the act
for which consent is given.

     (c)  The laws of the State of New York (without regard to rules governing
conflicts of law) shall govern the enforcement of this Agreement. Each party
hereby irrevocably waives the right to trial by jury in any proceeding to
enforce this Agreement.

     (d)  This Agreement constitutes the entire agreement of the parties with
respect to the subject matter hereof.

     (e)  This Agreement may be executed by each of the parties hereto in any
number of counterparts, each of which counterpart, when so executed and
delivered, shall be deemed to be an original and all such counterparts shall
together constitute one and the same agreement. Facsimile signatures shall be
sufficient to bind the parties hereto.

     (f)  Neither party shall be liable to the other for any incidental,
consequential or punitive damages (including but not limited to any alleged loss
of revenue or profits, cost of capital, business interruption, cost of
replacement services, claims of customers for service interruptions or
transmission problems or loss of opportunity) whether foreseeable or not, that
may result from or be related to any performance or breach of this Agreement.

     (g)  The obligations of the parties hereto shall be joint and several.

     (h)  If the Approval Order is obtained, nothing herein shall prevent a
party from commencing a separate action or actions for any breach of this
Agreement, and the bankruptcy court having jurisdiction over XO’s Chapter 11
proceedings shall retain jurisdiction to hear any such action or actions.

     (i)  The parties recognize that in the event any party hereto shall refuse
to perform its obligations under this Agreement, monetary damages will not be
adequate. The

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performing party shall therefore be entitled, in addition to any other remedies
which may be available, to obtain specific performance of the terms of this
Agreement by each non-performing party. In the event of any action to enforce
this Agreement, the non-performing party hereby waives the defense that there is
an adequate remedy at law. In the event a dispute arises as to the parties’
rights and obligations under this Agreement, the party who substantially
prevails in such dispute shall be entitled to be paid immediately by the
non-prevailing party the reasonable attorneys’ fees and costs incurred by the
prevailing party in connection with the adjudication or other resolution of such
dispute.

  9. Public Announcements.

     The parties hereto must agree to any public announcement made by either
relating to this Agreement, including the language thereof and the date upon
which such public announcement shall be made.

  10. Additional Documents.

     The parties agree to meet, confer, negotiate and execute any additional
documents that are reasonably required in order to effectuate the transactions
set forth in this Agreement. The parties further agree that the provisions of
this Agreement relate to agreements between various wholly owned direct and
indirect subsidiaries of each of Level 3 Communications, Inc. and XO
Communications, Inc. Each of Level 3 Communications, Inc. and XO Communications,
Inc. shall take such steps and execute such documents, and cause each and all of
its wholly-owned direct and indirect subsidiaries to take such steps and execute
such documents and agreements, as are necessary to effectuate the transactions
and commitments contained in this Agreement.

      LEVEL 3 COMMUNICATIONS, INC   XO COMMUNICATIONS, INC.   By: /s/ Thomas
Stortz                                    By: /s/ William Garrahan      
                            Title: Group Vice President                        
  Title:Vice President                                      LEVEL 3
COMMUNICATIONS, LLC   XO INTERCITY HOLDINGS NO. 2, LLC   By: /s/ John
Scarano                                           By: /s/ William
Garrahan                                   Title: Vice President          
                            Title: Vice President                        
         

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EXHIBIT A TO MASTER AGREEMENT

THIRD AMENDMENT TO COST SHARING AND IRU AGREEMENT

     This THIRD AMENDMENT TO COST SHARING AND IRU AGREEMENT (the “Amendment”) is
entered into this ______day of ______, 200_, and modifies that certain Cost
Sharing and IRU Agreement (the “IRU Agreement”) dated July 18, 1998 between
Level 3 Communications, LLC (“Grantor”) and XO Intercity Holdings No. 2, LLC
(“Grantee”) (formerly “Internext LLC”), as amended by the Second Amendment to
Cost Sharing and IRU Agreement dated June 28, 2001. Capitalized terms used but
not defined herein shall have the meanings assigned to them in the IRU
Agreement.

     Grantee and Grantor desire to amend the IRU Agreement so as to (a) reduce
Grantee’s obligation to pay the Recurring Charges otherwise due under the IRU
Agreement, and (b) eliminate the IRU (or any other rights, including ownership
rights) that Grantee has or may have in and to the Grantee Conduit and certain
of the Grantee Fibers conveyed to it under the IRU Agreement. In the event of
any inconsistency between the terms of this Amendment and the terms of the IRU
Agreement, the terms of this Amendment shall control.

     1.   Reduction in Recurring Charge. From and after January 1, 2003, the
Recurring Charge shall be five million dollars ($5,000,000) per year, due and
payable in equal installments on the first day of each month remaining in the
Term. The first payment of the Recurring Charge after the Effective Date shall
be due and payable on January 1, 2003, and shall be $416,666.66. The Recurring
Charge (as modified) shall no longer be subject to adjustment as provided in
sections 13.04, 13.05, 13.06 or 13.07, all of which sections are hereby deleted.
It is the intention of the parties that the Recurring Charge as stated above
shall be a fixed amount and any and all provisions of the IRU Agreement
indicating that the Recurring Charge is based on actual costs incurred by
Grantor are hereby deleted as of January 1, 2003, except that following the
expiration or termination of the Term of the IRU as to any Segment, the
Recurring Charge shall be decreased by $17.50 for each fiber mile represented by
Grantee Fibers in such Segment.

     2.   Relocations, Restorations and Impositions. From and after January 1,
2003, Grantee shall no longer be liable for the payment to Grantor of Grantee’s
proportionate share of relocation expenses (as provided for in Section 6.05 of
the Agreement), unscheduled maintenance and repair costs (as provided for in
Section 12.01 of the Agreement) and Impositions (as provided for in Section
14.03 and 14.04 of the Agreement), all of which shall be paid by Grantor.

