content: <PAGE>
  
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                        SECURITIES AND EXCHANGE COMMISSION
  
                              WASHINGTON, D.C. 20549
  
                                     FORM 8-K
  
                 CURRENT REPORT PURSUANT TO SECTION 13 OR 15(d) OF
                        THE SECURITIES EXCHANGE ACT OF 1934
  
        Date of Report (Date of earliest event reported):  January 09, 2000
  
                          CONCENTRIC NETWORK CORPORATION
  
                             _________________________
  
              (Exact name of Registrant as specified in its charter)
  
              Delaware                  000-22575                65-0257497
              --------                  ---------                ------------
  (State or other jurisdiction of     (Commission             (I.R.S. Employer
  incorporation or organization)      File Number)          Identification Number)
  
     1400 Parkmoor Avenue                                      94563
     San Jose, California 95126                              (Zip Code)
     (Address of Principal Executive
                Offices)
  
        Registrant's telephone number, including area code: (408) 817-2800
  
                                  Not Applicable
                           -----------------------------
           (Former name or former address, if changed since last report)
  
  
  ===============================================================================
  <PAGE>
  
  ITEM 5.  OTHER EVENTS
  
       On January 9, 2000 Concentric Network Corporation ("Concentric"), NEXTLINK
  Communications, Inc., a Delaware corporation ("NEXTLINK"), Eagle River
  Investments, L.L.C., a Washington limited liability company ("Eagle River") and
  NM Acquisition Corp., a Delaware corporation ("Newco") entered into an Agreement
  and Plan of Merger and Share Exchange Agreement (the "Agreement"), pursuant to
  which, subject to the terms and conditions set forth therein, NEXTLINK shall be
  merged with and into Newco and immediately thereafter Concentric shall be merged
  with and into Newco (the "Merger"), with Newco as the surviving corporation in
  the Merger (the "Surviving Corporation").  As a result of the Merger, among
  other things, each outstanding share of Concentric common stock shall be
  exchanged for that number of shares of Newco common stock determined by dividing
  $45.00 by the 20 day average trading price of NEXTLINK common stock prior to
  closing, but will not be less than .495 shares of Newco common stock (if
  NEXTLINK's stock price exceeds $90.91), or more than .650 shares of Newco common
  stock (if NEXTLINK's average stock price is less than $69.23).  It is
  contemplated that Newco will be renamed NEXTLINK Communications, Inc. following
  the Merger and its shares will trade on the Nasdaq National Market under the
  symbol "NXLK", currently the designation of NEXTLINK's common stock.
  
  
       In connection with the Agreement, NEXTLINK entered into individual Voting
  Agreements dated as of January 9, 2000 (the "Voting Agreements") with certain
  stockholders of Concentric (the "Concentric Stockholders").  The Voting
  Agreements relate to approximately 778,192 shares of Concentric Common Stock.
  Pursuant to the Voting Agreements, the Concentric Stockholders have agreed to
  vote all of the shares of Concentric Common Stock they own as of January 9, 2000
  and continue to hold at the time of the meeting of Concentric stockholders held
  to vote on the merger, in favor of the Merger and for approval and adoption of
  the Agreement.  Pursuant to the Agreement, Concentric has also entered into a
  voting agreement with Eagle River, in which Eagle River has agreed to deliver
  its consent as majority stockholder of NEXTLINK approving the Merger.
  
       The Boards of Directors of NEXTLINK and Concentric have unanimously
  approved the Merger.  The consummation of the transactions contemplated by the
  Agreement is subject to certain conditions, including (i) approval of the Merger
  by the holders of a majority of the shares of Concentric Common Stock at a
  special meeting of stockholders of Concentric (the "Concentric Special
  Meeting"); and (ii) certain other customary conditions in a transaction of this
  nature, including receipt of all necessary regulatory approvals.  A joint proxy
  statement/prospectus relating to the Merger and the shares to be issued
  thereunder will be distributed to the stockholders of Concentric prior to the
  Concentric Special Meeting.  After the Merger, the Concentric Common Stock will
  be delisted from trading on the Nasdaq National Market, and registration of the
  Concentric Common Stock will be terminated pursuant to Section 12(g)(4) of the
  Securities Exchange Act of 1934.  Concentric and NEXTLINK expect to obtain
  consent of Concentric's bond and preferred stock holders to the Merger, but have
  reserved the right to restructure the transaction so that these consents will
  not be required.
  
       A copy of each of the Agreement, dated January 9, 2000, and the joint press
  release of Concentric and NEXTLINK, dated January 10, 2000, issued in connection
  with execution of the Agreement is filed as an exhibit hereto and is
  incorporated herein by reference.
  
       (c)  EXHIBITS
  
            2.1   Agreement and Plan of Merger and Share Exchange Agreement dated
                  as of January 9, 2000, by and among Concentric Network
                  Corporation, NEXTLINK Communications, Inc., Eagle River
                  Investments, LLC, and NM Acquisition Corp.
  
                                        -2-
  <PAGE>
  
            99.1  Joint Press Release of Concentric Network Corporation and
                 NEXTLINK Communications, Inc., dated January 10, 2000.
  
                                        -3-
  <PAGE>
  
                                     SIGNATURE
  
       Pursuant to the requirements of the Securities Exchange Act of 1934, the
  registrant has duly caused this report to be signed on its behalf by the
  undersigned hereunto duly authorized.
  
