<PAGE>

                                                                   EXHIBIT 10.51

                              DEFINITIVE AGREEMENT

                                    BETWEEN

                              SBC OPERATIONS INC.

                                      AND

                         CONCENTRIC NETWORK CORPORATION








                            PROPRIETARY INFORMATION

The Information contained herein is for the use of SBC Operations, Inc. (and its
  Affiliated Companies) and Concentric Network Corporation only and is not for
                   disclosure without prior written approval.

                                      -1-
<PAGE>

PART I - DEAL TERMS ......................................  8
-------------------

  1.  SCOPE AND STRUCTURE OF RELATIONSHIP.................  8

  2.  DEFINITIONS.........................................  8

  3.  PRODUCT SPECIFICATION AND PLANNING..................  8

      3.1  SBC Acceptance of Products.....................  8
      3.2  Damages for delay..............................  9

  4.  NEW PRODUCTS AND SERVICES...........................  9

      4.1  New Products Committee......................... 10
      4.3  Upgrades....................................... 10

  6.  USE OF SBC TRANSPORT................................ 10

  7.  NATURE OF THE RELATIONSHIP.......................... 10

      7.2  Non-Exclusive Relationship..................... 11

  8.  PRICING AND PRICE PROTECTION........................ 11

      8.1  Systems and Billing............................ 11
      8.2  [*]............................................ 13

  9.  REPORTING........................................... 13

 10.  CHANNEL SYNERGIES AND CONFLICT...................... 13

      10.1  Channel Plan.................................. 14
      10.2  RFP Responses................................. 14
      10.3  Employee Conduct.............................. 14

 [*]...................................................... 14

 12.  FORECASTING AND FUTURE NETWORK COVERAGE............. 14

 13.  BRANDING............................................ 15

 [*]...................................................... 15

 15.  CUSTOMER CARE/TECH SUPPORT.......................... 15

 16.  SALES TRAINING...................................... 15

 17.  TERM OF AGREEMENT................................... 15

 18.  TERMINATION......................................... 15


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.



                                      -2-
<PAGE>

      18.1  Termination at End of Term.................... 15
      18.2  Termination For Convenience................... 16
      18.3  Termination for Cause......................... 16
      18.4  Termination evolving from Regulatory
            Contingencies................................. 17

 19.  CUSTOMER TRANSITION PROVISIONS...................... 17

      19.1  Customer Transition Plan...................... 17
      19.2  Customer Migration............................ 17

 20.  STEERING COMMITTEE AND STRATEGIC RELATIONSHIP....... 17

      20.1  Steering Committee............................ 17
      20.2  Additional Areas for Cooperation.............. 17
      20.3  Technology Committee.......................... 17

 21.  NETWORK OPERATIONS.................................. 18

 22.  PUBLICITY........................................... 18

PART II - STANDARD TERMS AND CONDITIONS................... 18
---------------------------------------

  1.  DEFINITIONS......................................... 18

  2.  BENEFIT OF AGREEMENT EXTENDING TO SBC AFFILIATES.... 21

  3.  NO INTERLATA  ACTIVITIES BY SBC..................... 21

  4.  REPRESENTATIONS AND WARRANTIES...................... 22

      4.1  Authorization.................................. 22
      4.2  Compliance with Specifications................. 22
      4.3  Year 2000...................................... 22
      4.4  Warranty Limits................................ 23

  5.  LIABILITY LIMITS.................................... 23

  6.  INDEMNIFICATION AND INFRINGEMENT.................... 23

      6.1  Infringement................................... 23
      6.2  Indemnification................................ 25

  7.  INSURANCE........................................... 26

  8.  CUSTOMER INFORMATION................................ 27

  9.  NON-SOLICITATION OF SBC CUSTOMERS................... 28

 10.  INTELLECTUAL PROPERTY RIGHTS........................ 28

      10.1  CNC Software.................................. 28
      10.2  SBC Software.................................. 29
      10.3  Trademarks.................................... 29


                           PROPRIETARY INFORMATION
    The Information contained herein is for the use of SBC Operations Inc.
    (and its Affiliates Companies) and Concentric Network Corporation only
           and is not for disclosure without prior written approval.