     3.   Surrender of Grantee Conduit. Grantee hereby surrenders as of the date
hereof the IRU in and to, and any and all other right, title and interest it may
have in, all of the Grantee Conduit (except for the portion of Grantee Conduit
described in the next following sentence). Grantee shall retain the IRU in and
to, and its rights under the Agreement with respect to, the Grantee Conduit
along approximately 6.48 miles from the “zero manhole” outside of Grantor’s
facilities located at 1 Aerojet Way, North Las Vegas, NV to manhole number
0007-5869 located

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at or near the intersection of the Grantor System and Bonneville Road in Las
Vegas, NV, as depicted in Exhibit A attached hereto. Grantee covenants and
agrees that Grantee will perform any and all acts necessary or reasonably
requested by Grantor to put Grantor in physical possession of the Grantee
Conduit surrendered pursuant hereto.

     4.   Surrender of Grantee Fibers. Grantee hereby surrenders as of the date
hereof the IRU in and to, and any and all other right, title and interest it may
have in, six (6) Grantee Fibers (the “Returned Fibers”) within and along each
Segment of the Grantor System where Grantee had previously purchased an IRU in
Grantee Fibers pursuant to the Agreement. Grantee shall retain an IRU in
eighteen (18) Grantee Fibers within and along each Segment. Grantee covenants
and agrees that Grantee will perform any and all acts necessary or reasonably
requested by Grantor to put Grantor in physical possession of the Returned
Fibers. Grantor’s obligation under Section 10.06 of the Agreement to deliver to
Grantee dedicated space and amenities in Regeneration Facilities and Opamp
Facilities within and along such Segment(s) shall be limited to providing the
amount of space and amenities required to support the remaining eighteen (18)
Grantee Fibers (pursuant to Section 10.06 of the IRU Agreement).

     5.   Grantor Retention of IRU Contribution. Notwithstanding the surrender
of Grantee Conduit and Returned Fibers accomplished herein, no portion of the
IRU Contribution paid by Grantee under the IRU Agreement shall be refunded to
Grantee, and Grantor shall be entitled to retain all of the IRU Contribution and
other sums, costs, fees or expenses paid by Grantee through the date hereof. In
consideration in part for the surrenders referred to in paragraphs 3 and 4
hereof, Grantor acknowledges that Grantee has fully prepaid all costs and
expenses relating to the Grantee Fibers of the nature described in clause (iv)
of Section 13.01 and all relocation costs.

     6.   Tag-Along Rights. Grantor hereby grants Grantee a one-time tag-along
right (“Tag-Along Right”) on fiber installed within and along each Segment or
portion of a Segment of the Grantor System where Grantee has purchased an IRU in
Grantee Fibers pursuant to the Agreement. The Tag-Along Right shall apply only
with respect to the next conduit within which Grantor installs fiber within and
along each Segment, and shall be subject to the following:

     A.   Grantor shall give Grantee no less than sixty (60) days prior written
notice of its intention to install a fiber optic cable in such conduit within
and along any Segment or portion of a Segment, which notice shall contain
Grantor’s good faith estimate of the Incremental Costs which will be payable by
Grantee if Grantee exercises the Tag-Along Right with respect to such Segment or
portion of a Segment. If Grantor installs fiber in one or more conduits
contemporaneously under circumstances where one of such conduits is subject to
the Tag-Along Right, Grantee may select which conduit is subject to the
Tag-Along Right hereunder.

     B.   Grantee may exercise the Tag-Along Right by delivering written notice
thereof to Grantor no less than thirty (30) days after delivery of Grantor’s
notice delivered under paragraph A above. The Tag-Along Right may be exercised
with respect to all of the Segment(s) or portions of Segment(s) which are
covered by Grantor’s written notice specified in paragraph A above. If the
Tag-Along Right is exercised, the additional fibers within which Grantee
acquires an IRU shall be “Additional Grantee Fibers” for purposes of the IRU
Agreement.

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     C.   The Tag-Along Right shall entitle Grantee to acquire an IRU in no more
than twenty five percent (25%) of the fiber actually installed by Grantor within
a conduit.

     D.   If Grantee exercises its Tag-Along Right hereunder, Grantee shall
reimburse Grantor for the Incremental Costs associated with the Additional
Grantee Fibers installed for Grantee as a result thereof. In addition, upon
completion of installation and testing of such Additional Grantee Fibers, the
annual Recurring Charge shall be increased by $17.50 per fiber per mile for each
Additional Grantee Fiber. The Term of the IRU respecting the Additional Grantee
Fibers shall be the remainder of the Term of Initial Grantee Fibers within the
same Segment in which such Additional Grantee Fibers are located.

     The Tag-Along Right shall only be applicable to the next conduit along each
Segment within which Grantor installs fiber after the date hereof. In the event
Grantee shall fail to timely exercise its Tag-Along Right with respect to a
Segment or portion of a Segment, or if Grantee notifies Grantor in writing of
its desire to surrender such right with respect to any Segment or portion of a
Segment, the Tag-Along Right with respect to such Segment (or portion thereof)
shall terminate. Once Grantee exercises, disclaims or fails to timely exercise
its Tag-Along Right with respect to a portion of a Segment, Grantee shall have
no further Tag-Along Right respecting other conduits within such portion of such
Segment, and once Grantee exercises, disclaims or fails to timely exercise its
Tag-Along Right with respect to an entire Segment, Grantee shall have no further
Tag-Along Right respecting other conduits within such Segment.