                                CONCENTRIC NETWORK CORPORATION
  
  Date: January 12, 2000        By: /s/ Michael Anthofer
                                    -----------------------------------------
                                    Michael Anthofer
                                    Senior Vice President and Chief Financial
                                    Officer
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text: ================================================================================
  
                        SECURITIES AND EXCHANGE COMMISSION
  
                              WASHINGTON, D.C. 20549
  
                                     FORM 8-K
  
                 CURRENT REPORT PURSUANT TO SECTION 13 OR 15(d) OF
                        THE SECURITIES EXCHANGE ACT OF 1934
  
        Date of Report (Date of earliest event reported):  January 09, 2000
  
                          CONCENTRIC NETWORK CORPORATION
  
                             _________________________
  
              (Exact name of Registrant as specified in its charter)
  
              Delaware                  000-22575                65-0257497
              --------                  ---------                ------------
  (State or other jurisdiction of     (Commission             (I.R.S. Employer
  incorporation or organization)      File Number)          Identification Number)
  
     1400 Parkmoor Avenue                                      94563
     San Jose, California 95126                              (Zip Code)
     (Address of Principal Executive
                Offices)
  
        Registrant's telephone number, including area code: (408) 817-2800
  
                                  Not Applicable
                           -----------------------------
           (Former name or former address, if changed since last report)
  
  ===============================================================================
  ITEM 5.  OTHER EVENTS
  
       On January 9, 2000 Concentric Network Corporation ("Concentric"), NEXTLINK
  Communications, Inc., a Delaware corporation ("NEXTLINK"), Eagle River
  Investments, L.L.C., a Washington limited liability company ("Eagle River") and
  NM Acquisition Corp., a Delaware corporation ("Newco") entered into an Agreement
  and Plan of Merger and Share Exchange Agreement (the "Agreement"), pursuant to
  which, subject to the terms and conditions set forth therein, NEXTLINK shall be
  merged with and into Newco and immediately thereafter Concentric shall be merged
  with and into Newco (the "Merger"), with Newco as the surviving corporation in
  the Merger (the "Surviving Corporation").  As a result of the Merger, among
  other things, each outstanding share of Concentric common stock shall be
  exchanged for that number of shares of Newco common stock determined by dividing
  $45.00 by the 20 day average trading price of NEXTLINK common stock prior to
  closing, but will not be less than .495 shares of Newco common stock (if
  NEXTLINK's stock price exceeds $90.91), or more than .650 shares of Newco common
  stock (if NEXTLINK's average stock price is less than $69.23).  It is
  contemplated that Newco will be renamed NEXTLINK Communications, Inc. following
  the Merger and its shares will trade on the Nasdaq National Market under the
  symbol "NXLK", currently the designation of NEXTLINK's common stock.
  
       In connection with the Agreement, NEXTLINK entered into individual Voting
  Agreements dated as of January 9, 2000 (the "Voting Agreements") with certain
  stockholders of Concentric (the "Concentric Stockholders").  The Voting
  Agreements relate to approximately 778,192 shares of Concentric Common Stock.
  Pursuant to the Voting Agreements, the Concentric Stockholders have agreed to
  vote all of the shares of Concentric Common Stock they own as of January 9, 2000
  and continue to hold at the time of the meeting of Concentric stockholders held
  to vote on the merger, in favor of the Merger and for approval and adoption of
  the Agreement.  Pursuant to the Agreement, Concentric has also entered into a
  voting agreement with Eagle River, in which Eagle River has agreed to deliver
  its consent as majority stockholder of NEXTLINK approving the Merger.
  
       The Boards of Directors of NEXTLINK and Concentric have unanimously
  approved the Merger.  The consummation of the transactions contemplated by the
  Agreement is subject to certain conditions, including (i) approval of the Merger
  by the holders of a majority of the shares of Concentric Common Stock at a
  special meeting of stockholders of Concentric (the "Concentric Special
  Meeting"); and (ii) certain other customary conditions in a transaction of this
  nature, including receipt of all necessary regulatory approvals.  A joint proxy
  statement/prospectus relating to the Merger and the shares to be issued
  thereunder will be distributed to the stockholders of Concentric prior to the
  Concentric Special Meeting.  After the Merger, the Concentric Common Stock will
  be delisted from trading on the Nasdaq National Market, and registration of the
  Concentric Common Stock will be terminated pursuant to Section 12(g)(4) of the
  Securities Exchange Act of 1934.  Concentric and NEXTLINK expect to obtain
  consent of Concentric's bond and preferred stock holders to the Merger, but have
  reserved the right to restructure the transaction so that these consents will
  not be required.
  
       A copy of each of the Agreement, dated January 9, 2000, and the joint press
  release of Concentric and NEXTLINK, dated January 10, 2000, issued in connection
  with execution of the Agreement is filed as an exhibit hereto and is
  incorporated herein by reference.
  
       (c)  EXHIBITS
  
            2.1   Agreement and Plan of Merger and Share Exchange Agreement dated
                  as of January 9, 2000, by and among Concentric Network
                  Corporation, NEXTLINK Communications, Inc., Eagle River
                  Investments, LLC, and NM Acquisition Corp.
  
                                        -2-
  99.1  Joint Press Release of Concentric Network Corporation and
                 NEXTLINK Communications, Inc., dated January 10, 2000.
  
                                        -3-
  SIGNATURE
  
       Pursuant to the requirements of the Securities Exchange Act of 1934, the
  registrant has duly caused this report to be signed on its behalf by the
  undersigned hereunto duly authorized.
  
                                CONCENTRIC NETWORK CORPORATION
  
  Date: January 12, 2000        By: /s/ Michael Anthofer
                                    -----------------------------------------
                                    Michael Anthofer
                                    Senior Vice President and Chief Financial
                                    Officer