                                      -3-
<PAGE>

      10.4  New Developments.............................. 30
      10.5  General....................................... 31

 11.  CONFIDENTIAL INFORMATION............................ 32

      11.1  SBC's Information............................. 32
      11.2  CNC's Information............................. 32

 13.  RECORDS AND AUDITS.................................. 33

 14.  CURE................................................ 34

 15.  DISPUTE ESCALATION.................................. 34

 16.  ARBITRATION......................................... 34

 17.  GOVERNING LAW....................................... 35

 18.  RELATIONSHIP OF PARTIES............................. 35

 19.  REMEDIES............................................ 35

 20.  AMENDMENT AND WAIVER................................ 35

 21.  RELEASES VOID....................................... 36

 22.  FORCE MAJEURE....................................... 36

 23.  ASSIGNMENT.......................................... 37

 24.  CONFLICT OF INTEREST................................ 37

 25.  LIENS PROHIBITED.................................... 38

 26.  SEVERABILITY........................................ 38

 27.  SURVIVAL............................................ 38

 28.  COMPLIANCE WITH LAWS/REGULATORY CONTINGENCY......... 38

      28.1  Compliance With Laws.......................... 38
      28.2  Regulatory Contingency........................ 39

 29.  TAXES............................................... 39

 30.  ACCESS TO PREMISES.................................. 40

 31.  USE OF SBC SYSTEMS.................................. 41

 32.  WORK DONE BY OTHERS................................. 41


                           PROPRIETARY INFORMATION
    The Information contained herein is for the use of SBC Operations Inc.
    (and its Affiliates Companies) and Concentric Network Corporation only
          and is not for disclosure without prior written approval.

                                      -4-
<PAGE>

 33.  SECURITY............................................ 41

 34.  INSPECTIONS......................................... 41

 35.  INDEPENDENT CONTRACTOR/ SBC AGENT................... 42

 36.  DIVERSION OF EMPLOYMENT............................. 43

 37.  UNIVERSAL DESIGN.................................... 43

 38.  HEADINGS............................................ 43

 39.  NOTICES............................................. 44

 40.  ENTIRE AGREEMENT/RELATIONSHIP AMONG DOCUMENTS....... 44

 41.  SIGNATURES.......................................... 47

ATTACHMENTS............................................... 47
-----------

 1.  MARKETING SERVICE DESCRIPTION........................ 47

 2.  TECHNICAL SERVICE DESCRIPTION........................ 47

 3.  DEPLOYMENT AND DELIVERY.............................. 47

 4.  USE OF SBC TRANSPORT................................. 47

 5.  CNC'S EXISTING STRATEGIC RELATIONSHIPS............... 47

 6.  BILLING.............................................. 47

 7.  SYSTEMS.............................................. 47

 8.  PRICING.............................................. 47

 9.  PERFORMANCE COMMITMENTS AND REPORTS.................. 47

10.  SALES CHANNEL CONFLICTS.............................. 47

11.  FORECASTING.......................................... 47

12.  BRANDING............................................. 47

13.  CUSTOMER CARE AND TECHNICAL SUPPORT.................. 47

14.  SALES TRAINING AND SUPPORT........................... 47


                           PROPRIETARY INFORMATION
    The Information contained herein is for the use of SBC Operations Inc.
    (and its Affiliates Companies) and Concentric Network Corporation only
           and is not for disclosure without prior written approval.

                                      -5-
<PAGE>

15.  NETWORK MANAGEMENT AND MONITORING.................... 47

16.  PROVISIONING......................................... 47

17.  SERVICE LEVEL AGREEMENTS............................. 47

18.  WHOLESALE DIAL ACCESS................................ 47

19.  EXECUTIVE ORDERS AND ASSOCIATED REGULATIONS.......... 47

20.  CONSULTING SERVICES.................................. 47

21.  ACCEPTANCE TEST PLAN AND PROCEDURES.................. 47


                           PROPRIETARY INFORMATION
    The Information contained herein is for the use of SBC Operations Inc.
    (and its Affiliates Companies) and Concentric Network Corporation only
           and is not for disclosure without prior written approval.