     7.   Option on Additional Conduit. For a period commencing upon execution
hereof and ending on July 31, 2007, Grantee shall have the option, subject at
all times to the conditions and provisions set forth below, on one or more
occasions to acquire a 20-year IRU in one (but only one) additional conduit
within and along any Segment or portion of a Segment of the Grantor System
(which additional conduit, when and if acquired, shall be “Grantee Conduit”
under the Agreement), provided that:

     A.   The IRU Contribution due and payable from Grantee to Grantor shall be
$15.00 per conduit foot, twenty five percent (25%) of which shall be due and
payable on the date when Grantee delivers written notice to Grantor exercising
the option granted herein, and the balance of which shall be due and payable no
later than fifteen (15) days after Grantor has completed installation and
testing of fiber within the Grantee Conduit. The IRU shall not commence, and
Grantee shall have no right to use the Grantee Conduit or the fibers installed
therein, until the full IRU Contribution has been paid by Grantee.

     B.   The Recurring Charge due and payable from Grantee to Grantor shall be
$25.00 per fiber mile per year (based on the total fiber count requested by
Grantee, less any fibers retained by Grantor under subparagraph (F) below), the
first monthly installment of which shall be due and payable no later than
fifteen (15) days after Grantor has completed installation and testing of fiber
within the Grantee Conduit.

     C.   The amounts payable by Grantee for the IRU Contribution and the
Recurring Charge shall be subject to a minimum of one mile (1) for each
contiguous portion of Grantee

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Conduit subject to this Section (so that, if Grantee purchased two portions of
non-contiguous Grantee Conduit of .5 mile each, the IRU Contribution and
Recurring Charges payable by Grantee for such portions would be based on two
(2) miles). The IRU Contribution and Recurring Charge fee due and payable from
Grantee shall be calculated from the locations of the interconnection points
requested by Grantee (provided that such interconnection points are reasonably
technically feasible); without regard to the location of the actual
interconnection points determined by Grantor.

     D.   Grantor shall be responsible for the procurement, installation and
testing of the fiber optic cable to be installed within the Grantee Conduit, and
Grantee shall reimburse Grantor for all Costs incurred by Grantor in connection
therewith, plus fifteen percent (15%). If Grantor does not intend to retain
fiber under subparagraph (f) below or in the event that Grantor desires to
retain fiber and requires the same fiber type as Grantee intends to use, then
Grantee may supply the fiber and Grantor shall use such fiber for installation
into the Grantee Conduit. If Grantor intends to retain fiber but requires a
different fiber type as Grantee intends to use, then Grantor shall procure the
fiber cable containing fibers meeting the requirements of each of Grantee and
Grantor, and Grantee and Grantor shall share in the costs thereof on a pro rated
basis based on fiber count. Prior to commencement of installation of such fiber,
Grantor shall deliver to Grantee a good faith estimate of the Costs (plus the
fifteen percent fee) that Grantor anticipates will be incurred in connection
with the installation, and Grantee shall pay such amount to Grantor prior to
commencement of installation of the fiber cable. The parties shall true-up the
amounts owed after completion of installation and full calculation of the Costs
incurred by Grantor in connection therewith. Grantee shall not be permitted to
pay amounts due and owing under this subparagraph through application of credits
granted to Grantee or its affiliates under and pursuant to that certain
Amendment to Workout Agreement dated of even date herewith.

     E.   Grantee shall have no option to acquire an IRU in Grantee Conduit
under this Section where the contiguous length of the Grantee Conduit is greater
than twenty (20) miles, and Grantee shall be permitted to obtain an IRU in no
greater than fifty (50) sections of the Grantor System during the life of the
option granted hereunder.

     F.   Grantor shall be permitted to retain title in and to up to fifty
percent (50%) of the fibers designated for installation by Grantee within the
Grantee Conduit, provided that (a) the IRU Contribution otherwise due and
payable from Grantee shall be reduced by a percentage equal to the percentage of
fibers retained by Grantor (and if Grantee has provided the fiber optic cable
pursuant to subparagraph (D) above, Grantor shall reimburse Grantee for such
percentage of the costs of such cable); and (b) the amounts due and payable by
Grantee for installation of the fiber shall be reduced by a percentage equal to
the percentage of fibers retained by Grantor (so that, if Grantor retained 40%
of the total fibers within the Grantee Conduit, the IRU Contribution would be
reduced by 40%). Grantor shall be responsible for determining, in its sole
discretion, the location of interconnection points and the manner of
interconnection to Grantee’s fiber or systems, provided that Grantor shall use
reasonable commercial efforts to accommodate Grantee’s requests as to location.
Grantor shall not be obligated to deliver the Grantee Conduit to Grantee
hereunder if Grantor has determined, in its reasonable judgment, that it is not
technically feasible or advisable to deliver such Grantee Conduit to Grantee.

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     G.   Notwithstanding the foregoing, Grantee shall have no option to acquire
an IRU in Grantee Conduit under this Section in any Segment or portion of a
Segment where Grantor has four (4) or fewer spare conduits within the Grantor
System.

     H.   The option granted hereunder may be exercised by Grantee by delivering
written notice thereof to Grantor, which written notice shall include a request
for installation of fiber (specifying the type of fiber to be installed and the
number of strands requested by Grantee) and payment of twenty five percent (25%)
of the IRU Contribution for the requested Grantee Conduit (calculated in
accordance with subparagraphs (A) and (C) above).

     I.   At any time after Grantee obtains an IRU in Grantee Conduit pursuant
to this section 7, it may elect upon not less than 30 days prior written notice
to Grantor to terminate such IRU, whereupon the Term shall expire with respect
to such Grantee Fibers and/or Grantee Conduit in such Segment and all title to
and/or rights to the use thereof shall revert to Grantor without reimbursement
of any of the IRU Contribution or other sums, costs, fees or expenses previously
made with respect thereto, and from and after such time Grantee shall have no
further rights, obligations or liabilities hereunder or any other liability with
respect thereto. Notwithstanding the foregoing, Grantee shall continue to
reimburse Grantor for any a pro rated share (based on fiber count) of the actual
and direct costs and expenses paid to providers of right-of-way resulting from
the installation of fiber within the surrendered Grantee Conduit; in no event,
however, shall such reimbursement exceed the Recurring Charge that would have
otherwise been paid by Grantee for the surrendered Grantee Conduit.

