Document:

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                                                                  EXHIBIT 4.11.2

                                                                  EXECUTION COPY

                          FIRST SUPPLEMENTAL INDENTURE

                  THIS FIRST SUPPLEMENTAL INDENTURE (the "Supplemental
Indenture") dated as of June 15, 2000 among Concentric Network Corporation, a
Delaware corporation ("the Company"), and Chase Manhattan Bank and Trust
Company, National Association, as trustee (the "Trustee").

                              W I T N E S S E T H:

                  WHEREAS, there has previously been executed and delivered to
the Trustee an Indenture (the "Indenture") dated as of December 18, 1997 between
the Company and the Trustee, providing for the issuance of $150,000,000 in
principal amount of the Company's 12 3/4% Senior Notes due 2007 (the
"Securities");

                  WHEREAS, the Company has entered into an Amended and Restated
Agreement and Plan of Merger and Share Exchange Agreement, dated as of May 10,
2000, with NEXTLINK Communications, Inc., a Delaware corporation ("NEXTLINK"),
Eagle River Investments, L.L.C., a Washington limited liability company, Craig
O. McCaw and NM Acquisition Corp., a Delaware corporation, pursuant to which the
Company and NEXTLINK will each merge with and into NM Acquisition Corp. and NM
Acquisition Corp. will be renamed "NEXTLINK Communications, Inc." (the
"Merger");

                  WHEREAS, in order to effect the Merger, the Company and
NEXTLINK have completed a solicitation of consents from the registered holders
(the "Holders") of the Securities to certain amendments to the Indenture, the
particulars of which are more fully set forth herein (the "Amendments"), and the
Holders of a majority in aggregate principal amount of the outstanding
Securities have consented to the Amendments in accordance with the provisions of
Section 902 of the Indenture;

                  WHEREAS, NM Acquisition Corp., as the surviving corporation in
the Merger will, upon effectiveness of the Merger, become the issuer of the
Securities and the "Company" for all purposes under the Indenture; and

                  WHEREAS, this Supplemental Indenture will become effective at
the effective time of the Merger, and will not become effective if the Merger is
not consummated.

                  NOW, THEREFORE, in consideration of the foregoing and of the
mutual premises and covenants contained herein and for other good and valuable
consideration, the parties hereto agree, for the equal and proportionate benefit
of the respective Holders from time to time of the Securities, as follows:

                                   ARTICLE ONE

                           AMENDMENT OF THE INDENTURE

         SECTION 1.1 Amendment of Section 101.

         (a)      Section 101 of the Indenture is hereby amended by:

                  (i)      deleting the definition of "Acquired Indebtedness"
         and inserting the following in lieu thereof:

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                           ""Acquired Debt" means, with respect to any specified
         Person, (i) Debt of any other Person existing at the time such Person
         merges with or into or consolidates with or becomes a Restricted
         Subsidiary of such specified Person and (ii) Debt secured by a Lien
         encumbering any asset acquired by such specified Person, which Debt was
         not Incurred in anticipation of, and was outstanding prior to, such
         merger, consolidation or acquisition."

                  (ii)     deleting the definition of "Affiliate" and inserting
         the following in lieu thereof:

                           ""Affiliate" of any specified Person means any other
         Person directly or indirectly controlling or controlled by or under
         direct or indirect common control with such specified Person. For the
         purposes of this definition, "control" when used with respect to any
         specified Person means the power to direct the management and policies
         of such Person, directly or indirectly, whether through the ownership
         of voting securities, by contract or otherwise; and the terms
         "controlling" and "controlled" have meanings correlative to the
         foregoing."

                  (iii)    deleting the definition of "Board of Directors" and
         inserting the following in lieu thereof:

                           ""Board of Directors" means either the board of
         directors of the Company or a Relevant Entity any duly authorized
         committee of that Board."

                  (iv)     deleting the definition of "Board Resolution" and
inserting the following in lieu thereof:

                           ""Board Resolution" means a copy of a resolution
         certified by the Secretary or an Assistant Secretary of the Company or
         a Relevant Entity to have been duly adopted by the Board of Directors
         and to be in full force and effect on the date of such certification,
         and delivered to the Trustee."

                  (v)      deleting the definition of "Capital Lease Obligation"
         and inserting the following in lieu thereof:

                           ""Capital Lease Obligation" of any Person means the
         obligation to pay rent or other payment amounts under a lease of (or
         other Debt arrangements conveying the right to use) real or personal
         property of such Person which is required to be classified and
         accounted for as a capital lease or a liability on the face of a
         balance sheet of such Person in accordance with generally accepted
         accounting principles (a "Capital Lease"). The stated maturity of such
         obligation shall be the date of the last payment of rent or any other
         amount due under such lease prior to the first date upon which such
         lease may be terminated by the lessee without payment of a penalty. The
         principal amount of such obligation shall be the capitalized amount
         thereof that would appear on the face of a balance sheet of such Person
         in accordance with generally accepted accounting principles."

                  (vi)     deleting the definition of "Common Stock" and
         inserting the following in lieu thereof:

                           ""Common Stock" means the Class A Common Stock, par
         value $0.02 per share, of NM Acquisition Corp., into which the common
         stock, par value $0.001 per share, of the Company will be converted
         pursuant to the Merger."

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                  (vii)    deleting the definition of "Company" and inserting
         the following in lieu thereof:

                           ""Company" means NM Acquisition Corp., as successor
         by merger to Concentric and NEXTLINK until a successor Person shall
         have become such pursuant to the applicable provisions of this
         Indenture and thereafter "Company" shall mean such successor Person"

                  (viii) deleting the definition of "Consolidated Income Tax
         Expense" and inserting the following in lieu thereof:

                           ""Consolidated Income Tax Expense" for any period
         means the consolidated provision for income taxes of the Relevant
         Entity and its Restricted Subsidiaries for such period calculated on a
         consolidated basis in accordance with generally accepted accounting
         principles."

                  (ix)     deleting the definition of "Consolidated Interest
         Expense" and inserting the following in lieu thereof:

                           ""Consolidated Interest Expense" means for any period
         the consolidated interest expense included in a consolidated income
         statement (excluding interest income) of the Relevant Entity and its
         Restricted Subsidiaries for such period calculated on a consolidated
         basis in accordance with generally accepted accounting principles,
         including without limitation or duplication (or, to the extent not so
         included, with the addition of), (i) the amortization of Debt
         discounts; (ii) any payments or fees with respect to letters of credit,
         bankers' acceptances or similar facilities; (iii) fees with respect to
         interest rate swap or similar agreements or foreign currency hedge,
         exchange or similar agreements; (iv) Preferred Dividends of the Company
         and its Restricted Subsidiaries (other than dividends paid in shares of
         Preferred Stock that is not Disqualified Stock) declared and paid or
         payable; (v) accrued Disqualified Stock dividends of the Company and
         its Restricted Subsidiaries, whether or not declared or paid; (vi)
         interest on Debt guaranteed by the Relevant Entity and its Restricted
         Subsidiaries; and (vii) the portion of any Capital Lease Obligation
         paid or accrued during such period that is allocable to interest
         expense."

                  (x)      deleting the definition of "Consolidated Net Income"
and inserting the following in lieu thereof:

                           ""Consolidated Net Income" for any period means the
         consolidated net income (or loss) of the Relevant Entity and its
         Restricted Subsidiaries for such period determined on a consolidated
         basis in accordance with generally accepted accounting principles;
         PROVIDED that there shall be excluded therefrom (a) the net income (or
         loss) of any Person acquired by the Relevant Entity or a Restricted
         Subsidiary of the Relevant Entity in a pooling-of-interests transaction
         for any period prior to the date of such transaction, (b) the net
         income (or loss) of any Person that is not a Restricted Subsidiary of
         the Relevant Entity except to the extent of the amount of dividends or
         other distributions actually paid to the Relevant Entity or a
         Restricted Subsidiary of the Relevant Entity by such Person during such
         period, (c) gains or losses on Asset Dispositions by the Relevant
         Entity or its Restricted Subsidiaries, (d) all extraordinary gains and
         extraordinary losses, (e) the cumulative effect of changes in
         accounting principles, (f) non-cash gains or losses resulting from
         fluctuations in currency exchange rates, (g) any non-cash gain or loss
         realized on the termination of any employee pension benefit plan and
         (h) the tax effect of any of the items described in clauses (a) through
         (g) above; PROVIDED, FURTHER, that for purposes of any determination
         pursuant to the provisions of Section 1009 there shall further be
         excluded therefrom the net income (but not net loss) of any Restricted
         Subsidiary of the Relevant Entity that is subject to a restriction
         which prevents the payment of dividends or the making of

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         distributions to the Relevant Entity or another Restricted Subsidiary
         of the Relevant Entity to the extent of such restriction."

                  (xi)     deleting the definition of "Consolidated Operating
Cash Flow" and inserting the following in lieu thereof in appropriate
alphabetical order:

                           ""Consolidated Cash Flow Available for Fixed Charges"
         for any period means the Consolidated Net Income of the Relevant Entity
         and its Restricted Subsidiaries for such period increased by the sum of
         (i) Consolidated Interest Expense of the Relevant Entity and its
         Restricted Subsidiaries for such period, plus (ii) Consolidated Income
         Tax Expense of the Relevant Entity and its Restricted Subsidiaries for
         such period, plus (iii) the consolidated depreciation and amortization
         expense included in the income statement of the Relevant Entity and its
         Restricted Subsidiaries for such period, plus (iv) any noncash expense
         for such period (excluding any noncash charge to the extent that it
         requires an accrual of or a reserve for cash disbursements in any
         future period), plus (v) any charge related to any premium or penalty
         paid in connection with redeeming or retiring any Debt prior to its
         stated maturity; PROVIDED, HOWEVER, that there shall be excluded
         therefrom the Consolidated Cash Flow Available for Fixed Charges (if
         positive) of any Restricted Subsidiary of the Relevant Entity
         (calculated separately for such Restricted Subsidiary in the same
         manner as provided above for the Relevant Entity) that is subject to a
         restriction which prevents the payment of dividends or the making of
         distributions to the Relevant Entity or another Restricted Subsidiary
         of the Company to the extent of such restriction."

                  (xii)    deleting the definition of "Disqualified Stock" and
inserting the following in lieu thereof:

                           ""Disqualified Stock" of any Person means any Capital
         Stock of such Person (other than Capital Stock outstanding on the
         Merger Effective Date) which, by its terms (or by the terms of any
         security into which it is convertible or for which it is exchangeable),
         or upon the happening of any event, matures or is mandatorily
         redeemable, pursuant to a sinking fund obligation or otherwise, or is
         redeemable at the option of the holder thereof, in whole or in part, on
         or prior to the final Maturity of the Securities (or, if earlier, the
         date as of which the Securities have been paid in full); PROVIDED,
         HOWEVER, that any Preferred Stock which would not constitute
         Disqualified Stock but for provisions thereof giving holders thereof
         the right to require the Company to repurchase or redeem such Preferred
         Stock upon the occurrence of an asset sale or a change of control
         occurring prior to the final Maturity of the Securities shall not
         constitute Disqualified Stock if the asset sale or change of control
         provisions applicable to such Preferred Stock are no more favorable to
         the holders of such Preferred Stock than the provisions applicable to
         the Securities contained in Section 1012 or Section 1014 and such
         Preferred Stock specifically provides that the Company will not
         repurchase or redeem any such stock pursuant to such provisions prior
         to the Company's repurchase of such Securities as are required to be
         repurchased pursuant to Section 1012 or Section 1014."

                  (xiii)   adding the phrase ", and the rules and regulations
thereunder" after the word "Statute" in the definition of "Exchange Act."