                                      -6-
<PAGE>

This Definitive Agreement ("Agreement"), effective as of April 1, 1999  (the
"Effective Date"), is made between SBC Operations Inc., a Delaware corporation,
(herein referred to as "SBC") with offices located at 175 East Houston Street,
San Antonio, TX 78215 and Concentric Network Corporation ("CNC"), a Delaware
corporation, with offices located at 1400 Parkmoor Avenue, San Jose, CA  95126.
For the purpose of this Agreement, SBC and CNC are each a "Party" and
collectively, the "Parties."

This Agreement is made with reference to the following facts, which form the
basis for this Agreement:

A.  SBC is one of a group of companies affiliated which share the parent company
SBC Communications, Inc. providing telecommunications and network services to
individual and business end-users worldwide.  Affiliates of SBC offer internet
access and network services on a IntraLATA basis (hereinafter defined).

B.  CNC is a network and Internet service provider offering IntraLATA and
InterLATA (hereinafter defined) services, with particular emphasis on Virtual
Private Network ("VPN") and similar services offered to small and medium sized
business customers.

C.  The Parties previously have entered into the Trial Agreement dated October
19, 1998 pursuant to which the Parties are operating a trial of certain
cooperatively offered products and services, the Stock Purchase Agreement dated
October 19, 1998 pursuant to which SBC has acquired a certain percentage of
CNC's stock, and the Web Hosting General Agreement dated October 9, 1998
("Hosting GA") pursuant to which CNC is providing web hosting services to SBC,
its affiliates and customers.  The Parties also have had discussions regarding a
broader strategic relationship.

D.  The Parties wish to formalize their previous discussions regarding a
strategic relationship pursuant to which SBC will purchase certain IntraLATA
products and services from CNC, and make such IntraLATA products and services
available to end user customers of SBC. The products and services delivered to
SBC's customers through this strategic relationship are intended to be
competitive offerings, achieved through successful planning, implementation and
improvement of technology, business processes, and strategic deployment of the
Parties' integrated products and services, with the goal of enhancing
competition and offering end user customers compelling, quality products and
services in a highly competitive, rapidly evolving marketplace.  In addition,
the Parties wish to set forth in this Agreement (including the attachments and
schedules attached hereto) all the terms and conditions governing the strategic
relationship between them.

NOW, THEREFORE, THE PARTIES AGREE AS FOLLOWS:



                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -7-
<PAGE>

PART I - DEAL TERMS
-------------------


1.  SCOPE AND STRUCTURE OF RELATIONSHIP

CNC hereby agrees to provide SBC with the Products and Services described in
ATTACHMENT 1, Marketing Service Description, on the terms and conditions set
forth herein. In addition, CNC agrees to make available to SBC for resale to end
users all enhancements and upgrades to Products and Services, as well as new
products and services which are related or complementary to the Products and
Services, as set forth in Part I, Section 4 below.

2.   DEFINITIONS

All capitalized terms not otherwise defined will have the meaning set forth in
Part II, Section 1, Definitions.

3.  PRODUCT SPECIFICATION AND PLANNING

All Products and Services to be delivered by CNC hereunder will conform in all
material respects to the descriptions and specifications set forth in ATTACHMENT
2, Technical Service Description, as such Attachment 2, Technical Service
Description or other applicable attachments, may be modified from time to time
by the express written agreement of the Parties.  CNC will provide electronic
and hard copies of all descriptions of the Products and Services and their
functionality, including all sales and technical materials prepared by CNC,
subject to the licensing requirements herein, including third party obligations.
SBC will have access to these Service descriptions and sales material and
authority to adapt, brand and publish them for use with its sales force and
other SBC personnel and SBC customers.

