MASTER AGREEMENT FOR MCI ENHANCED SERVICES

        This Master Agreement for MCI Enhanced Services, together with the ESA
Schedules annexed hereto (collectively, the "Agreement"), is made, effective as
of the Effective Date (as defined in Section 2 below), by and between MCI
Telecommunications Corporation and its appropriate affiliated companies
(collectively, "MCI") and Concentric Network Corporation with offices located at
10590 N. Tantau Avenue, Cupertino, California 95014 ("Customer").

1.      MCI Enhanced Service(s).
        ------------------------

1.1     As used in this Agreement, the term "MCI Enhanced Service(s)" means all
commercially available MCI non-Tariffed services and associated equipment
provided by MCI to Customer pursuant to this Agreement and for which an ESA
Schedule has been annexed hereto, and shall not include any MCI Tariffed
services provided pursuant to any filed tariff of MCI or an MCI affiliated
company. The descriptions of MCI Enhanced Services set forth in the respective
ESA Schedules are subject to revision by MCI from time to time.

2.      Service Term: Ramp Period
        -------------------------

2.1     The service term of this Agreement shall commence upon the Effective
Date and end thirty-six (36) months after the expiration of the Ramp Period (as
defined below) (the "Term"). After the expiration of the initial Term, the Term
shall be automatically extended on a month-to-month basis until such time as
either party provides the other with at least ninety (90) days prior written,
notice of termination. The "Effective Date" shall mean the first day of the
first full calendar month following the date upon which this Agreement is
executed by MCI, as indicated on the signature lines below

2.2     The Ramp Period shall begin on the Effective Date and end upon the
earlier of:

2.2.1   thirty (30) days after the date of Customer's written notice to MCI that
the Ramp Period be deemed terminated; or

2.2.2   eight (8) months after the Effective Date.

3.      Revenue Minimum: Underutilization Charges: New Technology
        ---------------------------------------------------------

3.1     During each annual billing period of the Term beginning after the
        expiration of the Ramp Period, (each, an "Annual Period"). Customer's
        Net Usage under this Agreement shall equal or exceed [*] (the "MVR").
        For purposes of this Agreement. "Net Usage" means recurring


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has been requested with respect to the omitted portions.

 
        charges accruing to Customer's account, after application of all
        discounts and credits, including without limitation charges for usage of
        services provided by MCI and for the rental and/or maintenance of
        equipment provided by MCI and excluding without limitation all charges
        expressly excluded in the applicable ESA Schedule and all taxes and
        surcharges.

3.2     If Customer does not satisfy the MVR for any Annual Period, then
        Customer will pay to MCI an underutilization charge (which Customer
        agrees is reasonable) equal to the [*].

3.3     Customer will not be liable for underutilization charges pursuant to
        Paragraph 3.2 above where such underutilization charges arise solely as
        a result of a [*], provided that in any case Customer shall use its best
        reasonable efforts to: (1) direct to MCI new traffic or traffic not
        currently carried by MCI in order to meet the MVR, and (2) retain MCI as
        the provider of the service required pursuant to the [*]. In the event
        Customer's Net Usage falls below [*] of the MVR during any Annual Period
        as the result of a New Technology Change. Customer and MCI shall use
        good faith efforts to renegotiate the terms and conditions of this
        Agreement, including but not limited to all rates and discounts;
        provided, however, that if a new agreement is not reached within sixty
        (60) days after MCI's notice to Customer that Customer's Net Usage has
        fallen below [*] of the MVR during an Annual Period, MCI may terminate
        this Agreement without liability to Customer, upon ninety (90) days
        prior written notice to Customer.

4.      Provision of ESA Services.
        ------------------------- 

4.1     Each MCI Enhanced Service provided under this Agreement shall have a
        corresponding ESA Schedule specifying the applicable rates, discounts
        and other terms and conditions on which MCI will provide such MCI
        Enhanced Service.

4.2     For all domestic and international access services provided in
        conjunction with the MCI Enhanced Services, MCI shall be entitled to
        immediately pass through to Customer any charges, fees, taxes and terms
        and conditions of service imposed by access suppliers, including but not
        limited to increases or decreases in telephone tariffs, communications
        charges and access charges that are imposed or enacted by access
        suppliers to MCI after the Effective Date.