     8.   Term and Ratification. The Term shall be unaffected by this Amendment.
Save as specifically modified by this Amendment, all terms of the IRU Agreement
are hereby ratified and confirmed.

      LEVEL 3 COMMUNICATIONS, LLC   XO INTERCITY HOLDINGS NO. 2, LLC  
By:                                                               By:          
                                                        Title:          
                                                 Title:          
                                                  

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August 5, 2002

Exhibit A to Third Amendment to Cost Sharing and IRU Agreement
Depiction of Retained Grantee Conduit

[w82714w82714a.gif]

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EXHIBIT B TO MASTER AGREEMENT

FIRST AMENDMENT TO

NATIONAL MASTER COMMUNICATIONS SERVICES AGREEMENT

     This FIRST AMENDMENT TO NATIONAL MASTER COMMUNICATIONS SERVICES AGREEMENT
(the “Amendment”) is entered into this ______day of ______, 200_ (“Amendment
Effective Date”), by and between Level 3 Communications, LLC, a Delaware limited
liability company (“Customer”) and XO Communications, Inc. on behalf of its
operating subsidiaries and affiliates, a Delaware corporation (formerly known as
“Nextlink Communications, Inc.” and hereinafter referred to as “Service
Provider”). This Amendment amends and modifies that certain National Master
Communications Services Agreement (the “Agreement”) between Customer and Service
Provider dated as of April 16, 1999. Capitalized terms used but not defined
herein shall have the same meanings as set forth in the Agreement.

     1.   Term and Renewal Options. Section 15.1 of the Agreement is hereby
modified so that the term of the Agreement shall be extended through June 1,
2005.

     2.   Taxes. Section 17.1 of the Agreement is hereby amended by deleting the
provision in its entirety and replacing it with the following:

  “The charges for Services are net of any applicable taxes and surcharges.
Customer shall be responsible for and shall pay any taxes, arising in any
jurisdiction, including without limitation, sales, use, excise, gross receipts,
value added, access, bypass, franchise, telecommunications, consumption, or
other taxes, fees, duties, charges or surcharges; however, designated, imposed
on or based on the provision, sale or use of the Services, including taxes
imposed directly on Service Provider. In the event Customer believes it or the
Services is exempt from any tax, Customer will provide Service Provider with a
properly executed exemption certificate evidencing such claimed exemption. In no
case shall Customer be responsible for any income taxes levied upon Services
Provider’s or any underlying carrier’s net income. Customer agrees that its
obligation to pay taxes under this Section shall survive the expiration or
earlier termination of the Agreement Term.”

     3.   Regulations. Section 18.1 of the Agreement is hereby amended by
deleting the provision in its entirety and replacing it with the following:

  “Each Party represents that it is not aware of any facts that would justify a
complaint to the Federal Communications Commission or any state regulatory
authority concerning the prices, terms or conditions of the transactions
contemplated by this Agreement. The Parties also agree that in the event of a
decision by a telecommunications regulatory authority at the federal, state or
local level that necessitates modifications in this Agreement, including
modifications to the pricing or compensation issues associated with the
provision of the

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  Services provided hereunder, the Parties will negotiate in good faith to
modify this Agreement in light of such decision (provided, however, that neither
Party shall be obligated to agree to any such modifications). If the Parties are
unable to agree on such modifications to this Agreement and (in the absence of
such modifications) the Service Provider determines that it would be technically
or economically infeasible for Service Provider to continue to provide service
hereunder, Service Provider may, upon ninety (90) days’ written notice,
terminate this Agreement without penalty.”

     4.   Services. The Parties agree that the Customer may only procure point
to point private line services under the terms of this Agreement. The Agreement
may be amended by mutual agreement between the Parties in accordance with the
terms herein in order to incorporate other Service Provider product/service
offerings during the term of this Agreement.

     5.   Notices. Section 21.1 of the Agreement is hereby amended by changing
the Service Provider’s address for notification purposes to the following:

  XO Communications, Inc. 11111 Sunset Hills Road Reston, VA 20190 Attn:
Assistant General Counsel

     6.   Credit. Customer is hereby granted a credit in the amount of two
million five hundred thousand dollars (U.S. $2,500,000.00) (the “Credit”) which
may be used by Customer to pay any invoice for Services ordered under the
Agreement (as amended) which are presently due and owing or hereafter delivered
(or, at Customer’s option, any other amounts due and owing to Service Provider
(or any of its affiliates or subsidiaries) under the Agreement or otherwise).
The Credit may be applied to any invoices for Services delivered under the
Agreement (regardless of whether such Services were ordered prior to or after
the date of execution hereof). All invoices for Services shall show the usage of
the Credit and the amount of the Credit remaining (Customer shall retain the
same rights as it has under the Agreement to dispute any invoiced amounts).

     6.   Ratification. Except as amended by this Amendment, the original terms
and provisions of the Agreement shall continue in full force and effect and the
Agreement, as amended by this Amendment, is hereby ratified and confirmed.

     7.   Entire Agreement/Amendment. This Amendment constitutes the entire and
final agreement and understanding between the Parties with respect to the
subject matter hereof and supersedes all prior agreements relating to the
subject matter hereof, which are of no further force or effect. The Agreement,
including this Amendment, may only be modified or supplemented by an instrument
in writing executed by a duly authorized representative of each Party. In no
event shall this Agreement, including this Amendment, be modified or amended in
anyway by
e-mail.

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IN WITNESS WHEREOF, the Parties hereto have caused this Agreement to be executed
by their duly authorized representatives as of the Amendment Effective Date.