     3.1  SBC Acceptance of Products

     CNC will deliver new Products and Services, as well as upgrades to Products
     and Services in accordance with ATTACHMENT 2, Technical Service Description
     or other applicable attachments, on or before the initial launch, new
     product or  upgrade  dates set forth in ATTACHMENT 3, Deployment and
     Delivery. A Product or Service or upgrade will be deemed delivered when SBC
     reasonably determines that the performance and reliability of such Product
     or Service conforms in all material respects to the descriptions and
     specifications set forth in ATTACHMENT 2, Technical Service Description, or
     other applicable attachments. Failure by SBC to respond or test the product
     within thirty (30)


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -8-
<PAGE>

     days from the date the Product or Service is delivered to SBC for testing,
     will relieve CNC from Damages for Delay. Notwithstanding any implication to
     the contrary, if after SBC accepts a Product or Service and it does not
     perform to its required specifications, CNC will bring such Product or
     Service into compliance as specified by the warranty provisions of this
     Agreement without cost to SBC. If, prior to acceptance by SBC, any of the
     Products or Services are found conform in all material respects with
     ATTACHMENT 2, Technical Service Description, SBC may require CNC to
     correct, at CNC's sole expense, any problems and otherwise ensure that the
     applicable Product or Service conforms in all material respects to
     ATTACHMENT 2, Technical Service Description, and damages for delay, as set
     forth in Part I, Section 3.2, Damages for Delay, will apply until
     acceptance by SBC.

     3.2  Damages for delay

     CNC understands that the achievement of the dates for development,
     delivery and launch of Products and Services  is critical for SBC's plans
     to provide Products and Services to its customers as rapidly as possible
     and for the success of the Parties' relationship.  CNC agrees to take all
     commercially reasonable actions to ensure that the planned delivery
     schedules are met for launch by SBC of the Products and Services set forth
     on ATTACHMENT 3, Deployment and Delivery.  Because it may be difficult to
     quantify the consequences of a failure to meet mutually agreed upon launch
     dates, the Parties agree to the penalties set forth on ATTACHMENT 3,
     Deployment and Delivery, for delays in commercial availability of Products
     and Services, where the delay has not been caused by SBC or its agents
     failing to timely perform its or their obligations expressly set forth
     under ATTACHMENT 2, Technical Service Description, or other applicable
     attachments.

4.  NEW PRODUCTS AND SERVICES

It is the intention of the Parties mutually to benefit from the relationship
established by this Agreement by giving SBC advance notice of, and the
opportunity to offer comments on, and potentially participate in the development
of, enhancements and upgrades to the Products and Services covered hereunder
(collectively "Upgrades") as well as new products and services related to or
complementary to the Products and Services hereunder, including without
limitation Virtual Private Network products and services (collectively "New
Products").


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -9-
<PAGE>

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



     4.1  New Products Committee

     The Parties will establish a New Products Committee, composed of
     representatives from both CNC and SBC. The New Products Committee will meet
     from time to time as the Parties determine, but no less frequently than
     quarterly during the term of this Agreement. The purpose of the New
     Products Committee will be for CNC to keep SBC apprised of potential
     Upgrades and New Products, and for SBC to offer comments and suggestions
     with respect to product development. If the Parties mutually agree, SBC may
     participate in financing or developing Upgrades or New Products, but such
     participation shall take place only pursuant to a fully executed Statement
     of Work pursuant to Attachment 20, Consulting Services or other written
     agreement of the Parties, addressing ownership of the developed
     intellectual property and other material issues.

     [*]

     4.3  Upgrades

     CNC agrees to make available to SBC for resale to end users all Upgrades,
     including without limitation upgraded or improved SLA's applicable to
     Products and Services, as soon as practicable, but no later than such
     upgrades are made available to any third party or offered to end users by
     CNC. Upgrades to any Product or Service will, when made available pursuant
     to the previous sentence and accepted by SBC pursuant to mutually approved
     criteria, be considered Products and Services hereunder.

[*]

6.  USE OF SBC TRANSPORT

Products and Services offered hereunder will incorporate transport components
(e.g., T1 and other high speed access lines, frame relay, and local access
transport components) provided by Affiliates of SBC, as set forth in more detail
in, and subject to the conditions contained in, ATTACHMENT 4, or [*]

7  NATURE OF THE RELATIONSHIP

     [*]



                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -10-
<PAGE>

     7.2  Non-Exclusive Relationship

     Except as set forth in Section 7.1 above, the relationship created by this
     Agreement is non-exclusive, and nothing contained herein will be deemed to
     restrict either Party from quoting, offering to sell or selling products or
     services to or acquiring products or services from others, entering into
     agreements with other entities for the provision of products and services.
     Furthermore, nothing in this Agreement will be deemed to restrict the
     prices at which any service may be offered or sold outside of this
     Agreement, or affect or influence in any way CNC's provision of InterLATA
     Products and Services.