                  (xiv)    deleting the definition of "Guarantee" and inserting
the following in lieu thereof:

                           ""Guarantee" by any Person means any obligation,
         contingent or otherwise, of such Person guaranteeing, or having the
         economic effect of guaranteeing, any Debt of any other Person (the
         "primary obligor") in any manner, whether directly or indirectly, and
         including,

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         without limitation, any obligation of such Person, (i) to purchase or
         pay (or advance or supply funds for the purchase or payment of) such
         Debt or to purchase (or to advance or supply funds for the purchase of)
         any security for the payment of such Debt, (ii) to purchase property,
         securities or services for the purpose of assuring the holder of such
         Debt of the payment of such Debt, or (iii) to maintain working capital,
         equity capital or other financial statement condition or liquidity of
         the primary obligor so as to enable the primary obligor to pay such
         Debt (and "Guaranteed", "Guaranteeing" and "Guarantor" shall have
         meanings correlative to the foregoing); PROVIDED, HOWEVER, that the
         Guarantee by any Person shall not include endorsements by such Person
         for collection or deposit, in either case, in the ordinary course of
         business; and PROVIDED, FURTHER, that the incurrence by a Restricted
         Subsidiary of the Company of a lien permitted under clause (iv) of the
         second paragraph of Section 1011 shall not be deemed to constitute a
         Guarantee by such Restricted Subsidiary of any Purchase Money Debt of
         the Company secured thereby."

                  (xv)     deleting the definition of "Incur" and inserting the
following in lieu thereof:

                           ""Incur" means, with respect to any Debt or other
         obligation of any Person, to create, issue, incur (by conversion,
         exchange or otherwise), assume, Guarantee or otherwise become liable in
         respect of such Debt or other obligation including by acquisition of
         Subsidiaries or the recording, as required pursuant to generally
         accepted accounting principles or otherwise, of any such Debt or other
         obligation on the balance sheet of such Person (and "Incurrence",
         "Incurred", "Incurrable" and "Incurring" shall have meanings
         correlative to the foregoing); PROVIDED, HOWEVER, that a change in
         generally accepted accounting principles that results in an obligation
         of such Person that exists at such time becoming Debt shall not be
         deemed an Incurrence of such Debt and that neither the accrual of
         interest nor the accretion of original issue discount shall be deemed
         an Incurrence of Debt; PROVIDED, FURTHER, HOWEVER, that the Company may
         elect to treat all or any portion of revolving credit debt of the
         Company or a Subsidiary as being incurred from and after any date
         beginning the date the revolving credit commitment is extended to the
         Company or a Subsidiary, by furnishing notice thereof to the Trustee,
         and any borrowings or reborrowings by the Company or a Subsidiary under
         such commitment up to the amount of such commitment designated by the
         Company as Incurred shall not be deemed to be new Incurrences of Debt
         by the Company or such Subsidiary."

                  (xvi)    deleting the definition of "Interest Rate Agreements"
and inserting the following in lieu thereof:

                           ""Interest Rate or Currency Protection Agreement" of
         any Person means any forward contract, futures contract, swap, option
         or other financial agreement or arrangement (including, without
         limitation, caps, floors, collars and similar agreements) relating to,
         or the value of which is dependent upon, interest rates or currency
         exchange rates or indices."

                  (xvii)   deleting the definition of "Investment" and inserting
the following in lieu thereof:

                           ""Investment" by any Person means any direct or
         indirect loan, advance or other extension of credit or capital
         contribution (by means of transfers of cash or other property to others
         or payments for property or services for the account or use of others,
         or otherwise) to, or purchase or acquisition of Capital Stock, bonds,
         notes, debentures or other securities or evidence of Debt issued by,
         any other Person, including any payment on a Guarantee of any
         obligation of such other Person, but excluding any loan, advance or
         extension of credit to an employee of the Company or any of its
         Restricted Subsidiaries in the ordinary course of business, accounts
         receivable and other commercially reasonable extensions of trade
         credit."

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                  (xviii)  deleting the definition of "Lien" and inserting the
following in lieu thereof:

                           ""Lien" means, with respect to any property or
         assets, any mortgage or deed of trust, pledge, hypothecation,
         assignment, Receivables Sale, deposit arrangement, security interest,
         lien, charge, easement (other than any easement not materially
         impairing usefulness or marketability), encumbrance, preference,
         priority or other security agreement or preferential arrangement of any
         kind or nature whatsoever on or with respect to such property or assets
         (including, without limitation, any conditional sale or other title
         retention agreement having substantially the same economic effect as
         any of the foregoing)."

                  (xix)    deleting the definition of "Permitted Investment" and
inserting the following in lieu thereof:

                           ""Permitted Investment" means (i) any Investment in a
         Joint Venture (including the purchase or acquisition of any Capital
         Stock of a Joint Venture), provided the aggregate amount of all
         outstanding Investments pursuant to this clause (i) in Joint Ventures
         in which the Company owns, directly or indirectly, a less than 50%
         interest shall not exceed $25 million, (ii) any Investment in any
         Person as a result of which such Person becomes a Restricted
         Subsidiary, or, subject to the proviso to clause (i) of this
         definition, becomes a Joint Venture of the Company, (iii) any
         Investment in Marketable Securities, (iv) Investments in Permitted
         Interest Rate or Currency Protection Agreements, (v) Investments made
         as a result of the receipt of noncash consideration from an Asset
         Disposition that was made pursuant to and in compliance with Section
         1012 of this Indenture and (vi) other Investments in an aggregate
         amount not to exceed the aggregate net proceeds received by the Company
         or any Restricted Subsidiary after the Merger Effective Date from the
         sale or liquidation of any Unrestricted Subsidiary or any interest
         therein (except to the extent that any such amount is included in the
         calculation of Consolidated Net Income)."

                  (xx)     deleting the definition of "Permitted Lien" and
inserting the following in lieu thereof:

                           ""Permitted Liens" means (a) Liens for taxes,
         assessments, governmental charges or claims which are not yet
         delinquent or which are being contested in good faith by appropriate
         proceedings, if a reserve or other appropriate provision, if any, as
         shall be required in conformity with generally accepted accounting
         principles shall have been made therefor; (b) other Liens incidental to
         the conduct of the Company's and its Restricted Subsidiaries' business
         or the ownership of its property and assets not securing any Debt, and
         which do not in the aggregate materially detract from the value of the
         Company's and its Restricted Subsidiaries' property or assets when
         taken as a whole, or materially impair the use thereof in the operation
         of its business; (c) Liens with respect to assets of a Restricted
         Subsidiary granted by such Restricted Subsidiary to the Company to
         secure Debt owing to the Company; (d) pledges and deposits made in the
         ordinary course of business in connection with workers' compensation,
         unemployment insurance and other types of statutory obligations
         (including to secure government contracts); (e) deposits made to secure
         the performance of tenders, bids, leases, and other obligations of like
         nature incurred in the ordinary course of business (exclusive of
         obligations for the payment of borrowed money); (f) zoning
         restrictions, servitudes, easements, rights-of-way, restrictions and
         other similar charges or encumbrances incurred in the ordinary course
         of business which, in the aggregate, do not materially detract from the
         value of the property subject thereto or interfere with the ordinary
         conduct of the business of the Company or its Restricted Subsidiaries;
         (g) Liens arising out of judgments or awards against the Company or any
         Restricted Subsidiary with respect to which the Company or such
         Restricted Subsidiary is prosecuting an appeal or

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         proceeding for review and the Company or such Restricted Subsidiary is
         maintaining adequate reserves in accordance with generally accepted
         accounting principles; (h) any interest or title of a lessor in the
         property subject to any lease other than a Capital Lease; and (i) any
         statutory warehousemen's, materialmen's or other similar Liens for sums
         not then due and payable (or which, if due and payable, are being
         contested in good faith and with respect to which adequate reserves are
         being maintained to the extent required by generally accepted
         accounting principles)."

                  (xxi)    inserting the words "or any other entity" after the
word "thereby" in the definition of "Person".

                  (xxii)   deleting the definition of "Preferred Stock" and
inserting the following in lieu thereof:

                           ""Preferred Stock" of any Person means Capital Stock
         of such Person of any class or classes (however designated) that ranks
         prior, as to the payment of dividends or as to the distribution of
         assets upon any voluntary or involuntary liquidation, dissolution or
         winding up of such Person, to shares of Capital Stock of any other
         class of such Person."

                  (xxiii)  deleting the definition of "Purchase Money
Obligation" and inserting the following in lieu thereof:

                           ""Purchase Money Debt" means (i) Acquired Debt
         Incurred in connection with the acquisition of Telecommunications
         Assets and (ii) Debt of the Relevant Entity or of any Restricted
         Subsidiary of the Relevant Entity (including, without limitation, Debt
         represented by Bank Credit Agreements, Capital Lease Obligations,
         Vendor Financing Facilities, mortgage financings and purchase money
         obligations) Incurred for the purpose of financing all or any part of
         the cost of construction, acquisition or improvement by the Relevant
         Entity or any Restricted Subsidiary of the Relevant Entity or any Joint
         Venture of any Telecommunications Assets of the Relevant Entity, any
         Restricted Subsidiary of the Relevant Entity or any Joint Venture, and
         including any related notes, Guarantees, collateral documents,
         instruments and agreements executed in connection therewith, as the
         same may be amended, supplemented, modified or restated from time to
         time."

                  (xxiv)   deleting the definition of "Sale and Leaseback
Transaction" and inserting the following in lieu thereof:

                           ""Sale and Leaseback Transaction" of any Person means
         an arrangement with any lender or investor or to which such lender or
         investor is a party providing for the leasing by such Person of any
         property or asset of such Person which has been or is being sold or
         transferred by such Person more than 365 days after the acquisition
         thereof or the completion of construction or commencement of operation
         thereof to such lender or investor or to any person to whom funds have
         been or are to be advanced by such lender or investor on the security
         of such property or asset. The stated maturity of such arrangement
         shall be the date of the last payment of rent or any other amount due
         under such arrangement prior to the first date on which such
         arrangement may be terminated by the lessee without payment of a
         penalty."

                  (xxv)    deleting the definition of "Subordinated
Indebtedness" and inserting the following in lieu thereof:

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<PAGE>   8

                           ""Subordinated Debt" means Debt of the Company as to
         which the payment of principal of (and premium, if any) and interest
         and other payment obligations in respect of such Debt shall be
         subordinate to the prior payment in full of the Securities to at least
         the following extent: (i) no payments of principal of (or premium, if
         any) or interest on or otherwise due in respect of such Debt may be
         permitted for so long as any default in the payment of principal (or
         premium, if any) or interest on the Securities exists; (ii) in the
         event that any other default that with the passing of time or the
         giving of notice, or both, would constitute an Event of Default exists
         with respect to the Securities, upon notice by the Holders of 25% or
         more in principal amount of the Securities to the Trustee, the Trustee
         shall have the right to give notice to the Company and the holders of
         such Debt (or trustees or agents therefor) of a payment blockage, and
         thereafter no payments of principal of (or premium, if any) or interest
         on or otherwise due in respect of such Debt may be made for a period of
         179 days from the date of such notice or for the period until such
         default has been cured or waived or ceased to exist and any
         acceleration of the Securities has been rescinded or annulled,
         whichever period is shorter (which Debt may provide that (A) no new
         period of payment blockage may be commenced by a payment blockage
         notice unless and until 360 days have elapsed since the effectiveness
         of the immediately prior notice, (B) no nonpayment default that existed
         or was continuing on the date of delivery of any payment blockage
         notice to such holders (or such agents or trustees) shall be, or be
         made, the basis for a subsequent payment blockage notice and (C)
         failure of the Company to make payment on such Debt when due or within
         any applicable grace period, whether or not on account of such payment
         blockage provisions, shall constitute an event of default thereunder);
         and (iii) such Debt may not (x) provide for payments of principal of
         such Debt at the stated maturity thereof or by way of a sinking fund
         applicable thereto or by way of any mandatory redemption, defeasance,
         retirement or repurchase thereof by the Company (including any
         redemption, retirement or repurchase which is contingent upon events or
         circumstances, but excluding any retirement required by virtue of
         acceleration of such Debt upon an event of default thereunder), in each
         case prior to the final Maturity of the Securities or (y) permit
         redemption or other retirement (including pursuant to an offer to
         purchase made by the Company) of such other Debt at the option of the
         holder thereof prior to the final Maturity of the Securities, other
         than a redemption or other retirement at the option of the holder of
         such Debt (including pursuant to an offer to purchase made by the
         Company) which is conditioned upon a change of control of the Company
         pursuant to provisions substantially similar to those of Section 1014
         (and which shall provide that such Debt will not be repurchased
         pursuant to such provisions prior to the Company's repurchase of the
         Securities required to be repurchased by the Company pursuant to the
         provisions of Section 1014)."