      LEVEL 3 COMMUNICATIONS, LLC   XO COMMUNICATIONS, INC.   BY:          
                                                       BY:             
                                                        TITLE:          
                                                 TITLE:          
                                                  

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EXHIBIT C TO MASTER AGREEMENT

THIRD AMENDMENT
MASTER WAVELENGTHS AGREEMENT

     THIS THIRD AMENDMENT TO MASTER WAVELENGTHS AGREEMENT (“Amendment”) is made
and entered into as of the ______day of ______, 200_, by and between LEVEL 3
COMMUNICATIONS, LLC, a Delaware limited liability company (“Level 3”) and XO
COMMUNICATIONS, INC., a Delaware corporation (“Customer”). This Amendment
modifies and amends that certain Master Wavelengths Agreement dated April 25,
2001 between Level 3 and Customer (as amended, the “Agreement”). Capitalized
terms used but not defined herein shall have the meanings set forth in the
Agreement.

ARTICLE 1

PO&M Charge

1.   Section 9.4 of the Agreement is hereby amended to read as follows:

       9.4    Power, Operations and Maintenance.

          (a) For any Route of Capacity provided pursuant to an IRU, Customer
shall pay to Level 3 commencing with the Acceptance Date of each Route and
continuing until the expiration of the Capacity Term with respect to such Route,
an annual charge (the “PO&M Charge”) equal to 1% of the IRU Route Price for each
U.S. Route of Capacity and 2.5% of the IRU Route Price for each European Route
of Capacity. The PO&M Charge is intended to cover the expenses of supplying
power, operations and maintenance services necessary to continue effective
availability of the Capacity for the Term.

          (b) The PO&M Charge shall be invoiced annually and shall be due and
payable by Customer on May 1 of each year for each and every Route of Capacity
in existence as of the date of invoice. The PO&M Charge for any new Route of
Capacity ordered hereunder shall be due on the Acceptance Date thereof and shall
be prorated to the next following April 30. In the event that the IRU Route
Price payable by Customer hereunder increases for any Route of Capacity, the
additional PO&M Charge related to such increase shall be due and payable on the
date such additional IRU Route Price is payable by Customer and shall be
prorated to the next following April 30.

2.   As of the date of this Amendment, the amount of the PO&M Charge due from
Customer for all Routes of Capacity in existence as of May 1, 2002 is $869,480
(which includes all PO&M Charges due for existing Routes of Capacity through
April 30, 2003). This amount has been paid

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by Customer upon execution of this Amendment as a result of the application
against the credit pursuant to Section 3 below.

3.   Pursuant to an agreement between the parties dated April 25, 2001, Level 3
purchased certain transmission gear owned by XO and in payment therefor provided
XO with a credit which could be used by XO to purchase of wavelength services
provided by Level 3 under the Agreement. The parties have subsequently had
discussions and negotiations regarding the amount of the credit granted and the
balance remaining with respect to the credit. The parties hereby agree that, as
a result of payment in full of the amounts listed in Section 2 above, and
payment in full of all of the invoices listed in Exhibit A (but only to the
extent that such invoices cover wavelength PO&M Charges and the IRU Route Price
due under the Agreement) attached hereto, a the balance of the credit remaining
to XO as of July 30, 2002 is $1,581,333.

4.   This Amendment may be executed in one or more counterparts, all of which
taken together shall constitute one and the same instrument.

5.   Except as amended by this Amendment, the original terms and provisions of
the Agreement shall continue in full force and effect and the Agreement, as
amended by this Amendment, is hereby ratified and confirmed.

6.   This Amendment constitutes the entire and final agreement and understanding
between the parties with respect to the subject matter hereof and supersedes all
prior agreements relating to the subject matter hereof, which are of no further
force or effect. The Agreement, including this Amendment, may only be modified
or supplemented by an instrument in writing executed by a duly authorized
representative of each party. In no event shall the Agreement, including this
Amendment, be modified or amended in anyway by e-mail.

IN WITNESS WHEREOF, Level 3 and Customer have executed this Amendment as of the
date first above written.

  LEVEL 3 COMMUNICATIONS, LLC,
A Delaware limited liability company (“Level 3”)   By            
                                                  Title:            
                                              XO COMMUNICATIONS, INC.,
A Delaware corporation   By                                   
                           Title:                               
                        

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Exhibit A to Third Amendment to Master Wavelengths Agreement
Wavelength IRU Route Price and PO&M Charges Invoices Paid In Full

                  Circuit #   Description / Segment A End   Description /
Segment Z End   Invoice #

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21   Atlanta — L3Gw   Nashville — SYN     533130   22   Atlanta — L3Gw  
Nashville — SYN     533130   47   Atlanta — L3Gw   Orlando — L3Gw     533130  
48   Atlanta — L3Gw   Orlando — L3Gw     590942   51   Austin – SYN   San
Antonio — SYN     486972   52   Austin — SYN+   San Antonio — SYN     486972   3
  Baltimore — L3Gw   Philadelphia — L3Gw     486972   9   Boston — L3Gw  
Cleveland — SYN     434706   10   Boston — L3Gw   Cleveland — SYN     434706  
103 (17/19)   Chicago   Nashville     590942 (434706) 104 (18/20)   Chicago  
Nashville     590942 (434706) 81   Chicago — L3Gw   Cincinnati — L3Gw     434706
  82   Chicago — L3Gw   Cincinnati — L3Gw     486972   25   Chicago — L3Gw   St.
Louis — L3Gw     486972   26   Chicago — L3Gw   St. Louis — L3Gw     486972   83
  Cincinnati — L3Gw   Nashville — SYN     434706   84   Cincinnati — L3Gw  
Nashville — SYN     434706   13   Cleveland – SYN   Detroit — L3Gw     486972  
14   Cleveland — SYN (b)   Detroit — L3Gw     533130   79   Cleveland – SYN  
Washington D.C. L3GW     486972   80   Cleveland – SYN   Washington D.C. L3GW  
  486972   11   Cleveland (b)   Washington D.C     533130   12   Cleveland (b)  
Washington D.C     533130   89   Dallas — L3Gw   Houston — L3Gw     486972   90
  Dallas — L3Gw   Houston — L3Gw     590942   39   Dallas — L3Gw   Houston —
L3Gw     486972   40   Dallas — L3Gw   Houston — L3Gw     486972   35   Dallas —
L3Gw   Memphis — SYN     486972   36   Dallas — L3Gw   Memphis — SYN     486972
  49   Dallas (b)   Austin     533130   50   Dallas (b)   Austin     533130   29
  Denver — L3Gw   Omaha — SYN     533130   30   Denver — L3Gw   Omaha — SYN    
533130   15   Detroit — L3Gw   Chicago — L3Gw     486972   16   Detroit — L3Gw  
Chicago — L3Gw     533130   93   Las Vegas – SYN   Salt Lake City — SYN    
533130   94   Las Vegas – SYN   Salt Lake City — SYN     533130   95   Los
Angeles — L3Gw   Las Vegas — SYN     533130   96   Los Angeles — L3Gw   Las
Vegas — SYN     533130   77   Los Angeles — L3Gw   Sacramento — SYN     486972  
78   Los Angeles — L3Gw   Sacramento — SYN     486972   59   Los Angeles   San
Diego     878541  