8.  PRICING AND PRICE PROTECTION

     8.1  Systems and Billing

     (a)  SBC is responsible for billing and collection of payment for all
          Products and Services, as specified in ATTACHMENTS 6 Billing and 7
          Systems.  CNC will provide SBC with billing information for Products
          and Services to allow SBC to accurately bill Customers.  CNC will
          invoice SBC for SBC Products and Services CNC provides directly to
          SBC.  CNC will provide customer invoice data to SBC for InterLATA
          Products and Services received. In connection with the billing of
          Customers, CNC agrees to comply with any rules, regulations, terms and
          conditions to which SBC may be subject, due to regulatory
          considerations.

     (b)  CNC will render an invoice in duplicate on a monthly basis, for all
          Products and Services purchased by SBC hereunder and will
          simultaneously provide SBC with all information necessary to allow SBC
          to bill on behalf of CNC with respect to all InterLATA Services. The
          invoice will conform in all respects to the requirements specified in
          ATTACHMENTS 6, Billing and 7, Systems, and will comply with any legal
          or regulatory requirements of which SBC notifies CNC from time to
          time.  SBC will pay CNC within sixty (60) days of the date of such
          invoices.

     (c)  CNC acknowledges that it is vital to SBC's relationship with end user
          customers that invoices to such customers be presented on a timely
          basis, in order to minimize customer inquiries and associated costs.
          Accordingly, CNC will use its best efforts to supply SBC with invoices



                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -11-
<PAGE>

          within 30 days after delivery of products and Services.  SBC will have
          no obligation to pay untimely invoices which are defined, for the
          purpose of this Section 8.1(C) as: (I) usage invoices received by SBC
          more than thirty (30) days after such usage is incurred and (ii)
          monthly recurring and non-recurring charge invoices received more than
          one hundred twenty (120) days after the delivery of Products or
          Services.  Provided, however, that if SBC in fact receives payment
          from end users on account of any such untimely CNC invoices, SBC will
          make payment to CNC of the corresponding amounts otherwise due to CNC
          hereunder.  If SBC disputes any invoice rendered or amount paid, SBC
          will so notify CNC and the Parties will use their best efforts to
          resolve such dispute expeditiously.  Notwithstanding the foregoing,
          the expectation is that all upgrades and changes to a Customer's
          Products or Services resulting in changes to recurring and non-
          recurring charges will be made by the Customer through SBC, so that
          SBC has adequate information to accurately bill such upgrades and
          changes, as specified in Attachment 6, Billing.

     (d)  All amounts payable to CNC hereunder, with respect to any InterLATA
          products and services as to which SBC bills end users on behalf of
          CNC, will be reduced by any bad debts and amounts uncollectible from
          end users.  SBC will bear the risk of all bad debts and uncollectible
          amounts in connection with IntraLATA Products and Services; provided,
          however, that the Parties will take the following steps to reduce the
          severity of customer complaints and associated churn and uncollectible
          amounts:

          (i)   From time to time and on a case by case basis, the Parties will
                cooperate in good faith to accommodate Customer complaints,
                including by means of credits and discounts, to the extent
                reasonable and necessary to maintain Customer satisfaction and
                reduce Customer terminations.

          (ii)  Without limiting subsection (I) above, from time to time and on
                a case by case basis, the Parties will adjust the Product and
                Service installation date for Customer billing purposes, if
                delays in installation for such Customer are attributable to
                CNC.

          (iii) The Parties will cooperate in good faith to develop an
                equitable plan for billing start dates and allocations for
                Customers with multiple sites.