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separately with the Securities and Exchange Commission. Confidential treatment
has been requested with respect to the omitted portions.

                                      -2-

 
4.3     If at any time during the Term MCI tariffs any of the MCI Enhanced
        Services provided pursuant to this Agreement (each a "Newly Tariffed
        Service"). Customer and MCI agree to promptly execute appropriate
        additional agreements and amendments to this Agreement the effect of
        which shall be to eliminate the Newly Tariffed Service from this
        Agreement and to incorporate such Newly Tariffed Service into an
        agreement for MCI Tariffed services. Customer acknowledges and agrees
        that MCI shall have no obligation to include any equipment provided
        under this Agreement or any charges payable for such equipment in any
        such agreement for Tariffed services.

5.      Installation Credits.
        -------------------- 

5.1     Customer shall be eligible for one-time credits in an aggregate amount
        not to exceed [*] to be applied against one-time installation and other
        one-time nonrecurring charges associated with implementation of MCI
        Enhanced Services under this Agreement (the "Install Waiver"). The
        application of the Install Waiver to each MCI Enhanced Service may be
        more specifically described in the applicable ESA Schedule.

6.      MCI Invoices; Payment; Surety.
        ----------------------------- 

6.1     MCI shall invoice Customer on a monthly basis in the month succeeding
        the applicable usage month. Customer shall pay the full amount of each
        invoice, within thirty (30) days after the date of the invoice. MCI may
        use amounts paid by Customer pursuant to any other agreement between the
        parties to offset amounts owed by Customer to MCI pursuant to this
        Agreement. Failure of MCI to timely invoice Customer for any amounts due
        hereunder shall not be deemed a waiver by MCI of its rights to payment
        therefor.

6.2     Upon MCI's reasonable request, Customer shall pay MCI weekly in advance
        for MCI services provided pursuant to this Agreement. Such payments
        shall be via electronic wire transfer to MCI's bank account. Such
        payments must be received by MCI on Friday of each week by 2:00 PM EST,
        if the payment date is a federal holiday, the payment must be received
        by 2:00 PM EST the next business day. Customer shall make each
        electronic wire transfer pursuant to the instructions set forth below,
        but MCI may modify these instructions upon at least thirty (30) days
        prior written notice to Customer. The amount of Customer's weekly wire
        transfers shall be in an amount which is one-fourth of MCI's good faith
        estimate of Customer's actual monthly usage, which estimate may be
        revised from time to time. If the amount of Customer's prepayments in a
        calendar month is less than its actual charges for said calendar month
        as invoiced by MCI pursuant to Paragraph 6.1 above. Customer shall pay
        the difference with its next wire transfer to MCI. Customer shall make
        each electronic wire


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                                      -3-

 
        transfer pursuant to the following instructions. and understands that
        failure to do so will result in cancellation of MCI services without
        further notification.

               Bank Name:  The First National Bank of Chicago
               ABA#:  
               DDA #: 
               Acct Name:  MCI Telecommunication Corporation
               Acct#: 
               Cust Name:  Concentric Network Corporation

6.3     Customer shall make a separate claim in writing, with adequate support,
        for any credit for service interruption to which Customer believes
        itself entitled under this Agreement, and MCI and Customer will promptly
        address and resolve the claim. The parties shall use reasonable
        commercial efforts to resolve any such claim within sixty (60) days
        after the date of MCI's notice thereof

6.4     Except as otherwise indicated herein, the charges specified in the
        attached Schedule(s) do not include, and Customer agrees to pay, all
        taxes levied by any duly constituted taxing authority against or upon
        MCI Enhanced Services or otherwise arising out of this Agreement
        (including, without limitation, any sales, gross receipts or value-added
        taxes), except any such income tax based on or measured in whole or in
        part by gross or net income, gross or net payments, profits, or net
        worth of MCI or its affiliates (the "Taxes"); so long as, in the case of
        foreign tax withholdings, Customer shall agree to cooperate with MCI in
        providing foreign tax receipts to MCI; utilize best efforts to comply
        with foreign tax laws; and utilize best efforts to provide MCI and/or a
        foreign taxing authority with additional information to support MCI's
        claim for foreign tax credit(s), as requested in writing by MCI.