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                  60   Los Angeles   San Diego     878541   75   Los Angeles —
L3Gw   San Jose — L3Gw     533130   76   Los Angeles — L3Gw   San Jose — L3Gw  
  533130   37   Memphis – SYN   Nashville — SYN     434706   38   Memphis – SYN
  Nashville — SYN     434706   41/43   Miami — L3Gw   Houston — L3Gw     533130
  42/44   Miami — L3Gw   Houston — L3Gw     533130   45   Miami — L3Gw   Orlando
— L3Gw     397405   46   Miami — L3Gw   Orlando — L3Gw     397405   85  
Nashville – SYN   Atlanta — L3Gw     533130   86   Nashville – SYN   Atlanta —
L3Gw     533130   87   Nashville – SYN   Dallas — L3Gw     486972   88  
Nashville – SYN   Dallas — L3Gw     486972   7   New York City — L3Gw — 111 8th
Ave   Boston — L3Gw     486972   8   New York City — L3Gw — 111 8th Ave   Boston
— L3Gw     486972   27   Omaha — SYN   St. Louis — L3Gw     486972   28   Omaha
— SYN   St. Louis — L3Gw     533130   4   Philadelphia — L3Gw   Baltimore — L3Gw
    486972   5   Philadelphia — L3Gw   New York City — L3Gw — 111 8th Ave    
486972   6   Philadelphia — L3Gw   New York City — L3Gw — 111 8th Ave     486972
  63   Phoenix — L3Gw   Stratford — RL     434706   64   Phoenix — L3Gw  
Stratford — RL     434706   69   Portland — SYN   Sacramento — SYN     486972  
70   Portland — SYN   Sacramento — SYN     486972   55   Salt Lake City — SYN  
Denver — L3Gw     533130   56   Salt Lake City — SYN (b)   Denver — L3Gw    
533130   57   Salt Lake City — SYN   Los Angeles — L3Gw     590942   58   Salt
Lake City — SYN   Los Angeles — L3Gw     533130   53   San Antonio — SYN  
Houston — L3Gw     486972   54   San Antonio — SYN   Houston — L3Gw     486972  
61   San Diego — L3Gw   Phoenix — L3Gw     486972   62   San Diego — L3Gw  
Phoenix — L3Gw     486972   105   San Francisco — L3Gw   Sacramento — SYN    
533130   106   San Francisco — L3Gw   Sacramento — SYN     533130   101   San
Francisco   Sacramento     878541/975563   102   San Francisco   Sacramento    
878541/975563   97   San Jose — L3Gw   Los Angeles — L3Gw     533130   98   San
Jose — L3Gw   Los Angeles — L3Gw     533130   65   Seattle — L3Gw   Salt Lake
City — SYN     486972   66   Seattle — L3Gw+   Salt Lake City — SYN     486972  
67   Seattle (b)   Portland     533130   68   Seattle (b)   Portland     533130
  33   Stratford — RL   Dallas — L3Gw     486972   34   Stratford — RL   Dallas
— L3Gw     486972   31   Stratford — RL   Denver — L3Gw     533130   32  
Stratford — RL   Denver — L3Gw     533130   91   Stratford — RL   Denver — L3Gw
    533130   92   Stratford — RL   Denver — L3Gw     533130   23   Washington
D.C. L3GW   Atlanta — L3Gw     533130   24   Washington D.C. L3GW   Atlanta —
L3Gw     533130  

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                  1   Washington D.C. L3GW   Baltimore — L3Gw     486972   2  
Washington D.C. L3GW   Baltimore — L3Gw     486972     IP Circuits              
    1   Dallas — L3Gw (a)   Atlanta — L3Gw     533130   2   Washington D.C. L3GW
(a)   Atlanta — L3Gw     533130   3   Chicago — L3Gw   Dallas — L3Gw (a)    
533130   4   Chicago — L3Gw   Denver — L3Gw (a)     533130   5   New York City —
L3Gw — 111 8th Ave (a)   Chicago — L3Gw     533130   6   Washington D.C. L3GW
(a)   Chicago — L3Gw     533130   7   Dallas — L3Gw (a)   Denver — L3Gw    
878541   8   Los Angeles — L3Gw (a)   Dallas — L3Gw     533130   9   Denver —
L3Gw   San Francisco — L3Gw (a)     533130   10   San Jose — L3Gw (a)   Los
Angeles — L3Gw     533130   11   Washington D.C. L3GW (a)   San Jose — L3Gw    
533130  

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EXHIBIT D TO MASTER AGREEMENT

FIRST AMENDMENT TO WORKOUT AGREEMENT

     This FIRST AMENDMENT TO WORKOUT AGREEMENT (the “Amendment”) is entered into
this ______day of ______, 200______(“Amendment Effective Date”), and modifies
that certain Workout Agreement (the “Workout Agreement”) dated October 30, 2001
between Level 3 Communications, Inc. (together with its affiliates and
subsidiaries, “Level 3”) and XO Communications, Inc. (together with its
affiliates and subsidiaries, “XO”). Capitalized terms used but not defined
herein shall have the meanings assigned to them in the Workout Agreement.