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -12-
<PAGE>

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


          (iv)  The Parties will cooperate in good faith to develop procedures
                to implement subsections (i) and (ii), including establishing a
                single point of contact, escalation procedures, and regular
                reviews of customer feedback

     (e)  All claims for moneys due from SBC will be subject to deduction by SBC
          for any credit due from CNC.  Notwithstanding the foregoing sentence,
          if credits have arisen under another contract between the Parties, the
          credits may be applied to this Agreement only if purchases under such
          other contract have ended, either as a result of expiration or
          termination of such contract.  Any amount due SBC that is not so
          applied against CNC's invoices for any reason will be paid to SBC by
          CNC within forty-five  (45) days after demand by SBC.

     8.2  [*]

          (e)  The Parties will establish single points of contact for pricing
               related issues and an escalation procedure for resolving pricing
               disputes.

          (f)  Notwithstanding anything to the contrary herein, including the
               foregoing Sections, nothing in this Section 7.2, will apply to
               the provision of InterLATA services by CNC. SBC will have no
               authority over any InterLATA transport charges to Customers, all
               of which will be established solely by CNC.

          [*]

9.  REPORTING

The Parties have defined reports which will be issued for use by Customers, and
for each of the Parties' internal use.  These reports will be issued when a
Customer's Service is launched and will continue for the duration of the
Service.  A list of such reports is attached and specified in ATTACHMENT 9,
Performance Commitments and Reports.  Such reports include but are not limited
to, web enabled customer facing reports and reports to each of the Parties' for
internal management needs.

10.  CHANNEL SYNERGIES AND CONFLICT


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -13-
<PAGE>

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



     10.1  Channel Plan

     SBC will be responsible for the sales and marketing of the Products and
     Services.  The Parties will resolve RFP conflicts as provided below in
     Section 10.2 and channel conflicts as provided in ATTACHMENT 10, Sales
     Channel Conflicts.  Any sales conflicts not resolved as provided herein,
     will be referred to the Steering Committee specified in Part 1, Section 20,
     Steering Committee.

     10.2  RFP Responses

     The Parties acknowledge that the spirit of the relationship established by
     this Agreement calls for CNC to involve SBC in RFP responses, and both
     Parties agree to use good faith efforts to cooperate to accomplish such
     objective. Without limiting the foregoing, CNC will, whenever reasonably
     possible, respond to RFP's with proposals which utilize SBC products,
     services and transport components in addition to those required to be
     utilized by CNC in providing service to CNC customers pursuant to
     Attachment 4, Use of SBC Transport hereto.


     10.3  Employee Conduct

     The employees of each Party will not disclose the strategic relationship
     between SBC and CNC in sales efforts, press releases, marketing materials,
     or other media unless mutually agreed in writing in advance.  The Parties
     recognize that in the marketplace, the Parties' channels may overlap and
     that selling on a comparative basis is not necessarily a violation of this
     Section 9.3.  In particular, employees of each of the Parties will be
     allowed to comparatively sell Products and Services.

[*]


12.  FORECASTING AND FUTURE NETWORK COVERAGE

ATTACHMENT 11, Forecasting, details a process by which SBC and CNC will use
reasonable efforts to accurately forecast Customer acquisition and network
usage.  ATTACHMENT 3,  Deployment and Delivery, details CNC network coverage and
expansion and deployment plans.


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -14-
<PAGE>

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


13.  BRANDING
[*]
[*]

15.  CUSTOMER CARE/TECH SUPPORT

Subject to the Act and as specified in ATTACHMENT 13, Customer Care/ Technical
Support, SBC and CNC will provide Customer Care (herein after defined in
ATTACHMENT 13, Customer Care/ Technical Support) on all Products and Services.
CNC will provide customer care on InterLATA Products and Services.  If, as a
result of a call to either SBC or CNC, it is determined that the trouble is not
attributable to the IntraLATA Products and Services, the call will be referred
to CNC for Customer Care.  Each Party will bear its own costs with respect to
the Customer Care inquiries it handles unless otherwise provided in a separate
agreement.


16.  SALES TRAINING

CNC will provide SBC with supplemental sales performance training on a monthly
basis, to help improve the capabilities of the sales team.  SBC sales trainers
will be instructed in this sales performance training, as specified in
ATTACHMENT 14,  Sales Training and Support, and at the fees identified therein,
where applicable.


