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                                                                  EXHIBIT 4.11.3


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                         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

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

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         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:


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         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.


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         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]


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         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]


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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]