     1.   Credits. Prior to July 31, 2002, XO has not used any of the credits
granted to it pursuant to Section 7 of the Workout Agreement. The parties hereby
agree to delete Section 7 of the Workout Agreement in its entirety (and the
credits granted therein are hereby cancelled) and replace it with the following:

  “XO is hereby granted a credit in the amount of seven million five hundred
thousand dollars (U.S. $7,500,000.00) which may be used by XO, in its sole
discretion, to pay any invoice for services ordered from or delivered by Level 3
which are presently due and owing or hereafter delivered (or, at XO’s option,
any other amounts due and owing to Level 3, including, but not limited to, the
“Recurring Charge” due after January 1, 2003 under and pursuant to that certain
Cost Sharing and IRU Agreement dated July 18, 1998, as amended, between XO
Intercity Holdings No. 2, LLC f/k/a INTERNEXT, LLC and Level 3 Communications,
LLC). The credit may be applied to any invoices for services delivered by Level
3 (regardless of whether such services were ordered prior to or after the date
of execution hereof). When utilizing the credit for a partial payment of an
invoice, XO shall give written notification to Level 3 regarding same. The
parties recognize that this new $7.5 million credit issued herein shall be
separate from the existing credits established in the Definitive Agreement, and
that the terms of this Workout Agreement shall govern XO’s utilization of this
new credit. The credit granted pursuant to this Section shall expire to the
extent that it is not used within eight (8) years after the Amendment Effective
Date of the First Amendment to this Workout Agreement. This credit provision
shall survive the termination of the Workout Agreement.”

     2.   Wavelength Purchase Exclusivity. XO hereby agrees to purchase all of
its requirements for wavelength services within the United States and Europe
exclusively from Level 3 for a period of five (5) years (subject to Level 3
providing competitive pricing and other terms and conditions and having capacity
available to meet XO’s requirements). Level 3 shall waive its rights to
exclusivity and XO shall be permitted to purchase wavelength services from other
carriers without violating this Section if (a) Level 3’s pricing increases over
Level 3’s current pricing, or (b) XO purchases such services from a reputable
carrier at a price with

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associated service level agreements (“SLAs”) which (after providing Level 3 the
opportunity to meet or beat such pricing and beat or meet such SLAs) is less
than the price with comparable SLAs offered by Level 3, or (c) XO is procuring
such wavelength services for the purpose of providing services which are route
diverse from existing wavelength services purchased from Level 3, and Level 3 is
unable to provide such diversely routed wavelength services. The foregoing
provisions regarding exclusivity shall not be construed as limiting the parties’
current agreements regarding wavelength pricing and SLAs set forth in the
Wavelength Agreement.

     3.   Payment of Boston Joint Build Obligations. Level 3 hereby acknowledges
and agrees that, as a result of execution and performance by XO of its
obligations under this Amendment , XO shall be deemed to have fully paid all
sums due and owing to Level 3 under and pursuant to certain agreements (a full
list of which is attached hereto as Exhibit A) relating to the construction of
communications facilities in Boston, MA (the “Boston Joint Build Agreements”).
Provided that XO is not then in default of this Amendment or of any of the
agreements executed contemporaneously herewith, Level 3 hereby agrees to convey
title to the facilities that XO agreed to purchase pursuant to the Boston Joint
Build Agreement (a) within five (5) days after the Amendment Effective Date for
any segments that have been completed prior to the Amendment Effective Date, or
(b) within five (5) days after the completion of any segments that are not
completed as of the Amendment Effective Date. In addition, XO is currently
negotiating a Participant Agreement with Level 3 and the Massachusetts Turnpike
Authority (“MTA”) which is not yet executed but which will, when and if
executed, convey certain rights to XO in and to eight (8) one and one quarter
inch innerducts across the Dalton Street Bridge and the Massachusetts Avenue
Bridge; when and if such agreement is executed, and as a result of execution and
performance by XO of its obligations under this Amendment, XO shall be deemed to
have fully paid all sums due and owing to Level 3 relating to reimbursement for
“Costs” (as defined in the Participants Agreement) incurred by Level 3 in
connection with the delivery and installation of the eight innerducts (but XO
shall not be released from any liability that XO may have to the MTA under the
Participant Agreement, and XO shall continue to be liable to the MTA for the
payment of fees due thereunder). Level 3 agrees to cooperate with XO in
connection with efforts to finalize and execute the Participant Agreement. Each
party acknowledges that it has no control over, or obligation to control, the
MTA in connection with the negotiation or execution of the Participant
Agreement.

     4.   Ratification. Except as amended by this Amendment, the original terms
and provisions of the Workout Agreement shall continue in full force and effect
and the Workout Agreement, as amended by this Amendment, is hereby ratified and
confirmed.

     5.   Entire Agreement/Amendment. This Amendment constitutes the entire and
final agreement and understanding between the parties with respect to the
subject matter hereof and supersedes all prior agreements relating to the
subject matter hereof, which are of no further force or effect. The Workout
Agreement, including this Amendment, may only be modified or supplemented by an
instrument in writing executed by a duly authorized representative of each
party. In no event shall this Workout Agreement, including this Amendment, be
modified or amended in anyway by e-mail.

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IN WITNESS WHEREOF, the parties hereto have caused this Workout Agreement to be
executed by their duly authorized representatives as of the Amendment Effective
Date.