17.  TERM OF AGREEMENT

This Agreement is effective upon the Effective Date and unless sooner terminated
as provided herein, will remain in full force and effect for an initial term of
three (3) years following the Effective Date.  Thereafter, the Agreement will be
automatically renewed for three (3) successive one (1) year terms, unless either
party has provided the other party with notice of termination, in accordance
with Part I, Section 18, Termination, herein.  Termination is complete only upon
each Parties' satisfaction of its obligations under this Agreement.


18.  TERMINATION

     18.1  Termination at End of Term

     Either Party may terminate this Agreement without penalty by providing not
     less than sixty (60) days written notice to the other Party, provided that
     the


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -15-
<PAGE>

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



     termination date does not become effective until the completion of the
     initial three year term or any extension thereof.


     18.2  Termination For Convenience

     Commencing six (6) months after the Effective Date, SBC may, at its sole
     option, terminate this Agreement, or unilaterally modify ATTACHMENTS 1,
     Marketing Service Description and 2 Technical Service Description, in order
     to remove from the scope of this Agreement any Product or Service offered
     pursuant to this Agreement, at any time. SBC acknowledges that if SBC
     terminates the IntraLATA portion of the Products and Services, CNC will
     terminate its arrangements with end-user customers to provide the InterLATA
     portion of all Products and Services, subject to an appropriate transition
     period which matches the transition period provided for IntraLATA Products
     and Services provided in Part I, Section 19.1, Customer Transition Plan.
     Upon termination, pursuant to this Section18.2, (other than pursuant to
     Attachment 9, Performance Commitments), as liquidated damages for such
     termination, SBC will compensate CNC for the difference between the amount
     of revenue CNC would have received on the total number of outstanding
     Customer term contracts (including set-up and monthly fees) had a greater
     term discount not been applied.
     [*]


     18.3  Termination for Cause

     Either Party may terminate this Agreement in the event of a material breach
     of contract which remains unremedied for a period of thirty (30)  days
     following written notice of such breach.  If SBC terminates this Agreement
     on account of an uncured CNC breach, then SBC will have no further
     obligations to CNC hereunder, including without limitation pursuant to Part
     I, Sections 5 SBC Revenue Commitment and 19.2, and Termination for
     Convenience, except for any invoices due and owing to CNC for services
     provided.  If CNC terminates this Agreement on account of an uncured SBC
     breach, then CNC will have no further obligations to SBC hereunder, except
     as provided in Part 1, Section 19, Customer Transition Provisions.


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -16-
<PAGE>

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


     18.4  Termination evolving from Regulatory Contingencies

     Through no fault of either Party, there may be instances in which this
     Agreement terminates due to government laws or regulations.  Such
     termination is addressed in Part II, Section 27.7, Regulatory Contingency.


19.  CUSTOMER TRANSITION PROVISIONS

     19.1  Customer Transition Plan

           [*]

     19.2  Customer Migration

           [*]

20.  STEERING COMMITTEE AND STRATEGIC RELATIONSHIP

     20.1  Steering Committee

     The Parties have established a "Steering Committee" consisting of two (2)
     representatives of management from each Party who will collectively oversee
     and address issues relating to the continuation of the contractual
     relationship.  The Steering Committee will meet from time to time to ensure
     that the goals of the relationship are being met.  Each Party may change
     its representative to the Steering Committee from time to time in its sole
     discretion.

     20.2  Additional Areas for Cooperation

     [*]

     20.3  Technology Committee

     The Steering Committee will establish a technology committee, to meet no
     less frequently than quarterly, with the intention that, subject to all
     applicable legal and regulatory requirements, the Parties will engage in
     joint discussions to allow coordination of technology issues and
     integration of technology, hardware and software to maintain
     interoperability and otherwise enhance the Parties' ability to perform
     their respective obligations under this Agreement and to provide
     competitive, innovative, high quality services to end users.