7.      Customer Obligations
        --------------------

7.1     Customer shall be responsible for obtaining, installing, inter-
        connecting and maintaining all equipment, software and/or communications
        services necessary for inter-connection with MCI's network or otherwise
        for use in conjunction with the MCI Enhanced Services. Customer shall
        ensure that such equipment, software and/or services, including but not
        limited to computer operating systems, are compatible with MCI's
        requirements and that they continue to be compatible with subsequent
        revision levels of MCI-provided equipment, software and services. MCI
        shall exercise reasonable business efforts to provide Customer with
        prior notice of an modifications in MCI's requirements that may affect
        compatibility. Unless otherwise expressly agreed in writing. MCI shall
        have no responsibility for the availability, capacity and/or condition
        of any equipment, software or services obtained by

                                      -4-

 
        Customer hereunder. Should Customer undertake to connect any MCI
        products or services to any other service or network, Customer shall
        indemnify and hold harmless MCI from any damages, costs, liabilities and
        expenses resulting from such connection or attempted connection,
        including but not limited to damages resulting from unauthorized use of,
        or access to, MCI's network.

7.2     Customer shall take, at its sole expense, all physical and information
        systems security measures reasonably necessary to protect all equipment,
        software and systems provided by MCI and its subcontractors in
        connection with the MCI Enhanced Services, whether owned by Customer,
        MCI, or MCI's subcontractors. Further, Customer shall be responsible for
        ensuring the security of Customer's data, and Customer acknowledges and
        agrees that MCI shall have no liability for any loss resulting from any
        unauthorized third-party access to any Customer data.

7.3     Customer agrees to provide MCI and its subcontractors and their
        respective employees and agents access to Customer's sites where any MCI
        Enhanced Services are provided (including access to associated
        equipment) as reasonably necessary for MCI and its subcontractors to
        perform the MCI Enhanced Services ordered hereunder.

7.4     Customer shall be responsible for obtaining any and all local permits
        and licenses necessary for performance under this Agreement (including
        each ESA Schedule annexed hereto). MCI shall reasonably cooperate with
        Customer to assist in the obtaining of permits. Customer shall indemnify
        and hold harmless MCI, its subcontractors and their respective agents
        and affiliates from and against all damages, costs, liabilities and
        expenses (including reasonable attorneys' fees) arising out of
        Customer's failure to obtain the appropriate permits and licenses.

7.5     Customer is responsible for preparing all equipment installation areas
        at Customer sites with adequate wiring, power sources, telephone
        connections and other physical facilities and services that may be
        necessary to implement the MCI Enhanced Services, and MCI shall have no
        obligation to provide any MCI Enhanced Service that requires equipment
        to be installed at any site(s) where such site requirements have not
        been fulfilled. MCI shall cooperate with Customer to provide site
        requirements.

8.      Software and Documentation.
        -------------------------- 

8.1     All rights, including but not limited to copyright, patent, trademark
        and other intellectual property rights, in any software and/or
        documentation provided by MCI in connection with any MCI Enhanced
        Service shall remain the exclusive property of MCI or its licensor(s).
        MCI grants to Customer a non-exclusive license to use such software and
        documentation solely for Customer's internal business purposes in
        accordance with the terms of this Agreement. No portion of such software
        or documentation shall be copied, decompiled, downloaded, translated, or
        delivered to a third party without MCI's prior written consent,

                                      -5-

 
        except that Customer shall be permitted to copy MCI-provided software
        for Customer's internal emergency use.

9.      Termination

9.1     A party may terminate this Agreement immediately upon notice to the
        other party if (i) such other party dissolves, discontinues or
        terminates its business to which this Agreement pertains; (ii) such
        other party fails to pay its debts as they become due or admits that it
        is, or is reasonably believed to be, unable to pay its debts as they
        become due; (iii) any bankruptcy, reorganization, insolvency,
        dissolution or similar proceeding is instituted against such other
        party; (iv) such other party makes any assignment for the benefit of
        creditors; or (v) such other parry takes any corporate action in
        furtherance of any of the foregoing.