      LEVEL 3 COMMUNICATIONS, LLC   XO COMMUNICATIONS, INC.   By:          
                                                       By:             
                                                        Title:          
                                                    Title:              
                                                  

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Exhibit A to First Amendment to Workout Agreement
Boston Joint Build Agreements

1.     Joint Build Agreement dated April 15, 1998 between Level 3
Communications, LLC and Nextlink Communications, Inc., as amended.

2.     Scope of Work — Ware Street Joint Build dated May 18, 1999 between Level
3 Communications, LLC and Nextlink Massachusetts, Inc.
3.     Scope of Work — Bent Street Joint Build — Segment A-1 dated April 25,
2000 between Level 3 Communications, LLC and Nextlink Massachusetts, Inc.
4.     Scope of Work — Bent Street to the Longfellow Bridge Joint Build —
Segments A-2, A-3 and A-4 dated April 25, 2000 between Level 3 Communications,
LLC and Nextlink Massachusetts, Inc.
5.     Scope of Work — Longfellow Bridge Joint Build — Segment A-5 dated
April 25, 2000 between Level 3 Communications, LLC and Nextlink Massachusetts,
Inc.
6.     Participants Agreement — Cambridge Street from Longfellow Bridge to
Scollay Square from Somerset Street to Tremont Street — 3,600 Feet dated
December 13, 1999 between Level 3 Communications, LLC and Nextlink
Communications, Inc.
7.     Participants Agreement — 650 Feet in Kingston Street from Essex Street to
Summer Street; 600 Feet in Summer Street from Kingston Street to High Street;
770 Feet in High Street from Summer Street to Congress Street; 3120 Feet in
Congress Street from High Street to Sudbury Street; 950 Feet in Sudbury Street
from Congress Street to Scollay Square dated December 13, 1999 between Level 3
Communications, LLC and Nextlink Communications, Inc.
8.     Participants Agreement — 550 Feet in Charles Street South from Boylston
Street to Stuart Street; 550 Feet in Stuart Street from Charles Street South to
Washington Street; 900 Feet in Kneeland Street from Washington Street to
Harrison Avenue; 650 Feet in Harrison Avenue from Kneeland Street to Essex
Street; 450 Feet in Essex Street from Harrison Avenue to Kingston Street dated
December 13, 1999 between Level 3 Communications, LLC and Nextlink
Communications, Inc.
9.     Participants Agreement — 6000 Feet in Boylston Street, from Massachusetts
Avenue to Charles Street dated December 13, 1999 between Level 3 Communications,
LLC and Nextlink Communications, Inc.
10.     Participants Agreement — 250 Feet in Massachusetts Avenue from Boylston
Street to Belvidere Street; 750 Feet in Belvidere Street from Massachusetts
Avenue to Dalton Street; 650 Feet in Dalton Street from Belvidere Street to
Boylston Street dated July 31, 2001 between Level 3 Communications, LLC and XO
Communications, Inc.
11.     Participants Agreement — 720 Feet in Commonwealth Avenue from
Massachusetts Avenue to Hereford Street; 150 Feet in Hereford Street from
Commonwealth Avenue to Commonwealth Avenue; 720 Feet in Commonwealth Avenue from
Hereford Street to Massachusetts Avenue dated December 13, 1999 between Level 3
Communications, LLC and Nextlink Communications, Inc.
12.     Participants Agreement — 1600 Feet in Massachusetts Avenue from the
Harvard Bridge to Boylston Street dated December 13, 1999 between Level 3
Communications, LLC and Nextlink Communications, Inc.
13.     Scope of Work — The Harvard Bridge Joint Build — Segment A-13 dated
April 25, 2000 between Level 3 Communications, LLC and Nextlink Massachusetts,
Inc.

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Confidential
August 5, 2002

14.     Scope of Work — Bent Street to the Harvard Bridge Joint Build — Segments
A-14, A-15, A-16, A-17 & A-18 dated April 25, 2000 between Level 3
Communications, LLC and Nextlink Massachusetts, Inc.

15.     Scope of Work — Fulkerson Street Joint Build — Segment B1-1 dated
April 25, 2000 between Level 3 Communications, LLC and Nextlink Massachusetts,
Inc.
16.     Scope of Work — Kirkland Street Joint Build — Segment B-5 dated
April 25, 2000 between Level 3 Communications, LLC, and Nextlink Massachusetts,
Inc., as amended.
17.     Scope of Work — Line Street & Cambridge Street Joint Build — Segments
B-6 & B-7 dated April 25, 2000 between Level 3 Communications, LLC and Nextlink
Massachusetts, Inc.
18.     Scope of Work — Third Street Joint Build — Segment B-8 dated April 25,
2000 between Level 3 Communications, LLC and Nextlink Massachusetts, Inc.
19.     Scope of Work — Concord Turnpike to Massachusetts Avenue Joint Build -
Segment C-5A dated December 6, 2000 between Level 3 Communications, LLC and
Nextlink Communications, Inc.
20.     Scope of Work — Somerville Avenue and Washington Street Joint Build -
Segment C-5E dated December 6, 2000 between Level 3 Communications, LLC and
Nextlink Communications, Inc.
21.     Scope of Work — Alewife Brook Parkway from Massachusetts Avenue to
Broadway Joint Build — Segment C-5F dated December 6, 2000 between Level 3
Communications, LLC and Nextlink Communications, Inc.
22.     Scope of Work — Alewife Brook Parkway to Somerville Avenue Joint Build -
Segment C-5G dated December 6, 2000 between Level 3 Communications, LLC and
Nextlink Communications, Inc.
23.     Scope of Work — Somerville Avenue from Central Street to Sacramento
Street Joint Build — Segment E-1 dated December 6, 2000 between Level 3
Communications, LLC and Nextlink Communications, Inc.

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