                  (xxvi)   deleting the definition of "Unrestricted Subsidiary"
and inserting the following in lieu thereof:

                           ""Unrestricted Subsidiary" means (1) any subsidiary
         of the Company designated as such by the Board of Directors as set
         forth below where (a) neither the Company nor any of its other
         subsidiaries (other than another Unrestricted Subsidiary) (i) provides
         credit support for, or Guarantee of, any Debt of such subsidiary or any
         subsidiary of such subsidiary (including any undertaking, agreement or
         instrument evidencing such Debt) or (ii) is directly or indirectly
         liable for any Debt of such subsidiary or any subsidiary of such
         subsidiary, and (b) no default with respect to any Debt of such
         subsidiary or any subsidiary of such subsidiary (including any right
         which the holders thereof may have to take enforcement action against
         such subsidiary) would permit (upon notice, lapse of time or both) any
         holder of any other Debt of the Company and its Restricted Subsidiaries
         to declare a default on such other Debt or cause the payment thereof to
         be accelerated or payable prior to its final scheduled maturity and (2)
         any subsidiary of an Unrestricted Subsidiary. The Board of Directors
         may designate any subsidiary to be an Unrestricted Subsidiary unless
         such subsidiary owns any Capital Stock of, or owns or holds any

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<PAGE>   9

         Lien on any property of, any other subsidiary of the Company which is
         not a subsidiary of the subsidiary to be so designated or otherwise an
         Unrestricted Subsidiary, PROVIDED that either (x) the subsidiary to be
         so designated has total assets of $1,000 or less or (y) immediately
         after giving effect to such designation, the Company could Incur at
         least $1.00 of additional Debt pursuant to the first paragraph of
         Section 1008 and PROVIDED, FURTHER, that the Company could make a
         Restricted Payment in an amount equal to the greater of the fair market
         value and the book value of such subsidiary pursuant to Section 1009
         and such amount is thereafter treated as a Restricted Payment for the
         purpose of calculating the aggregate amount available for Restricted
         Payments thereunder. The Board of Directors may designate any
         Unrestricted Subsidiary to be a Restricted Subsidiary, PROVIDED that if
         such Unrestricted Subsidiary has Debt outstanding at such time, either
         (a) immediately after giving effect to such designation, the Company
         could Incur at least $1.00 of additional Debt pursuant to the first
         paragraph of Section 1008 or (b) the Company or such Restricted
         Subsidiary could Incur such Debt hereunder (other than as Acquired
         Debt)."

                  (xxvii)  adding the following definitions in appropriate
alphabetical order:

                           ""Asset Disposition" by the Company or any Restricted
         Subsidiary means any transfer, conveyance, sale, lease or other
         disposition (other than a creation of a Lien) by such Person (including
         a consolidation or merger or other sale of any such Restricted
         Subsidiary with, into or to another Person in a transaction in which
         such Restricted Subsidiary ceases to be a Restricted Subsidiary of the
         Company, but excluding a disposition by a Restricted Subsidiary of the
         Company to the Company or a Restricted Subsidiary of the Company or by
         the Company to a Restricted Subsidiary of the Company) of (i) shares of
         Capital Stock or other ownership interests of a Restricted Subsidiary
         of the Company (including the issuance of Capital Stock by a Restricted
         Subsidiary), other than as permitted by the provisions of Section 1018
         or pursuant to a transaction in compliance with Section 801, (ii)
         substantially all of the assets of the Company or any of its Restricted
         Subsidiaries representing a division or line of business (other than as
         part of a Permitted Investment) or (iii) other assets or rights of the
         Company or any of its Restricted Subsidiaries other than (A) in the
         ordinary course of business or (B) that constitutes a Restricted
         Payment which is permitted by the provisions of Section 1009; PROVIDED
         that a transaction described in clauses (i), (ii) and (iii) shall
         constitute an Asset Disposition only if the aggregate consideration for
         such transfer, conveyance, sale, lease or other disposition is equal to
         $5 million or more in any 12-month period."

                           ""Attributable Value" means, as to any particular
         lease under which any Person is at the time liable other than a Capital
         Lease Obligation, and at any date as of which the amount thereof is to
         be determined, the total net amount of rent required to be paid by such
         Person under such lease during the initial term thereof as determined
         in accordance with generally accepted accounting principles, discounted
         from the last date of such initial term to the date of determination at
         a rate per annum equal to the discount rate which would be applicable
         to a Capital Lease Obligation with like term in accordance with
         generally accepted accounting principles. The net amount of rent
         required to be paid under any such lease for any such period shall be
         the aggregate amount of rent payable by the lessee with respect to such
         period after excluding amounts required to be paid on account of
         insurance, taxes, assessments, utility, operating and labor costs and
         similar charges. In the case of any lease which is terminable by the
         lessee upon the payment of penalty, such net amount shall also include
         the lesser of the amount of such penalty (in which case no rent shall
         be considered as required to be paid under such lease subsequent to the
         first date upon which it may be so terminated) or the rent which would
         otherwise be required to be paid if such lease is not so terminated."

                                      -9-
<PAGE>   10

                           ""Bank Credit Agreement" means any one or more credit
         agreements (which may include or consist of revolving credits) between
         the Company or any Restricted Subsidiary of the Company and one or more
         banks or other financial institutions providing financing for the
         business of the Company and its Restricted Subsidiaries."

                           ""Common Equity" of any Person means Capital Stock of
         such Person that is not Disqualified Stock, and a "sale of Common
         Equity" includes any sale of Common Equity effected by private sale or
         public offering."

                           ""Consolidated Capital Ratio" of any Person as of any
         date means the ratio of (i) the aggregate consolidated principal amount
         of Debt (or in the case of Debt issued at a discount the accreted
         amount thereof) of such Person then outstanding (which amount of Debt
         shall be reduced by any amount of cash or cash equivalent collateral
         securing on a perfected basis and dedicated for disbursement
         exclusively to the payment of principal of and interest on such Debt)
         to (ii) the aggregate consolidated Capital Stock (other than
         Disqualified Stock) and paid in capital (other than in respect of
         Disqualified Stock) of such Person as of such date."

                           ""Consolidated Net Worth" of any Person means the
         consolidated stockholders' equity of such Person, determined on a
         consolidated basis in accordance with generally accepted accounting
         principles, less amounts attributable to Disqualified Stock of such
         Person; PROVIDED that, with respect to the Company, adjustments
         following the date of this Indenture to the accounting books and
         records of the Company in accordance with Accounting Principles Board
         Opinions Nos. 16 and 17 (or successor opinions thereto) or otherwise
         resulting from the acquisition of control of the Company by another
         Person shall not be given effect to."

                           ""Consolidated Tangible Assets" of any Person means
         the total amount of assets (less applicable reserves and other properly
         deductible items) which under generally accepted accounting principles
         would be included on a consolidated balance sheet of such Person and
         its Restricted Subsidiaries after deducting therefrom all goodwill,
         trade names, trademarks, patents, unamortized debt discount and expense
         and other like intangibles, which in each case under generally accepted
         accounting principles would be included on such consolidated balance
         sheet; PROVIDED that, with respect to the Company, adjustments
         following the date of this Indenture to the accounting books and
         records of the Company in accordance with Accounting Principles Board
         Opinions Nos. 16 and 17 (or successor opinions thereto) or otherwise
         resulting from the acquisition of control of the Company by another
         Person shall not be given effect to."

                           ""corporation" means a corporation, association,
         company, limited liability company, joint-stock company or business
         trust."

                           ""Debt" means (without duplication), with respect to
         any Person, whether recourse is to all or a portion of the assets of
         such Person and whether or not contingent, (i) every obligation of such
         Person for money borrowed, (ii) every obligation of such Person
         evidenced by bonds, debentures, notes or other similar instruments,
         including any such obligations Incurred in connection with the
         acquisition of property, assets or businesses, (iii) every
         reimbursement obligation of such Person with respect to letters of
         credit, bankers' acceptances or similar facilities issued for the
         account of such Person, (iv) every obligation of such Person issued or
         assumed as the deferred purchase price of property or services
         (including securities repurchase in the ordinary course of business
         which are not overdue or which are being contested in good faith), (v)
         every Capital Lease Obligation of such Person, (vi) all Receivables
         Sales of such Person, together with any obligation of such Person to
         pay any discount, interest, fees, indemnities, penalties, recourse,
         expenses or other amounts in connection therewith, (vii) all
         obligations to redeem Disqualified

                                      -10-
<PAGE>   11

         Stock issued by such Person, (viii) every obligation under Interest
         Rate or Currency Protection Agreements of such Person and (ix) every
         obligation of the type referred to in clauses (i) through (viii) of
         another Person and all dividends of another Person the payment of
         which, in either case, such Person has Guaranteed. The "amount" or
         "principal amount" of Debt at any time of determination as used herein
         represented by (a) any Debt issued at a price that is less than the
         principal amount at maturity thereof, shall be the amount of the
         liability in respect thereof determined in accordance with generally
         accepted accounting principles, (b) any Receivables Sale, shall be the
         amount of the unrecovered capital or principal investment of the
         purchaser (other than the Company or a Wholly-Owned Restricted
         Subsidiary of the Company) thereof, excluding amounts representative of
         yield or interest earned on such investment, (c) any Disqualified
         Stock, shall be the maximum fixed redemption or repurchase price in
         respect thereof, (d) any Capital Lease Obligation, shall be determined
         in accordance with the definition thereof, or (e) any Permitted
         Interest Rate or Currency Protection Agreement, shall be zero. In no
         event shall Debt include any liability for taxes."

                           "Eligible Institution" means a commercial banking
         institution that has combined capital and surplus of not less than $500
         million or its equivalent in foreign currency, whose debt is rated
         "A-3" or higher, "A" or higher or "A" or higher according to Moody's
         Investors Service, Inc., Standard & Poor's Ratings Group or Duff &
         Phelps Credit Rating Co. (or such similar equivalent rating by at least
         one "nationally recognized statistical rating organization" (as defined
         in Rule 436 under the Securities Act)) respectively, at the time as of
         which any investment or rollover therein is made.

                           ""Eligible Receivables" means, at any time,
         Receivables of the Relevant Entity and its Restricted Subsidiaries, as
         evidenced on the most recent quarterly consolidated balance sheet of
         the Company as at a date at least 45 days prior to such time arising in
         the ordinary course of business of the Company or any Restricted
         Subsidiary of the Company."

                           ""Government Securities" means direct obligations of,
         or obligations guaranteed by, the United States of America for the
         payment of which obligations or guarantee the full faith and credit of
         the United States is pledged and which have a remaining weighted
         average life to maturity of not more than 18 months from the date of
         Investment therein."

                           ""Joint Venture" means a corporation, partnership or
         other entity engaged in one or more Telecommunications Businesses
         (which shall include for purposes of this definition of "Joint Venture"
         all businesses in which the Relevant Entity or any of its subsidiaries
         was engaged on the Merger Effective Date; provided that, as used in
         this definition of "Joint Venture," the determination of what
         constitutes a "Telecommunications Business" shall be made in good faith
         by the Board of directors, which determination shall be conclusive) as
         to which the Company (directly or through one or more Restricted
         Subsidiaries) exercises managerial control and in which the Company
         owns (i) a 50% or greater interest, or (ii) a 30% or greater interest,
         together with options or other contractual rights, exercisable not more
         than seven years after the Company's initial Investment in such Joint
         Venture, to increase its interest to not less than 50%."

                           ""Marketable Securities" means: (i) Government
         Securities; (ii) any time deposit account, money market deposit and
         certificate of deposit maturing not more than 365 days after the date
         of acquisition issued by, or time deposit of, an Eligible Institution;
         (iii) commercial paper maturing not more than 365 days after the date
         of acquisition issued by a corporation (other than an Affiliate of the
         Company) with a rating, at the time as of which any investment therein
         is made, of "P-1" or higher according to Moody's Investors Service,
         Inc., "A-1" or higher according to Standard & Poor's Ratings Group or
         "A-1" or higher according to Duff & Phelps Credit Rating

                                      -11-
<PAGE>   12

         Co. (or such similar equivalent rating by at least one "nationally
         recognized statistical rating organization" (as defined in Rule 436
         under the Securities Act)); (iv) any banker's acceptances or money
         market deposit accounts issued or offered by an Eligible Institution;
         (v) repurchase obligations with a term of not more than 7 days for
         Government Securities entered into with an Eligible Institution; (vi)
         auction-rate preferred stocks of any corporation maturing within 90
         days after the date of acquisition by the Company thereof, having a
         rating of at least AA by Standard & Poor's; and (vii) any fund
         investing exclusively in investments of the types described in clauses
         (i) through (vi) above."