9.2     MCI may terminate this Agreement immediately upon notice to Customer if
        Customer fails to meet any payment obligation hereunder and such failure
        is not cured within ten (10) business days after Customer's receipt of
        written notice from MCI notifying Customer of such failure. MCI may also
        terminate this Agreement upon thirty (30) days written notice to
        Customer if Customer fails to comply with any other material term of
        this Agreement and Customer does not cure such failure within such
        thirty (30) day period.

9.3     In addition to MCI's rights of termination under Sections 9.1 and 9.2
        above, MCI may, immediately upon written notice to Customer, interrupt,
        suspend and/or terminate any facility, equipment, or service provided
        under this Agreement if:

9.3.1   MCI determines in its sole discretion that continued provision of such
        facility, equipment, or service would contravene any national or
        international regulation, law, or tariff; or

9.3.2   MCI determines that interruption or termination of an MCI Enhanced
        Service is necessary to prevent or protect against fraud or otherwise
        protect its personnel, agents, facilities, or services; or

9.3.3   Any third-party subcontractor or vendor to MCI is unable to continue to
        provide such facility, equipment, or service for any reason; provided,
        however, that where such third party has ceased to provide any facility,
        equipment, or service, MCI will use reasonable efforts to continue to
        provide to Customer a comparable facility, equipment, or service by or
        through another vendor under comparable terms and conditions.

9.4     Customer may terminate this Agreement without further liability to MCI
        upon thirty (30) days written notice to MCI if:

9.4.1   MCI fails to comply with a material term of this Agreement and does not
        cure such failure within such thirty (30) day period; or

                                      -6-

 
9.4.2   as of the expiration of the Ramp Period, MCI fails to make the following
        MCI locations available to Customer (per those specifications
        communicated by Customer to MCI as of August 13,1996) for inclusion in
        that certain Support Services Agreement by and between the parties,
        dated August 9,1996; provided that such termination right shall be 
                             --------
        deemed to have expired upon Customer's written notice to MCI pursuant to
        Paragraph 2.2.1 above that the Ramp Period be deemed terminated:
        Auburndale, Florida; Austell, Georgia; Downers Grove, Illinois:
        Charlton, Massachusetts and Hillburn, New York.

9.5     In the event Customer terminates this Agreement other than in accordance
        with Section 9.4. then, in addition to any other rights and remedies
        available to MCI. Customer shall pay to MCI an Early Termination Charge
        (which Customer agrees is reasonable) equal to [*].

9.6     Either party may terminate this Agreement without further liability in
        the event that the parties have not, within sixty (60) days after MCI
        Tariffs a service provided pursuant to this Agreement, executed
        appropriate additional agreements and/or Amendments to this Agreement as
        set forth in Paragraph 4.3 above. All of the terms and conditions of
        this Agreement shall apply during said sixty (60) day period, provided
        that such application is permissible pursuant to regulatory and legal
        constraints.

10.     Warranty.
        -------- 

10.1    MCI's warranty obligations, if any, with respect to each Enhanced
        Service shall be set forth in the applicable ESA Schedule. EXCEPT AS
        SPECIFICALLY SET FORTH IN THIS AGREEMENT (INCLUDING THE ESA SCHEDULES),
        MCI MAKES NO WARRANTIES, EXPRESS OR IMPLIED, AS TO ANY MCI ENHANCED
        SERVICES. MCI SPECIFICALLY DISCLAIMS ANY AND ALL EXPRESS OR IMPLIED
        WARRANTIES, INCLUDING WITHOUT LIMITATION ANY WARRANTIES OF
        MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE AND ANY INTELLECTUAL
        PROPERTY WARRANTIES OF ANY TYPE

11.     Indemnification
        ---------------

11.1    Customer agrees to indemnify MCI and its affiliates and their respective
        employees, officers, directors, agents and subcontractors, and hold them
        harmless against any damages and expenses incurred by any of them
        arising out of Customer's acts, omissions and/or breach of its
        obligations hereunder and/or Customer's use of any MCI Enhanced Services
        in a manner other than as contemplated herein, including without
        limitation any use that gives rise to infringement of any patent,
        copyright, trademark, or other proprietary right of a third party,
        except to the extent that such use constitutes a direct infringement by
        Customer of a third party proprietary right that was solely contributed
        to or solely induced by MCI.