                           ""Merger" means the merger of Concentric Network
         Corporation with and into NM Acquisition Corp. and NEXTLINK pursuant to
         the Agreement and Plan of Merger, dated as of January 9, 2000, by and
         among the Concentric Network Corporation, NEXTLINK, Eagle River
         Investments, L.L.C. and NM Acquisition Corp."

                           ""Merger Effective Date" means the date on which the
         Merger becomes effective."

                           ""NEXTLINK" means NEXTLINK Communications, Inc. a
         Delaware corporation, prior to the Merger.

                           ""Permitted Interest Rate or Currency Protection
         Agreement" of any Person means any Interest Rate or Currency Protection
         Agreement entered into with one or more financial institutions in the
         ordinary course of business that is designed to protect such Person
         against fluctuations in interest rates or currency exchange rates with
         respect to Debt Incurred and which shall have a notional amount no
         greater than the payments due with respect to the Debt being hedged
         thereby and not for purposes of speculation."

                           ""Preferred Dividends" for any Person means for any
         period the quotient determined by dividing the amount of dividends and
         distributions paid or accrued (whether or not declared) on Preferred
         Stock of such Person during such period calculated in accordance with
         generally accepted accounting principles, by 1 minus the maximum
         statutory income tax rate then applicable to the Company (expressed as
         a decimal)."

                           ""Receivables" means receivables, chattel paper,
         instruments, documents or intangibles evidencing or relating to the
         right to payment of money in respect of the sale of goods or services."

                           ""Receivables Sale" of any Person means any sale of
         Receivables of such Person (pursuant to a purchase facility or
         otherwise), other than in connection with a disposition of the business
         operations of such Person relating thereto or a disposition of
         defaulted Receivables for purpose of collection and not as a financing
         arrangement."

                           ""Related Person" of any Person means any other
         Person directly or indirectly owning (a) 10% or more of the issued and
         outstanding Common Equity of such Person (or, in the case of a Person
         that is not a corporation, 10% or more of the equity interest in such
         Person) or (b) 10% or more of the combined voting power of the Voting
         Stock of such Person."

                           "Relevant Entity" means NEXTLINK for periods prior to
         the consummation of the Merger and the Company for periods from and
         after the consummation of the Merger.

                                      -12-
<PAGE>   13

                           ""Restricted Subsidiary" of the Company or the
         Relevant Entity, as applicable means any subsidiary, whether existing
         on or after the Merger Effective Date with respect to the Company, or
         prior to the Merger Effective Date with respect to the Relevant Entity,
         unless such subsidiary is an Unrestricted Subsidiary.

                           "Significant Subsidiary" means a Restricted
         Subsidiary that is a "significant subsidiary" as defined in Rule
         1-02(w) of Regulation S-X under the Securities Act and the Exchange
         Act."

                           ""Telecommunications Assets" means all assets, rights
         (contractual or otherwise) and properties, whether tangible or
         intangible, used or intended for use in connection with a
         Telecommunications Business (which shall include for purposes of this
         definition of "Telecommunications Assets" all businesses in which the
         Relevant Entity or any of its subsidiaries was engaged on the Merger
         Effective Date; provided that, as used in this definition of
         "Telecommunications Assets," the determination of what constitutes a
         "Telecommunications Business" shall be made in good faith by the Board
         of Directors, which determination shall be conclusive)."

                           ""Vendor Financing Facility" means any agreements
         between the Company or a Restricted Subsidiary of the Company and one
         or more vendors or lessors of equipment or other capital assets to the
         Company or any of its Restricted Subsidiaries (or any affiliate of any
         such vendor or lessor) providing financing for the acquisition by the
         Company or any such Restricted Subsidiary of equipment or other capital
         assets from any such vendor or lessor."

                           ""Wholly-Owned Restricted Subsidiary" of any Person
         means a Restricted Subsidiary of such Person 99% or more of the
         outstanding Capital Stock or other ownership interests of which (other
         than directors' qualifying shares) shall at the time be owned by such
         Person or by one or more Wholly-Owned Restricted Subsidiaries of such
         Person or by such Person and one or more Wholly-Owned Restricted
         Subsidiaries of such Person."

                  (xxx)    deleting the following terms and their respective
definitions:

                           "Acquired Person," "Acquisition," "Average Life to
         Stated Maturity," "Cumulative Operating Cash Flow," "Debt to Annualized
         Operating Cash Flow," "Disinterested Director," "Guaranteed Debt,"
         "Guarantor," "Qualified Capital Stock," "Redeemable Capital Stock,"
         "Total Consolidated Indebtedness," "Unrestricted Subsidiary
         Indebtedness" and "Wholly Owned Subsidiary."

         SECTION 1.2 Amendment of Section 102.

                  Section 102 of the Indenture is hereby amended by (a) deleting
the definitions of "Basket," "Permitted Indebtedness, "Permitted Payment,"
"Refinancing, "Restricted Payments" and "Security Amount" and (b) deleting the
cross-references to Section "1015" in the definitions of "Change of Control
Offer," "Change of Control Purchase Date," "Change of Control Purchase Notice"
and "Change of Control Purchase Price" and replacing such cross-references with
cross-references to Section "1014".

         SECTION 1.3 Amendment of Section 501.

                  Section 501 of the Indenture is hereby amended by:

                                      -13-
<PAGE>   14

                  (a)      (i) deleting the number "30" in clause (i) in
         paragraph (c) of such Section and inserting the number "60" in lieu
         thereof and (ii) deleting the words "Article Eight" in clause (ii) of
         such Section and inserting the words "Section 801" in lieu thereof;

                  (b)      deleting paragraph (d) of such Section and inserting
         the following in lieu thereof:

                           "(d) a default or defaults under any bond(s),
         debenture(s), note(s) or other evidence(s) of Debt by the Company or
         any Significant Subsidiary of the Company or under any mortgage(s),
         indenture(s) or instrument(s) under which there may be issued or by
         which there may be secured or evidenced any Debt of such type by the
         Company or any such Significant Subsidiary with a principal amount then
         outstanding, individually or in the aggregate, in excess of $10
         million, whether such Debt now exists or shall hereafter be created,
         which default or defaults shall constitute a failure to pay such Debt
         when due at the final maturity thereof, or shall have resulted in such
         Debt becoming or being declared due and payable prior to the date on
         which it would otherwise have become due and payable;";

                  (c)      deleting paragraph (e) of such Section;

                  (d)      deleting paragraph (f) of such Section and inserting
         the following in lieu thereof:

                           "(e) a final judgment or final judgments (not subject
         to appeal) for the payment of money are entered against the Company or
         any Significant Subsidiary in an aggregate amount in excess of $10
         million by a court or courts of competent jurisdiction, which judgments
         remain undischarged or unstayed for a period (during which execution
         shall not be effectively stayed) of 45 days after the right to appeal
         all such judgments has expired;";

                  (e)      deleting paragraph (g) of such Section;

                  (f)      deleting paragraph (h) of such Section and inserting
         the following in lieu thereof:

                           "(f) the entry by a court having jurisdiction in the
         premises of (A) a decree or order for relief in respect of the Company
         or any Significant Subsidiary in an involuntary case or proceeding
         under any applicable Federal or State bankruptcy, insolvency,
         reorganization or other similar law or (B) a decree or order adjudging
         the Company or any Significant Subsidiary a bankrupt or insolvent, or
         approving as properly filed a petition seeking reorganization,
         arrangement, adjustment or composition of or in respect of the Company
         or any Significant Subsidiary under any applicable Federal or State
         law, or appointing a custodian, receiver, liquidator, assignee,
         trustee, sequestrator or other similar official of the Company or any
         Significant Subsidiary or of any substantial part of its property, or
         ordering the winding up or liquidation of its affairs, and the
         continuance of any such decree or order for relief or any such other
         decree or order unstayed and in effect for a period of 60 consecutive
         days;";

                  (h)      deleting paragraph (i) of such Section and inserting
         the following in lieu thereof:

                           "(g) the commencement by the Company or any
         Significant Subsidiary of a voluntary case or proceeding under any
         applicable Federal or State bankruptcy, insolvency, reorganization or
         other similar law or of any other case or proceeding to be adjudicated
         a bankrupt or insolvent, or the consent by it to the entry of a decree
         or order for relief in respect of the Company or any Significant
         Subsidiary in an involuntary case or proceeding under any applicable
         Federal or State bankruptcy, insolvency, reorganization or other
         similar law or to the commencement of any bankruptcy or insolvency case
         or proceeding against it, or the filing by it

                                      -14-
<PAGE>   15

         of a petition or answer or consent seeking reorganization or relief
         under any applicable Federal or State law, or the consent by it to the
         filing of such petition or to the appointment of or taking possession
         by a custodian, receiver, liquidator, assignee, trustee, sequestrator
         or other similar official of the Company or any Significant Subsidiary
         or of any substantial part of its property, or the making by it of an
         assignment for the benefit of creditors, or the admission by it in
         writing of its inability to pay its debts generally as they become due,
         or the taking of corporate action by the Company or any Significant
         Subsidiary in furtherance of any such action; or"; and

                  (i)      by redesignating paragraph "(j)" of such Section as
         paragraph "(h)".

         SECTION 1.4 Amendment of Section 801.

                  Section 801 of the Indenture is hereby amended by deleting
such section and inserting the following in lieu thereof:

                  "Section 801. Mergers, Consolidations And Certain Sales Of
                                   Assets.

                           (a)      The Company may not, in a single transaction
         or a series of related transactions, (i) consolidate with or merge into
         any other Person or permit any other Person to consolidate with or
         merge into the Company (other than a consolidation or merger of a
         Wholly-Owned Restricted Subsidiary organized under the laws of a State
         of the United States into the Company), or (ii) directly or indirectly,
         transfer, sell, lease or otherwise dispose of all or substantially all
         of its assets (determined on a consolidated basis for the Company and
         its Restricted Subsidiaries taken as a whole and provided that the
         creation of a Lien on or in any of its assets shall not in and of
         itself constitute the transfer, sale, lease or disposition of the
         assets subject to the Lien), unless: (1) in a transaction in which the
         Company does not survive or in which the Company sells, leases or
         otherwise disposes of all or substantially all of its assets to any
         other Person, the successor entity to the Company shall be a
         corporation organized under the laws of the United States of America or
         any State thereof or the District of Columbia and shall expressly
         assume, by a supplemental indenture executed and delivered to the
         Trustee in form satisfactory to the Trustee, all of the Company's
         obligations under this Indenture; (2) immediately after giving pro
         forma effect to such transaction as if such transaction had occurred at
         the beginning of the last full fiscal quarter immediately prior to the
         consummation of such transaction with the appropriate adjustments with
         respect to the transaction being included in such pro forma calculation
         and treating any Debt which becomes an obligation of the Company or a
         Subsidiary as a result of such transaction as having been Incurred by
         the Company or such Subsidiary at the time of the transaction, no
         Default or Event of Default shall have occurred and be continuing; (3)
         immediately after giving effect to such transaction, the Consolidated
         Net Worth of the Company (or other successor entity to the Company) is
         equal to or greater than that of the Company immediately prior to the
         transaction; (4) if, as a result of any such transaction, property or
         assets of the Company would become subject to a Lien prohibited by the
         provisions of Section 1011, the Company or the successor entity to the
         Company shall have secured the Securities as required by Section 1011;
         (5) the Company has delivered to the Trustee an Officer's Certificate
         and an Opinion of Counsel, each in form and substance satisfactory to
         the Trustee stating that such consolidation, merger, conveyance,
         transfer, lease or acquisition and, if a supplemental indenture is
         required in connection with such transaction, such supplemental
         indenture, complies with this Article and that all conditions precedent
         herein provided for relating to transaction have been complied with,
         and, with respect to such Officer's Certificate, setting forth the
         manner of determination of the Consolidated Net Worth in accordance
         with Clause (3) of Section 801, of the Company or, if applicable, of
         the Successor Company as required pursuant to the foregoing.