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        [*] Certain information on this page has been omitted and filed
separately with the Securities and Exchange Commission. Confidential treatment
has been requested with respect to the omitted portions.


                                      -7-

 
12.     Export and Legal Compliance
        ---------------------------

12.1    The parties acknowledge that certain equipment, software and technical
        data which may be provided hereunder may be subject to export and re-
        export controls under the U.S. Export Administration Regulations and/or
        similar regulations of the U.S. or any other country. No party shall
        export or re-export any such equipment, software, technical data or any
        direct product thereof in violation of any such laws.

12.2    Customer is responsible for complying with alt laws and regulations,
        including but not limited to import and customs laws and regulations.
        MCI shall provide reasonable assistance to Customer and its affiliates
        to facilitate such compliance. Such assistance may include preparation
        of import and customs forms and/or, where requested by Customer, acting
        as Customer's agent in the import process.

13.     Limitation Of Liability.
        ----------------------- 

13.1    NEITHER PARTY SHALL BE LIABLE TO THE OTHER FOR ANY CONSEQUENTIAL,
        EXEMPLARY, SPECIAL, INCIDENTAL OR PUNITIVE DAMAGES EVEN IF IT HAS BEEN
        ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

13.2    WITHOUT LIMITATION OF THE PROVISIONS OF PARAGRAPH 13.1 ABOVE, THE TOTAL
        LIABILITY OF MCI TO CUSTOMER IN CONNECTION WITH THIS AGREEMENT SHALL BE
        LIMITED TO THE LESSER OF (A) DIRECT DAMAGES PROVEN BY CUSTOMER OR (B)
        THE AGGREGATE AMOUNTS PAID BY CUSTOMER TO MCI UNDER THIS AGREEMENT FOR
        THE ONE (1) MONTH PERIOD PRIOR TO ACCRUAL OF SUCH CAUSE OF ACTION AND
        FOR THE SPECIFIC PRODUCT OR SERVICE WHICH FORMS THE BASIS FOR SUCH CAUSE
        OF ACTION. THE FOREGOING LIMITATION APPLIES TO ALL CAUSES OF ACTIONS AND
        CLAIMS, INCLUDING WITHOUT LIMITATION BREACH OF CONTRACT, BREACH OF
        WARRANTY, NEGLIGENCE, STRICT LIABILITY, MISREPRESENTATION AND OTHER
        TORTS, FURTHER, NO CAUSE OF ACTION WHICH ACCRUED MORE THAN ONE (1) YEAR
        PRIOR TO THE INSTITUTION OF A LEGAL PROCEEDING ALLEGING SUCH CAUSE OF
        ACTION MAY BE ASSERTED BY EITHER PARTY AGAINST THE OTHER.

        MCI SHALL NOT BE RESPONSIBLE FOR PROTECTION OF TRANSMISSION FACILITIES
        AND CUSTOMER PREMISES EQUIPMENT FROM UNAUTHORIZED ACCESS. MCI SHALL NOT
        BE LIABLE, EITHER IN CONTRACT OR IN TORT, FOR UNAUTHORIZED ACCESS TO, OR
        ALTERATION, THEFT, OR DESTRUCTION OF, DATA FILES, PROGRAMS, OR
        INFORMATION OF CUSTOMER WHETHER OCCURRING THROUGH ACCIDENT OR FRAUD.

                                      -8-

 
        MCI'S LIABILITY WITH RESPECT TO INDIVIDUAL MCI ENHANCED SERVICES MAY
        ALSO BE LIMITED PURSUANT TO THE TERMS AND CONDITIONS OF THE APPLICABLE
        ESA SCHEDULE.

14.     Confidential Information.
        ------------------------ 

14.1    Neither party shall disclose to any third party during the Term of this
        Agreement, or during the three (3) year period after expiration or
        termination of this Agreement, any of the terms and conditions of this
        Agreement unless such disclosure is lawfully required by any federal
        governmental agency, is otherwise required to be disclosed by law, is
        necessary in any legal proceeding establishing rights and obligations
        under this Agreement or is made to a third party under similar
        confidentiality obligations. Each party reserves the right to terminate
        this Agreement by giving written notice to the other party in the event
        of any unpermitted third party disclosure hereunder.