                                      -15-
<PAGE>   16

                           (b)      In the event of any transaction (other than
         a lease) described in and complying with the immediately preceding
         paragraph in which the Company is not the surviving Person and the
         surviving Person assumes all the obligations of the Company under the
         Securities and this Indenture pursuant to a supplemental indenture,
         such surviving Person shall succeed to, and be substituted for, and may
         exercise every right and power of, the Company, and the Company will be
         discharged from its obligations under this Indenture and the
         Securities; PROVIDED that solely for the purpose of calculating amounts
         under Section 1009(iv)(3), any such surviving Person shall only be
         deemed to have succeeded to and be substituted for the Company with
         respect to the period subsequent to the effective time of such
         transaction, and the Company (before giving effect to such transaction)
         shall be deemed to be the "Company" for such purposes for all prior
         periods."

         SECTION 1.5  Amendment of Section 901

         Section 901 of the Indenture is hereby amended by adding the following
phrase immediately after the phrase "which may be defective or inconsistent with
any other provision herein" in paragraph (c) of such Section:

         "or therein or with the intent of the First Supplemental Indenture
hereto, dated June 15, 2000,"

         SECTION 1.6 Amendment of Section 1008.

         Section 1008 of the Indenture is hereby amended by deleting such
section and inserting the following in lieu thereof:

                  "Section 1008.  Limitation On Consolidated Debt.

                  The Company may not, and may not permit any Restricted
         Subsidiary of the Company to, Incur any Debt unless either (a) the
         ratio of: (i) the aggregate consolidated principal amount of Debt of
         the Relevant Entity outstanding as of the most recent available
         quarterly or annual balance sheet, after giving pro forma effect to the
         Incurrence of such Debt and any other Debt Incurred since such balance
         sheet date and the receipt and application of the proceeds thereof to
         (ii) Consolidated Cash Flow Available for Fixed Charges for the four
         full fiscal quarters next preceding the Incurrence of such Debt for
         which consolidated financial statements are available, determined on a
         pro forma basis as if (x) any such Debt had been Incurred and the
         proceeds thereof had been applied at the beginning of such four fiscal
         quarters; (y) the net income (or loss) for such period of any Person or
         related to any assets disposed of by the Relevant Entity or a
         Restricted Subsidiary of the Relevant Entity prior to the end of such
         period had been excluded from Consolidated Net Income; and (z) the net
         income (or loss) for such period of any Person or related to any assets
         acquired by the Relevant Entity or any Restricted Subsidiary prior to
         the end of such period had been included in Consolidated Net Income,
         would be less than 5.5 to 1 for such four-quarter periods ending on or
         prior to December 31, 1999 and 5.0 to 1 for such periods ending
         thereafter; or (b) the Relevant Entity's Consolidated Capital Ratio as
         of the most recent available quarterly or annual balance sheet, after
         giving pro forma effect to the Incurrence of such Debt, any issuance of
         Capital Stock (other than Disqualified Stock) since such balance sheet
         date, any increase in paid in-capital (other than in respect of
         Disqualified Stock) since such balance sheet date and the Incurrence of
         any other Debt since such balance sheet date and the receipt and
         application of the proceeds thereof, is less than 2.0 to 1.

                  Notwithstanding the foregoing limitation, the Company and any
         Restricted Subsidiary may Incur the following:

                                      -16-
<PAGE>   17

                  (i)      Debt under any one or more Bank Credit Agreements or
         Vendor Financing Facilities in an aggregate principal amount at any one
         time not to exceed the greater of:

                  (x)      $250 million and

                  (y)      85% of the Eligible Receivables, and any renewal,
         extension, refinancing or refunding thereof in an amount which,
         together with any principal amount remaining outstanding or available
         under all Bank Credit Agreements and Vendor Financing Facilities of the
         Company and its Restricted Subsidiaries, plus the amount of any premium
         required to be paid in connection with such refinancing pursuant to the
         terms of any Bank Credit Agreement so refinanced plus the amount of
         expenses incurred in connection with such refinancing, does not exceed
         the aggregate principal amount outstanding or available under all such
         Bank Credit Agreements and Vendor Financing Facilities of the Company
         and its Restricted Subsidiaries immediately prior to such renewal,
         extension, refinancing or refunding;

                  (ii)     Purchase Money Debt Incurred to finance the
         construction, acquisition or improvement of Telecommunications Assets,
         PROVIDED that the net proceeds of such Purchase Money Debt do not
         exceed 100% of the cost of construction, acquisition or improvement
         price of the applicable Telecommunications Assets;

                  (iii)    Debt owed by the Company to any Restricted Subsidiary
         of the Company or Debt owed by a Restricted Subsidiary of the Company
         to the Company or a Restricted Subsidiary of the Company; PROVIDED,
         HOWEVER, that upon either (x) the transfer or other disposition by such
         Restricted Subsidiary or the Company of any Debt so permitted to a
         Person other than the Company or another Restricted Subsidiary of the
         Company or (y) the issuance (other than directors' qualifying shares),
         sale, lease, transfer or other disposition of shares of Capital Stock
         (including by consolidation or merger) of such Restricted Subsidiary as
         a result of which the obligor of such Debt ceases to be a Restricted
         Subsidiary, the provisions of this clause (iii) shall no longer be
         applicable to such Debt and such Debt shall be deemed to have been
         Incurred at the time of such transfer or other disposition;

                  (iv)     Debt Incurred to renew, extend, refinance or refund
         (each, a "refinancing") Debt of the Relevant Entity and any of its
         Restricted Subsidiaries outstanding at the Merger Effective Date (after
         giving effect to the Merger) or Incurred by the Relevant Entity
         pursuant to the preceding paragraph or clause (ii) of this paragraph or
         the Securities in an aggregate principal amount not to exceed the
         aggregate principal amount of and accrued interest on the Debt so
         refinanced plus the amount of any premium required to be paid in
         connection with such refinancing pursuant to the terms of the Debt so
         refinanced or the amount of any premium reasonably determined by the
         Company as necessary to accomplish such refinancing by means of a
         tender offer or privately negotiated repurchase, plus the amount of
         expenses of the Company incurred in connection with such refinancing;
         PROVIDED, HOWEVER, that Debt the proceeds of which are used to
         refinance the Securities or Debt which is PARI PASSU to the Securities
         or debt which is subordinate in right of payment to the Securities
         shall only be permitted if:

                  (A)      in the case of any refinancing of the Securities or
         Debt which is PARI PASSU to the Securities, the refinancing Debt is
         made PARI PASSU to the Securities or subordinated to the Securities,
         and, in the case of any refinancing of Debt which is subordinated to
         the Securities, the refinancing Debt constitutes Subordinated Debt and

                                      -17-
<PAGE>   18

                  (B)      in either case, the refinancing Debt by its terms, or
         by the terms of any agreement or instrument pursuant to which such Debt
         is issued, (x) does not provide for payments of principal of such Debt
         at the stated maturity thereof or by way of a sinking fund applicable
         thereto or by way of any mandatory redemption, defeasance, retirement
         or repurchase thereof by the Company (including any redemption,
         retirement or repurchase which is contingent upon events or
         circumstances, but excluding any retirement required by virtue of
         acceleration of such Debt upon any event of default thereunder), in
         each case prior to the time the same are required by the terms of the
         Debt being refinanced and (y) does not permit redemption or other
         retirement (including pursuant to an offer to purchase made by the
         Company) of such Debt at the option of the holder thereof prior to the
         final stated maturity of the Debt being refinanced, other than a
         redemption or other retirement at the option of the holder of such Debt
         (including pursuant to an offer to purchase made by the Company) which
         is conditioned upon a change substantially similar to the provisions of
         Section 1014 or which is pursuant to provisions substantially similar
         to the provisions of Section 1012;

                  (v)      Debt consisting of Permitted Interest Rate and
         Currency Protection Agreements;

                  (vi)     Debt outstanding under the Securities;

                  (vii)    Subordinated Debt invested by: (a) a group of
         employees of the Relevant Entity, which includes the Chief Executive
         Officer of the Relevant Entity, who own, directly or indirectly,
         through an employee stock ownership plan or arrangement, shares of the
         Relevant Entity's Capital Stock or (b) any other Person that controls
         the Relevant Entity (i) on the Merger Effective Date or (ii) after a
         Change of Control, PROVIDED that the Company is not in default with
         respect to its obligations under Section 1016;

                  (viii)   Debt consisting of performance and other similar
         bonds and reimbursement obligations Incurred in the ordinary course of
         business securing the performance of contractual, franchise or license
         obligations of the Company or a Restricted Subsidiary, or in respect of
         a letter of credit obtained to secure such performance; and

                  (ix)     Debt not otherwise permitted to be Incurred pursuant
         to clauses (i) through (viii) above, which, together with any other
         outstanding Debt Incurred pursuant to this clause (ix), has an
         aggregate principal amount (or, in the case of Debt issued at a
         discount, an accreted amount (determined in accordance with generally
         accepted accounting principles) at the time of Incurrence) not in
         excess of $10 million at any time outstanding.

                  For purposes of determining compliance with this Section 1008,
         in the event that an item of Debt meets the criteria of more than one
         of the types of Debt the Company is permitted to incur pursuant to the
         foregoing clauses (i) through (ix) or the first paragraph of this
         Section 1008, the Company shall have the right, in its sole discretion,
         to classify such item of Debt and shall only be required to include the
         amount and type of such Debt under the clause or paragraph permitting
         the Debt as so classified. The determination of any particular amount
         of Debt under such covenant shall be made without duplication for
         Guarantees or Liens supporting Debt otherwise included in the
         determination of a particular amount."

         SECTION 1.7 Amendment of Section 1009.

         Section 1009 of the Indenture is hereby amended by deleting such
section and inserting the following in lieu thereof:

                                      -18-
<PAGE>   19

                  "Section 1009.  Limitation On Restricted Payments.

                  The Company:

                  (i)      may not, directly or indirectly, declare or pay any
         dividend, or make any distribution, in respect of its Capital Stock or
         to the holders thereof (in their capacity as such), excluding any
         dividends or distributions payable solely in shares of its Capital
         Stock (other than Disqualified Stock) or in options, warrants or other
         rights to acquire its Capital Stock (other than Disqualified Stock);

                  (ii)     may not, and may not permit any Restricted Subsidiary
         to, purchase, redeem, or otherwise retire or acquire for value:

                  (a)      any Capital Stock of the Company or any Related
         Person of the Company; or

                  (b)      any options, warrants or rights to purchase or
         acquire shares of Capital Stock of the Company or any Related Person of
         the Company or any securities convertible or exchangeable into shares
         of Capital Stock of the Company or any Related Person of the Company;

                  (iii)    may not make, or permit any Restricted Subsidiary to
         make, any Investment in, or payment on a Guarantee of any obligation
         of, any Person, other than the Company or a Restricted Subsidiary of
         the Company, except for Permitted Investments; and

                  (iv)     may not, and may not permit any Restricted Subsidiary
         to, redeem, defease, repurchase, retire or otherwise acquire or retire
         for value, prior to any scheduled maturity, repayment or sinking fund
         payment, Debt of the Company which is subordinate in right of payment
         to the Securities (each of clauses (i) through (iv) being a "Restricted
         Payment") if:

                  (1)      a Default or an Event of Default shall have occurred
         and is continuing; or

                  (2)      upon giving effect to such Restricted Payment, the
         Company could not Incur at least $1.00 of additional Debt pursuant to
         the provisions of the first paragraph of Section 1008; or

                  (3)      upon giving effect to such Restricted Payment, the
         aggregate of all Restricted Payments made by the Relevant Entity from
         April 25, 1996 exceeds the sum of: (a) 50% of cumulative Consolidated
         Net Income (or, in the case Consolidated Net Income shall be negative,
         less 100% of such deficit) since the end of the last full fiscal
         quarter prior to April 25, 1996 through the last day of the last full
         fiscal quarter ending immediately preceding the date of such Restricted
         Payment; plus (b) $5 million; plus (c) 100% of the net reduction in
         Investments of the Relevant Entity in any Unrestricted Subsidiary since
         the end of the last full fiscal quarter prior to April 25, 1996
         resulting from payments of interest on Debt, dividends, repayments of
         loans or advances, or other transfers of assets, in each case to the
         Relevant Entity or any Restricted Subsidiary of the Relevant Entity
         from such Unrestricted Subsidiary (except to the extent that any such
         payment is included in the calculation of Consolidated Net Income) or
         from redesignations of Unrestricted Subsidiaries as Restricted
         Subsidiaries; PROVIDED that the amount included in this clause (c)
         shall not exceed the amount of Investments previously made by the
         Relevant Entity and its Restricted Subsidiaries in such Unrestricted
         Subsidiary; PROVIDED, FURTHER, that the Company or a Restricted
         Subsidiary of the Company may make any Restricted Payment with the
         aggregate net proceeds received by the Relevant Entity after April 25,
         1996, including the fair value of property other than cash (determined
         in good faith by the Board of Directors, as

                                      -19-
<PAGE>   20

         conclusively evidenced by a Board Resolution (which, with respect to
         such Board Resolutions after the Merger Effective Date, shall be filed
         with the Trustee)), as capital contributions to the Relevant Entity or
         from the issuance by the Relevant Entity (other than to a Restricted
         Subsidiary) of Capital Stock (other than Disqualified Stock) of the
         Relevant Entity and warrants, rights or options on Capital Stock (other
         than Disqualified Stock) of the Relevant Entity and the principal
         amount of Debt of the Relevant Entity that has been converted into
         Capital Stock (other than Disqualified Stock and other than by a
         Restricted Subsidiary) of the Relevant Entity after April 25, 1996.

                  Notwithstanding the foregoing, (i) the Company may pay any
         dividend on Capital Stock of any class within 60 days after the
         declaration thereof if, on the date when the dividend was declared, the
         Company could have paid such dividend in accordance with the foregoing
         provisions; (ii) the Company may repurchase any shares of its Common
         Equity or options to acquire its Common Equity from Persons who were
         formerly officers or employees of the Relevant Entity, PROVIDED that
         the aggregate amount of all such repurchases made pursuant to this
         clause (ii) shall not exceed $2 million, plus the aggregate cash
         proceeds received by the Relevant Entity since April 25, 1996 from
         issuances of its Common Equity or options to acquire its Common Equity
         to members, officers, managers, directors and employees of the Relevant
         Entity or any of its Subsidiaries; (iii) the Company and its Restricted
         Subsidiaries may refinance any Debt otherwise permitted by clause (iv)
         of the second paragraph of Section 1008; and (iv) the Company and its
         Restricted Subsidiaries may retire or repurchase any Capital Stock or
         Subordinated Debt of the Company in exchange for, or out of the
         proceeds of the substantially concurrent sale (other than to a
         Restricted Subsidiary of the Company) of, Capital Stock (other than
         Disqualified Stock) of the Company. If the Company makes a Restricted
         Payment which, at the time of the making of such Restricted Payment,
         would in the good faith determination of the Company be permitted under
         this Indenture, such Restricted Payment shall be deemed to have been
         made in compliance with this Indenture notwithstanding any subsequent
         adjustments made in good faith to the Company financial statements
         affecting Consolidated Net Income for any period.

                  In determining the aggregate amount expended or available for
         Restricted Payments in accordance with clause (3) of the first
         paragraph above, (1) no amounts expended under clauses (iii) or (iv) of
         the immediately preceding paragraph shall be included, (2) 100% of the
         amounts expended under clauses (i) and (ii) of the immediately
         preceding paragraph shall be included, and (3) no amount shall be
         credited in respect of issuances of Capital Stock in transactions under
         clause (iv) of the immediately preceding paragraph."

         SECTION 1.8 Amendment of Section 1010.

         Section 1010 of the Indenture is hereby amended by deleting such
section and inserting the following in lieu thereof:

                  "Section 1010. Transactions With Affiliates And Related
                                    Persons.

                  The Company may not, and may not permit any Restricted
         Subsidiary of the Company to, enter into any transaction (or series of
         related transactions) with an Affiliate or Related Person of the
         Company (other than the Company or a Wholly-Owned Restricted Subsidiary
         of the Company), including any Investment, but excluding transactions
         pursuant to employee compensation arrangements approved by the Board of
         Directors, either directly or indirectly, unless such transaction is on
         terms no less favorable to the Company or such Restricted Subsidiary
         than those that could reasonably be obtained in a comparable
         arm's-length transaction

                                      -20-
<PAGE>   21

         with an entity that is not an Affiliate or Related Person and is in the
         best interests of such Company or such Restricted Subsidiary. For any
         transaction that involves in excess of $1 million but less than or
         equal to $15 million, the Chief Executive Officer of the Company shall
         determine that the transaction satisfies the above criteria and shall
         evidence such a determination by an Officer's Certificate filed with
         the Trustee. For any transaction that involves in excess of $15
         million, the Company shall also either (x) obtain the approval of the
         transaction from the Board of Directors including a majority of the
         disinterested members of the Board of Directors or (y) obtain an
         opinion from a nationally recognized investment bank or other expert
         with experience in appraising the terms and conditions, taken as a
         whole, of the type of transaction (or series of related transactions)
         for which the opinion is required stating that such transaction (or
         series of related transactions) is on terms and conditions, taken as a
         whole, no less favorable to the Company or such Restricted Subsidiary
         than those that could be obtained in a comparable arm's-length
         transaction with an entity that is not an Affiliate or Related Person
         of the Company, which opinion shall be filed with the Trustee. This
         covenant shall not apply to Investments by an Affiliate or a Related
         Person of the Company in the Capital Stock (other than Disqualified
         Stock) of the Company or any Restricted Subsidiary of the Company."

         SECTION 1.9 Amendment of Section 1011.

         Section 1011 of the Indenture is hereby amended by deleting such
section and inserting the following in lieu thereof:

                  "Section 1011.  Limitation On Liens.

                  The Company may not, and may not permit any Restricted
         Subsidiary of the Company to, Incur or suffer to exist any Lien on or
         with respect to any property or assets now owned or hereafter acquired
         to secure any Debt without making, or causing such Restricted
         Subsidiary to make, effective provision for securing the Securities:
         (x) equally and ratably with (or prior to) such Debt as to such
         property for so long as such Debt will be so secured or (y) in the
         event such Debt is Debt of the Company which is subordinate in right of
         payment to the Securities, prior to such Debt as to such property for
         so long as such Debt will be so secured.

                  The foregoing restrictions shall not apply to: (i) Liens of
         the Relevant Entity existing on the Merger Effective Date (after giving
         effect to the Merger) and securing Debt of the Relevant Entity
         outstanding on the Merger Effective Date (after giving effect to the
         Merger) or securing the Securities or Liens securing Debt of the
         Relevant Entity Incurred pursuant to any Bank Credit Agreement or
         Vendor Financing Facility (whether or not such Bank Credit Agreement or
         Vendor Financing Facility was outstanding on the Merger Effective Date
         (after giving effect to the Merger)); (ii) Liens securing Debt in an
         amount which, together with the aggregate amount of Debt then
         outstanding or available under the Bank Credit Agreement and Vendor
         Financing Facility (or under refinancings or amendments of such
         agreements), does not exceed 1.5 times the Company's Consolidated Cash
         Flow Available for Fixed Charges for the four full fiscal quarters
         preceding the Incurrence of such Lien for which consolidated financial
         statements are available, determined on a pro forma basis as if such
         Debt had been Incurred and the proceeds thereof had been applied at the
         beginning of such four fiscal quarters; (iii) Liens in favor of the
         Company or any Wholly-Owned Restricted Subsidiary of the Company; (iv)
         Liens on real or personal property of the Company or a Restricted
         Subsidiary of the Company acquired, constructed or constituting
         improvements made after the Merger Effective Date to secure Purchase
         Money Debt which is Incurred for the construction, acquisition and
         improvement of Telecommunications Assets and is otherwise permitted
         under this Indenture, PROVIDED, HOWEVER, that (a) the net proceeds of
         any Debt secured by such a Lien does not exceed 100% of such purchase
         price or cost of

                                      -21-
<PAGE>   22

         construction or improvement of the property subject to such Lien, (b)
         such Lien attaches to such property prior to, at the time of or within
         180 days after the acquisition, completion of construction or
         commencement of operation of such property and (c) such Lien does not
         extend to or cover any property other than the property (or
         identifiable portions thereof) acquired, constructed or constituting
         the improvements made with the proceeds of such Purchase Money Debt (it
         being understood and agreed that all Debt owed to any single lender or
         group of lenders or outstanding under any single credit facility shall
         be considered a single Purchase Money Debt, whether drawn at one time
         or from time to time); (v) Liens to secure Acquired Debt, PROVIDED,
         HOWEVER, that (a) such Lien attaches to the acquired asset prior to the
         time of the acquisition of such asset and (b) such Lien does not extend
         to or cover any other asset; (vi) Liens to secure Debt Incurred to
         extend, renew, refinance or refund (or successive extensions, renewals,
         refinancings or refundings), in whole or in part, Debt secured by any
         Lien referred to in the foregoing clauses (i), (ii), (iv) and (v) so
         long as such Lien does not extend to any other property and the
         principal amount of Debt so secured is not increased except as
         otherwise permitted under clause (iv) of Section 1008; (vii) Liens
         securing Debt not otherwise permitted by the foregoing clauses (i)
         through (vi) in an amount not to exceed 5% of the Company's
         Consolidated Tangible Assets determined as of the most recent available
         quarterly or annual balance sheet; and (viii) Permitted Liens.

         SECTION 1.10 Amendment of Section 1013.

         Section 1013 of the Indenture is hereby amended by deleting such
section in its entirety.

         SECTION 1.11 Amendment of Section 1015.

         Section 1015 of the Indenture is hereby amended by deleting such
section and inserting the following in lieu thereof:

                  "Section 1015.  Limitation On Sale And Leaseback Transactions.

                  The Company may not, and may not permit any Restricted
         Subsidiary to, enter into any Sale and Leaseback Transaction unless

                  (i)      the Company or such Restricted Subsidiary would be
           entitled to Incur a Lien to secure Debt by reason of the provisions
           of Section 1011, equal in amount to the Attributable Value of the
           Sale and Leaseback Transaction without equally and ratably securing
           the Securities; or

                  (ii)     the Sale and Leaseback Transaction is treated as an
           Asset Sale and all of the conditions of Section 1012 (including the
           provisions concerning the application of Net Cash Proceeds) are
           satisfied with respect to such Sale and Leaseback Transaction,
           treating all of the consideration received in such Sale and Leaseback
           Transaction in the same manner as consideration in respect of an
           Asset Sale for purposes of such covenant."

         SECTION 1.12 Amendment of Section 1016.

         Section 1016 of the Indenture is hereby amended by deleting such
section and inserting the following in lieu thereof:

                                      -22-
<PAGE>   23

                  "Section 1016. Limitation On Issuances And Sales Of Capital
                                    Stock Of Restricted Subsidiaries.

                  The Company may not, and may not permit any Restricted
         Subsidiary of the Company to, issue, transfer, convey, sell or
         otherwise dispose of any shares of Capital Stock of a Restricted
         Subsidiary of the Company or securities convertible or exchangeable
         into, or options, warrants, rights or any other interest with respect
         to, Capital Stock of a Restricted Subsidiary of the Company to any
         person other than the Company or a Wholly-Owned Restricted Subsidiary
         of the Company except (i) in a transaction that complies with the
         provisions of Section 1012; (ii) if required, the issuance, transfer,
         conveyance, sale or other disposition of directors' qualifying shares;
         (iii) in a transaction in which, or in connection with which, the
         Company or a Restricted Subsidiary acquires at the same time sufficient
         Capital Stock of such Restricted Subsidiary to at least maintain the
         same percentage ownership interest it had prior to such transaction;
         (iv) constituting the issuance of Preferred Stock permitted by the
         provisions of Section 1018; and (v) Disqualified Stock issued in
         exchange for, or upon conversion of, or the proceeds of the issuance of
         which are used to redeem, refinance, replace or refund shares of
         Disqualified Stock of such Restricted Subsidiary, provided that the
         amounts of the redemption obligations of such Disqualified Stock shall
         not exceed the amounts of the redemption obligations of, and such
         Disqualified Stock shall have redemption obligations no earlier than
         those required by, the Disqualified Stock being exchanged, converted,
         redeemed, refinanced, replaced or refunded."