15.     Miscellaneous
        -------------

15.1    Neither party may assign this Agreement, or any rights or obligations
        hereunder, without the prior written consent of the other party, which
        consent shall not be unreasonably withheld. Any attempted assignment
        without such prior written consent shall be void. Notwithstanding the
        foregoing, MCI may assign this Agreement to its parent or any of their
        subsidiaries or affiliates.

15.2    Other than as the result of an assignment permitted in accordance with
        Paragraph 15.1 above, this Agreement shall not be deemed to provide
        third parties with any remedy, claim, right of action, or other right

15.3    The construction, interpretation and performance of this Agreement, and
        all causes of action arising out of this Agreement, whether in contract,
        indemnity, warranty, strict liability, tort, or otherwise, shall be
        governed as follows: (1) subject to the Communications Act of 1934, as
        amended ("the Act"), by the Act; and (2) as to elements not controlled
        by the Act, by the domestic law of the State of New York without regard
        to its choice of law principles.

15.4    Any dispute arising out of or related to this Agreement, which cannot be
        resolved by negotiations shall be settled by binding arbitration in
        accordance with the J.A.M.S/ENDISPUTE Arbitration Rules and Procedures
        ("Endispute Rules"), as amended by this Agreement. The costs of
        arbitration, including the fees and expenses of the arbitrator, shall be
        shared equally by the parties unless the arbitration award provides
        otherwise. Each party shall bear the cost of preparing and presenting
        its case. The parties agree that this provision and the Arbitrator's
        authority to grant relief shall be subject to the United States
        Arbitration Act. 9 U.S.C. 1-16 et seq. ("USAA"), the provisions of this
        Agreement, and the ABA-AAA Code of Ethics for Arbitrators in Commercial
        Disputes. The parties agree that the arbitrator shall have no power or
        authority to make awards or issue orders of any kind except as expressly
        permitted by this Agreement, and in no event shall the arbitrator have
        the

                                      -9-

 
        authority to make any award that provides for punitive or exemplary
        damages. The Arbitrator's decision shall follow the plain meaning of the
        relevant documents, and shall be final and binding. The award may be
        confirmed and enforced in any court of competent jurisdiction. All post-
        award proceedings shall be governed by the USAA.

15.5    If any paragraph or clause of this Agreement shall be held to be invalid
        or unenforceable by any body or entity of competent jurisdiction, then
        the remainder of the Agreement shall remain in full force and effect and
        the parties shall promptly negotiate a replacement provision or agree
        that no replacement is necessary.

15.6    If either party fails, at any time, to enforce any right or remedy
        available to it under this Agreement, that failure shall not be
        construed to be a waiver of such party's right to enforce each and every
        provision of this Agreement.

15.7    Any notice or other communication required to be given to the other
        party under this given in Agreement shall be in writing, in the English
        language and either (1) delivered in person, (2) sent by United States
        certified or registered mail, postage prepaid, or (3) sent by an
        overnight courier service, to the following addresses:

15.7.1  If to MCI:

        MCI Telecommunications Corporation
        201 Spear Street
        San Francisco, California 94105
        FAX:[*]
        Attn: Legal Department

15.7.2  with copy to:

        MCI Telecommunications Corporation
        2560 North First Street, Bldg. F. 2d Floor,
        San Jose, California 95131
        FAX: [*]
        Attn: Branch Manager


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        [*]Certain information on this page has been omitted and filed
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                                      -10-

 
15.7.3  If to Customer:

        Concentric Network Corporation
        10590 N. Tantau Avenue
        Cupertino, California 95014
        FAX: 408 342-2810
        Attn: Chief Financial Officer

15.7.4  The address for notice may be changed by giving written notice in
        accordance with this Section. If mailed in accordance with this Section,
        notice shall be deemed given three (3) days after mailing. If sent by an
        overnight courier service, notice shall be deemed given one (1) day
        after deposit with the courier service.

15.8    Any delay in or failure of performance by either party under this
        Agreement (other than a failure to comply with payment obligations)
        shall not be considered a breach of this Agreement if and to the extent
        caused by events beyond the reasonable control of the party affected,
        including but not limited to acts of God, embargoes, governmental
        restrictions, strikes (other than those only affecting Customer),
        subcontractor failures or delays, riots, wars or other military action,
        civil disorders, rebellion, vandalism, or sabotage. Market conditions
        and/or fluctuations (including a downturn of Customer's business) shall
        not be deemed force majeure events. The party whose performance is
        affected by such events shall promptly notify the other party giving
        details of the force majeure circumstances, and the obligations of the
        party giving such notice shall be suspended during but not longer than
        the continuance of the force majeure, and the time for performance of
        the affected obligation hereunder shall be extended by the time of the
        delay caused by the force majeure event. In the event that a force
        majeure event occurs, each party has the duty to mitigate or its
        damages.

15.9    Nothing in this Agreement shall create in either party any rights in any
        trademark, trade name, service mark, insignia, symbol, identification
        and/or logotype of the other party. Before either party uses any such
        mark of the other party, it shall obtain the prior written consent of
        the other party.

15.10   This Agreement has been written in the English language and, in the
        event of any conflict or inconsistency between the English-language
        version and any translation hereof the English language version shall
        prevail.

15.11   The headings and captions of the various sections of this Agreement are
        included solely for convenience and shall not be deemed to be a part of
        this Agreement or considered in construing the terms and conditions
        hereof.

15.12   Unless otherwise expressly provided herein, all rights and remedies
        provided for in this Agreement shall be cumulative and in addition to
        every other right and remedy available under this Agreement or otherwise
        at law or in equity.

                                      -11-

 
15.13   This Agreement, including the ESA Schedules (which are incorporated
        herein by this reference), constitutes the entire agreement between the
        parties hereto with respect to the subject matter hereof, and there are
        no representations, understandings or agreements relative thereto which
        are not fully expressed herein. No amendment, change, waiver, or
        discharge hereof shall be valid unless in writing and signed by an
        authorized representative of the party against which such amendment,
        change, waiver, or discharge is sought to be enforced.

15.14   The parties have duly executed and agreed to be bound by this Agreement
        as evidenced by the signatures of their authorized representatives
        below. Each party represents and warrants to the other that the
        signatory identified beneath its name below has full authority to
        execute this Agreement on its behalf.

15.15   Customer must sign and return to MCI this document in duplicate on or
        before September 18, 1996. After such time, Customer's ability to make
        an offer to MCI consistent with the terms and conditions stated in this
        document shall be deemed terminated.

MCI Telecommunications Corporation     Concentric Network Corporation


By: /s/ Edward W. Smith                By:  /s/ Michael F. Authofer
    -------------------------------         --------------------------------

Name: Edward W. Smith                  Name: Michael F. Authofer
      ---------------                        -------------------------------

Title: Director                        Title: VP & CFO
       --------                               ------------------------------

Date: 10/23/96                         Date: Sept. 11, 1996
      --------                               -------------------------------

                                      -12-

 
                               ESA Schedule No. 1

               MCI Hyperstream Asynchronous Transfer Mode Service

1.      Description: MCI Hyperstream Asynchronous Transfer Mode (ATM) is a
        connection oriented public/private network data service. Hyperstream 
        ATM - is a cell-based broadband technology which allows seamless high
        speed wide area and local area network connections. A wide range of data
        applications connect with HyperStream ATM including local area network
        (LAN) interconnections, high-speed transmission of digitized, medical
        imaging across the country' and desktop videoconferencing enabling users
        to share multimedia applications.

2.      Rates & Charges; Net Usage.

        [*]



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separately with the Securities and Exchange Commission. Confidential treatment
has been requested with respect to the omitted portions.

                                      -13-

 
2.0.1   Monthly Recurring Charges (simplex):

[*]


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separately with the Securities and Exchange Commission. Confidential treatment
has been requested with respect to the omitted portions.

                                      -14-

 
2.1     "Net Usage" of MCI Hyperstream Asynchronous Transfer Mode Service shall
        include only Recurring Port and Usage Charges, as set forth above. "Net
        Usage" shall not include Nonrecurring Port and Usage Charges or Access
        charges, as set forth above.

3.      [*]



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     [*]  Certain information on this page has been omitted and filed
separately with the Securities and Exchange Commission. Confidential treatment
has been requested with respect to the omitted portions.

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