         SECTION 1.13 Amendment of Section 1017.

         Section 1017 of the Indenture is hereby amended by deleting such
section and inserting the following in lieu thereof:

                  "Section 1017. Limitation On Dividend And Other Payment
                                    Restrictions Affecting Restricted
                                    Subsidiaries.

                  The Company may not, and may not permit any Restricted
         Subsidiary to, directly or indirectly, create or otherwise cause or
         suffer to exist or become effective any consensual encumbrance or
         restriction on the ability of any Restricted Subsidiary of the Company
         (i) to pay dividends (in cash or otherwise) or make any other
         distributions in respect of its Capital Stock owned by the Company or
         any other Restricted Subsidiary of the Company or pay any Debt or other
         obligation owed to the Company or any other Restricted Subsidiary; (ii)
         to make loans or advances to the Company or any other Restricted
         Subsidiary; or (iii) to transfer any of its property or assets to the
         Company or any other Restricted Subsidiary.

                  Notwithstanding the foregoing, the Company may, and may permit
         any Restricted Subsidiary to, suffer to exist any such encumbrance or
         restriction: (a) pursuant to any agreement of the Relevant Entity in
         effect on the Merger Effective Date (after giving effect to the
         Merger); (b) pursuant to an agreement relating to any Acquired Debt,
         which encumbrance or restriction is not applicable to any Person, or
         the properties or assets of any Person, other than the Person so
         acquired and its subsidiaries; (c) pursuant to any one or more Bank
         Credit Agreements or Vendor Financing Facilities (and renewals,
         extensions, refinancings or refundings thereof) which is permitted to
         be outstanding under clause (i) or (ii) of the second paragraph of
         Section 1008 or clause (vi) or (ix) of Section 1018, PROVIDED that such
         restriction is consistent with, and not materially more restrictive (as
         conclusively determined in good faith by the Chief Financial Officer of
         the Company), taken as a whole, than, comparable provisions included in
         similar agreements or facilities extended to comparable credits engaged
         in the Telecommunications Business (which shall include for purposes of
         this Section 1017 all business in which the

                                      -23-
<PAGE>   24

         Relevant Entity or any of its subsidiaries was engaged on the Merger
         Effective Date (after giving effect to the Merger); provided that, as
         used in this Section 1017, the determination of what constitutes a
         "Telecommunications Business" shall be made in good faith by the Board
         of directors, which determination shall be conclusive) and PROVIDED
         FURTHER that, in the case of any such Bank Credit Agreement or Vendor
         Financing Facility entered into by a Restricted Subsidiary under clause
         (ii) of Section 1008 or clause (ix) of Section 1018, such encumbrances
         or restrictions do not prohibit dividends, distributions, loans or
         advances by such Restricted Subsidiary to the Company or another
         Restricted Subsidiary to the extent that the failure to make such
         distribution, loan or advance would result in the Company defaulting in
         the payment of principal or interest on its indebtedness; (d) pursuant
         to an agreement effecting a renewal, refunding or extension of Debt
         Incurred pursuant to an agreement referred to in clause (a) or (b)
         above or (e) below, PROVIDED, HOWEVER, that the provisions contained in
         such renewal, refunding or extension agreement relating to such
         encumbrance or restriction are not materially more restrictive (as
         conclusively determined in good faith by the Chief Financial Officer of
         the Company), taken as a whole, than the provisions contained in the
         agreement the subject thereof; (e) in the case of clause (c) above,
         restrictions contained in any security agreement (including a Capital
         Lease Obligation) securing Debt of the Company or a Restricted
         Subsidiary otherwise permitted under this Indenture, but only to the
         extent such restrictions restrict the transfer of the property subject
         to such security agreement; (f) in the case of clause (c) above,
         customary nonassignment provisions entered into in the ordinary course
         of business in leases and other agreements; (g) any restriction with
         respect to a Restricted Subsidiary of the Company imposed pursuant to
         an agreement which has been entered into for the sale or disposition of
         all or substantially all of the Capital Stock or assets of such
         Restricted Subsidiary, provided that consummation of such transaction
         would not result in a Default or an Event of Default, that such
         restriction terminates if such transaction is not consummated and that
         such consummation or abandonment of such transaction occurs within one
         year of the date such agreement was entered into; (h) pursuant to
         applicable law or regulations; (i) pursuant to this Indenture and the
         Securities; or (j) any restriction on the sale or other disposition of
         assets or property securing Debt as a result of a Permitted Lien on
         such assets or property."

         Section 1.14 Amendment to Section 1018.

         Section 1018 of the Indenture is hereby amended by deleting such
section and inserting the following in lieu thereof:

                  "Section 1018. Limitation On Debt And Preferred Stock Of
                                    Restricted Subsidiaries.

                  The Company may not permit any Restricted Subsidiary of the
         Company (other than a Restricted Subsidiary that has fully and
         unconditionally Guaranteed the Securities on an unsubordinated basis)
         to Incur or suffer to exist any Debt or issue any Preferred Stock
         except:

                  (i)      Debt or Preferred Stock of the Relevant Entity
           outstanding on the Merger Effective Date (after giving effect to the
           Merger);

                  (ii)     Debt Incurred or Preferred Stock issued to and held
           by the Company or a Restricted Subsidiary of the Company (provided
           that such Debt or Preferred Stock is at all times held by the Company
           or a Restricted Subsidiary of the Company);

                  (iii)    Debt Incurred or Preferred Stock issued by a Person
           prior to the time:

                  (A)      such Person became a Restricted Subsidiary of the
           Company,

                                      -24-
<PAGE>   25

                  (B)      such Person merges into or consolidates with a
           Restricted Subsidiary of the Company, or

                  (C)      another Restricted Subsidiary of the Company merges
           into or consolidates with such Person (in a transaction in which such
           Person becomes a Restricted Subsidiary of the Company), which Debt or
           Preferred Stock was not Incurred or issued in anticipation of such
           transaction and was outstanding prior to such transaction;

                  (iv)     Debt consisting of Permitted Interest Rate and
           Currency Protection Agreements;

                  (v)      Debt or Preferred Stock of a Joint Venture;

                  (vi)     Debt under any one or more Bank Credit Agreements or
           Vendor Financing Facilities (and renewals, extensions, refinancings
           or refundings thereof) which is permitted to be outstanding under
           clause (i) of Section 1008;

                  (vii)    Debt consisting of Guarantees of the Securities;

                  (viii)   Debt or Preferred Stock which is exchanged for, or
           the proceeds of which are used to refinance, refund or redeem, any
           Debt or Preferred Stock permitted to be outstanding pursuant to
           clauses (i), (iii) and (ix) hereof (or any extension or renewal
           thereof) (for purposes hereof, a "refinancing"), in an aggregate
           principal amount, in the case of Debt, or with an aggregate
           liquidation preference, in the case of Preferred Stock, not to exceed
           the aggregate principal amount of the Debt so refinanced or the
           aggregate liquidation preference of the Preferred Stock so
           refinanced, plus the amount of any premium required to be paid in
           connection with such refinancing pursuant to the terms of the Debt or
           Preferred Stock so refinanced or the amount of any premium reasonably
           determined by the Company as necessary to accomplish such refinancing
           by means of a tender offer or privately negotiated repurchase, plus
           the amount of expenses of the Company and the Restricted Subsidiary
           incurred in connection therewith and provided the Debt or Preferred
           Stock incurred or issued upon such refinancing by its terms, or by
           the terms of any agreement or instrument pursuant to which such Debt
           or Preferred Stock is Incurred or issued, (x) does not provide for
           payments of principal or liquidation value at the stated maturity of
           such Debt or Preferred Stock or by way of a sinking fund applicable
           to such Debt or Preferred Stock or by way of any mandatory
           redemption, defeasance, retirement or repurchase of such Debt or
           Preferred Stock by the Company or any Restricted Subsidiary of the
           Company (including any redemption, retirement or repurchase which is
           contingent upon events or circumstances, but excluding any retirement
           required by virtue of acceleration of such Debt upon an event of
           default thereunder), in each case prior to the time the same are
           required by the terms of the Debt or Preferred Stock being refinanced
           and (y) does not permit redemption or other retirement (including
           pursuant to an offer to purchase made by the Company or a Restricted
           Subsidiary of the Company) of such Debt or Preferred Stock at the
           option of the holder thereof prior to the stated maturity of the Debt
           or Preferred Stock being refinanced, other than a redemption or other
           retirement at the option of the holder of such Debt or Preferred
           Stock (including pursuant to an offer to purchase made by the Company
           or a Restricted Subsidiary of the Company) which is conditioned upon
           the change of control of the Company pursuant to provisions
           substantially similar to the provisions of Section 1014 or which is
           pursuant to provisions substantially similar to the provisions of
           Section 1012, and PROVIDED, FURTHER, that in the case of any exchange
           or redemption of Preferred Stock of a Restricted Subsidiary of the
           Company, such Preferred Stock may only be exchanged for or redeemed
           with Preferred Stock of such Restricted Subsidiary;

                                      -25-
<PAGE>   26

                  (ix)     Purchase Money Debt Incurred to finance the
           construction, acquisition or improvement of Telecommunications
           Assets, PROVIDED that the net proceeds of such Purchase Money Debt do
           not exceed 100% of the cost of construction, acquisition or
           improvement price of the applicable Telecommunications Assets; and

                  (x)      Debt consisting of performance and other similar
           bonds and reimbursement obligations Incurred in the ordinary course
           of business securing the performance of contractual, franchise or
           license obligations of the Company or a Restricted Subsidiary, or in
           respect of a letter of credit obtained to secure such performance;
           and

                  (xi)     Debt not otherwise permitted to be incurred pursuant
           to clauses (i) through (x) above, which, together with any other
           outstanding Debt incurred pursuant to this clause (xi), has an
           aggregate principal amount (or, in the case of Debt issued at a
           discount, an accreted amount (determined in accordance with generally
           accepted accounting principles) at the time of Incurrence) not in
           excess of $10 million at any time outstanding.

                  For purposes of determining compliance with this Section
           1018, in the event that an item of Debt meets the criteria of more
           than one of the types of Debt a Restricted Subsidiary of the Company
           is permitted to incur pursuant to the foregoing clauses (i) through
           (xi), the Company shall have the right, in its sole discretion, to
           classify such item of Debt and shall be only required to include the
           amount and type of such Debt under the clause permitting the Debt as
           so classified. The determination of any particular amount of Debt
           under such covenant shall be made without duplication for Guarantees
           or Liens supporting Debt otherwise included in the determination of a
           particular amount."

         Section 1.15 Amendment to Section 1022.

                  Section 1022 of the Indenture is hereby amended by deleting
         such section in its entirety.

                                   ARTICLE TWO

                                  MISCELLANEOUS

         SECTION 2.1. Effect. The provisions set forth in this Supplemental
Indenture shall be deemed to be, and shall be construed as part of, the
Indenture to the same extent as if set forth fully therein. This Supplemental
Indenture shall become effective at the effective time of the Merger at which
time the surviving Corporation of the Merger will execute and deliver a second
supplemental indenture assuming the obligations of the Company hereunder. The
effectiveness of this Supplemental Indenture is expressly conditioned on the
consummation of the Merger, and shall be of no force or effect in the event that
the Merger is not consummated. All references to the "Indenture" in the
Indenture or in any other agreement, document or instrument delivered in
connection therewith or pursuant thereto shall be deemed to refer to the
Indenture as amended by this Supplemental Indenture. Except as amended hereby,
the Indenture shall remain in full force and effect.

         SECTION 2.2. Definition of Subsidiary. Notwithstanding any provision of
this Supplemental Indenture, the Indenture is hereby further amended so that all
references to "Subsidiary" in the Indenture shall be deemed to be references to
"Restricted Subsidiary," except that the term "Subsidiary" as used in Section
1012 of the Indenture shall not be so amended but shall mean "Subsidiary" as
defined in the Indenture.

                                      -26-
<PAGE>   27

         SECTION 2.3. Trust Indenture Act Controls. If any provision of this
Supplemental Indenture limits, qualifies or conflicts with another provision
that is required by or deemed to be included in this Supplemental Indenture by
the Trust Indenture Act (as such term is defined in the Indenture), the required
or incorporated provision shall control.

         SECTION 2.4. Governing Law. THIS SUPPLEMENTAL INDENTURE SHALL BE
GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK
(WITHOUT GIVING EFFECT TO THE CONFLICT OF LAWS PRINCIPLES THEREOF).

         SECTION 2.5. Counterparts. This Supplemental Indenture may be executed
in any number of counterparts, each of which shall be original, but such
counterparts shall together constitute but one and the same instrument.

         SECTION 2.6. Severability. In case any provision in this Supplemental
Indenture shall be invalid, illegal or unenforceable, the validity, legality and
enforceability of the remaining provisions shall not in any way be affected or
impaired thereby.

         SECTION 2.7. Effect of Headings. The Article and Section headings
herein are for convenience only and shall not affect the construction hereof.

                                      -27-
<PAGE>   28

                  IN WITNESS WHEREOF, the parties hereto have caused this
Supplemental Indenture to be duly executed as of the day and year first above
written.

                                       CONCENTRIC NETWORK CORPORATION
                                       By /s/ Henry R. Nothhaft
                                         --------------------------------------
                                           Name: Henry R. Nothhaft
                                           Title: Chairman, President and CEO

                                       CHASE MANHATTAN BANK AND TRUST
                                       COMPANY, NATIONAL ASSOCIATION, as
                                       Trustee

                                       By /s/ Cecil D. Bobey
                                         --------------------------------------
                                           Name: Cecil D. Bobey
                                           Title: AVP<PAGE>   1

                                                                  EXHIBIT 4.11.3

================================================================================

                         CONCENTRIC NETWORK CORPORATION

                                     Issuer

                                       and

                              NM ACQUISITION CORP.

                  (to be the successor by merger to the Issuer)

                                       and

                    CHASE MANAHATTAN BANK AND TRUST COMPANY,
                              NATIONAL ASSOCIATION

                                     Trustee

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

                          SECOND SUPPLEMENTAL INDENTURE

                            Dated as of June 15, 2000

                                    Amending

                                    INDENTURE

                          Dated as of December 18, 1997

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

                                  $150,000,000

                          12 3/4% Senior Notes Due 2007

================================================================================

<PAGE>   2

         THIS SECOND SUPPLEMENTAL INDENTURE, dated as of June 15, 2000 (herein
called the "Supplement"), is by and among CONCENTRIC NETWORK CORPORATION, a
corporation organized and existing under the laws of the State of Delaware
("Concentric" or the "Issuer"), having its principal office at 1400 Parkmoor
Avenue, San Jose, CA 95126, NM ACQUISITION CORP. (to be known, following the
Merger (as defined below), as NEXTLINK Communications, Inc.), a corporation
organized and existing under the laws of the State of Delaware (the "Company"),
having its principal office at 1505 Farm Credit Drive, McLean, VA 22102, and
CHASE MANHATTAN BANK AND TRUST COMPANY, NATIONAL ASSOCIATION, a national banking
association, as trustee (herein called the "Trustee").

                                    RECITALS

         WHEREAS, pursuant to the terms of the Indenture, dated as of December
18, 1997 (the "Original Indenture") (as supplemented by the First Supplemental
Indenture, dated as of even date herewith (the "First Supplemental Indenture"),
herein called the "Indenture"), between Concentric and the Trustee, $150,000,000
principal amount of 12 3/4% Senior Notes due 2007 (herein called the
"Securities") were issued; and

         WHEREAS, pursuant to the terms of that certain Amended and Restated
Agreement and Plan of Merger and Share Exchange Agreement, dated as of May 10,
2000, by and among Concentric, NEXTLINK Communications, Inc., a corporation
organized and existing under the laws of the State of Delaware ("NEXTLINK"),
Eagle River Investments, L.L.C., a Washington limited liability company, Craig
O. McCaw and the Company, NEXTLINK will be merged with and into the Company (at
which time the Company will change its name to "NEXTLINK Communications, Inc.")
and, immediately thereafter, Concentric will be merged with and into the
Company, in each case with the Company as the surviving corporation (the
"Merger"); and

         WHEREAS, the First Supplemental Indenture and this Supplement will
become effective upon consummation of the Merger; and

         WHEREAS, pursuant and subject to the provisions of Sections 801 and 901
of the Original Indenture and assuming the requirements of such Sections are
satisfied, Concentric is permitted, without the consent of the Holders of the
Securities, to merge with and into the Company through a merger transaction in
which Concentric is not the surviving corporation, provided that the Company, as
the surviving Company in the Merger, assumes, by execution of a supplemental
indenture, all of the obligations of Concentric under the Securities, the
Indenture, the Escrow Agreement (as defined in the Original Indenture) and the
Registration Rights Agreement (as defined in the Original Indenture);

         WHEREAS, pursuant to the provisions of Section 801 the Company wishes
by this Supplement to evidence its succession to Concentric and its assumption,
upon consummation of the Merger, of the obligations of Concentric under the
Securities, the Indenture, the Escrow Agreement and the Registration Rights
Agreement; and

         WHEREAS, all things necessary to make this Supplement, when executed
and delivered by the Trustee, the valid agreement of the Company in accordance
with its terms have been done.

         NOW, THEREFORE, THIS SUPPLEMENTAL INDENTURE WITNESSETH:

<PAGE>   3

         SECTION 101. Definitions. Except as otherwise expressly provided
herein, all capitalized words and terms used herein shall have the respective
meanings ascribed thereto in Article One of the Indenture.

         SECTION 102. Representations of the Company. The Company hereby
represents and warrants to the Trustee that as of the date hereof:

         (a) The Company is a corporation validly existing and in good standing
under the laws of the State of Delaware; and

         (b) No Default or Event of Default will result from the Merger or the
execution and delivery of this Supplement.

         SECTION 103. Assumption of Obligations. The Company hereby assumes,
upon consummation of the Merger, all of the obligations of Concentric under the
Securities, the Indenture, the Escrow Agreement and the Registration Rights
Agreement (to the extent that any obligations of Concentric remain under such
agreement).

         SECTION 104. Construction with Original Indenture. All of the
covenants, agreements and provisions of this Supplement shall be deemed to be
and construed as part of the Indenture and VICE VERSA to the same extent as if
fully set forth verbatim therein and herein and shall be fully enforceable in
the manner provided in the Indenture. Except as provided in this Supplement, the
Indenture shall remain in full force and effect and the terms and conditions
thereof are hereby confirmed.

         SECTION 105. Conflict with Trust Indenture Act. If any provision hereof
limits, qualifies or conflicts with a provision of the Trust Indenture Act that
is required under such Act to be part of and govern the Indenture or this
Supplement, the latter provision shall control. If any provision hereof modifies
or excludes any provision of the Trust Indenture Act that may be so modified or
excluded, the latter provision shall be deemed to apply to this Supplement as so
modified or to be excluded, as the case may be.

         SECTION 106. Effect of Headings. The Section headings herein are for
convenience only and shall not affect the construction hereof.

         SECTION 107. Separability Clause. In case any provision in this
Supplement shall be invalid, illegal or unenforceable, the validity, legality
and enforceability of the remaining provisions shall not in any way be affected
or impaired thereby, it being intended that all of the provisions hereof shall
be enforceable to the full extent permitted by law.

         SECTION 108. Benefits of Supplement and Indenture. Nothing in this
Supplement or the Indenture or in the Securities, the Escrow Agreement or the
Registration Rights Agreement, express or implied, shall give to any Person
other than the parties hereto and thereto and their successors hereunder and
thereunder and the Holders of Securities, any benefit or any legal or equitable
right, remedy or claim under this Supplement or the Indenture. Neither this
Supplement nor the Indenture may be used to interpret another indenture, loan
agreement or debt agreement of the Company or any of its Subsidiaries. No such
other indenture or loan or debt agreement may be utilized to interpret this
Supplement or the Indenture.

<PAGE>   4

         SECTION 109. GOVERNING LAW. THIS SUPPLEMENT SHALL BE GOVERNED BY AND
CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.

         SECTION 110. No Recourse Against Others. A director, member, managing
member officer, employee, stockholder or incorporator, as such, of the Issuer or
the Company shall not have any liability for any obligations of the Company
under this Supplement or for any claim based on, in respect or by reason of such
obligations or their creation.

         SECTION 111. Duplicate Originals. All parties may sign any number of
copies or counterparts of this Supplement. Each signed copy or counterpart shall
be an original, but all of them together shall represent the same agreement.

                            [Signature pages follow]

<PAGE>   5

         SECTION 112. Effectiveness. This Supplement shall become effective upon
consummation of the Merger and otherwise in accordance with the provisions of
Article Nine of the Original Indenture., and shall be of no force and effect if
the Merger is not consummated by the End Date (as defined in the Merger
Agreement).

         IN WITNESS WHEREOF, the parties hereto have caused this Supplement to
be duly executed as of the day and year first above written.

                                       CONCENTRIC NETWORK CORPORATION

                                       By: /s/ Henry R. Nothhaft
                                          --------------------------------
                                             Name: Henry R. Nothhaft
                                             Title: Chairman, President and CEO

Attest:

 /s/ Peter J. Bergeron
-----------------------------
Name: Peter J. Bergeron
Title: Corporate Secretary

                                       NM ACQUISITION CORP.

                                       By: /s/ Gary D. Begeman
                                          --------------------------------------
                                           Name: Gary D. Begeman
                                           Title: Vice President

Attest:

/s/ Richard Montfort
-----------------------------
Name: Richard Montfort
Title: Assistant Secretary

                                       CHASE MANHATTAN BANK AND
                                       TRUST COMPANY, NATIONAL
                                       ASSOCIATION, Trustee

                                       By: /s/ Cecil D. Bobey
                                          --------------------------------------
                                               Name: Cecil D. Bobey
                                               Title: AVP
Attest:

N/A
-----------------------------
Name:
Title:

                [Signature page to Second Supplemental Indenture]

<PAGE>   6

STATE OF CALIFORNIA)

                    )ss.:

COUNTY OF SANTA CLARA)

         On the 15th day of June, 2000, before me personally came Henry
Nothhaft, to me known, who, being by me duly sworn, did depose and say that he
is the Chairman, President and CEO of Concentric Network Corporation, a Delaware
corporation, one of the corporations described in and which executed the
foregoing instrument, and duly acknowledged to me that he executed the same by
authority of the Board of Directors of said corporation.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal the
day and year first above written.

                                       /s/ Teresa F. Foye
                                       ----------------------------------------
                                       NOTARY PUBLIC in and for the State of
                                       California, residing at Santa Clara
                                                               ----------------
                                       My appointment expires 1-11-02
                                                             ------------------
                                       Print Name Teresa F. Foye
                                                 ------------------------------

STATE OF VIRGINIA     )

                      )ss.:

COUNTY OF FAIRFAX          )

         On the 15th day of June, 2000, before me personally came Gary D.
Begeman, to me known, who, being by me duly sworn, did depose and say that
he/she is the Vice President of NM Acquisition Corp., a Delaware corporation,
one of the corporations described in and which executed the foregoing
instrument, and duly acknowledged to me that he executed the same by authority
of the Board of Directors of said corporation.

         IN WITNESS WHEREOF, I have hereunto set my hand and official seal the
day and year first above written.

                                       /s/ Cathryn O. Lowe
                                       ----------------------------------------
                                       NOTARY PUBLIC in and for the State of
                                       Virginia, residing at Fairfax, VA
                                                             ------------------
                                       My appointment expires 08-31-04
                                                             ------------------
                                       Print Name Cathryn O. Lowe
                                                  -----------------------------

                [Signature page to Second Supplemental Indenture]

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