                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -17-
<PAGE>

21.  NETWORK OPERATIONS

     21.1 As specified in ATTACHMENT 15, Network Management and Monitoring, SBC
          will provide all single points of contact ("SPOCs") and tier one
          network operations support for IntraLATA transmission elements, non-
          transmission oriented services, and administrative services, provided
          that SBC may, in its discretion, require CNC to provide network
          operations support where the problem is isolated to Products and
          Services developed by CNC.  CNC will provide network operations
          support for InterLATA Products and Services, for CNC's network and for
          network elements leased by CNC from a third party to deliver InterLATA
          Products and Services.

     21.2 Each Party will bear its own costs with respect to network operations
          support it handles.  The Parties have established processes to
          coordinate network operations support to be provided by the Parties
          under this Agreement.  Such processes are specified in ATTACHMENT 15,
          Network Management and Monitoring/Network Operations Center.


22.  PUBLICITY

Neither Party will use the other Party's name or any language, pictures or
symbols which could imply the other Party's identity in any (i) written or oral
advertising presentation or communication, or (ii) brochure, press release,
newsletter, book or other written material of whatever nature, without the other
Party's prior written consent.  To the extent CNC believes that the execution of
any of the potential agreements discussed in this Agreement is a financially
disclosable event, CNC will notify SBC, with the exception of confidentiality
constraints in accordance with SEC regulations and laws, of CNC's opinion in
advance of execution of such agreement.



PART II - STANDARD TERMS AND CONDITIONS
---------------------------------------


1.  DEFINITIONS

All capitalized terms not otherwise defined will have the meaning set forth
below:


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -18-
<PAGE>

"Act" - means the Telecommunications Act of 1996 (47 U.S.C. 151 et seq.), Pub.
L. No. 104-104, 110 Stat. 56, as amended from time to time.

"Affiliate" -  means (i) any corporation or other entity owning, either directly
or indirectly, today or in the future, a majority of the outstanding stock of a
Party ("Parent"), or (ii) any corporation or other entity in which a majority of
the ownership interest is held, either directly or indirectly, today or in the
future, by the Parent of a Party or by such Party; but only so long as such
majority ownership exists.


"Confidential Information" - means non-public information treated by the
disclosing Party as confidential, including trade secrets, technical, financial
and business information, names of existing and potential customers or partners,
existing and potential business ventures, reports, plans, technology, products,
services, processes, methods, data or any other confidential and proprietary
information. In order to be treated as confidential hereunder, information must
be:  (i) reduced to tangible form and marked clearly as confidential or
proprietary, or (ii) with respect to any oral presentation, such presentation
must deal with information previously or concurrently designated in writing as
confidential. Confidential information will not include any information in the
public domain other than as a result of a Party's failure to comply with its
obligations hereunder, information already known to or in the possession of the
disclosee, information independently developed by the disclosee or information
which is obtained by the disclosee without restriction on disclosure.

"Consulting Services"- means any work, direction of work, technical information,
technical consulting, or other technical services (including but not limited to
design services, analytical services, quality assurance, and other specialized
services furnished by CNC under this Agreement as specified in a mutually agreed
Statement of Work.

"Customer" - means any person or business who purchases Products and Services
(hereinafter defined) from SBC, as described in this Agreement.

"Customer Information"- means information which identifies a Customer which
includes but is not limited to name, phone number, email address, IP address,
Customer call records, data transmitted by the Customer, credit card account
information, street address, or any other information which is unique to
individual Customers.

"Documentation"- means all material associated with Products, Services and
InterLATA (all hereinafter defined) Products and Services, including, but not
limited to, test data, flow charts, documentation, manuals, data file listings,
input and output


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -19-
<PAGE>

formats, user instructions, training materials and any other written documents
normally supplied to customers of CNC.

"Global Service Provider" or "GSP" means a provider of communication services
for InterLATA transport of Customer data.

"Hosting General Agreement" -means the agreement signed between Southwestern
Bell Communications Internet Services and Concentric Network Corporation on
October 9, 1998 for the provision of Web Hosting and Consulting Services.

"IntraLATA" - means those services, revenues, and functions related to
telecommunications originating and terminating within a single Local Access and
Transport Area ("LATA") within SBC's Service Region (hereinafter defined).

"InterLATA" - means those services, revenues, and functions related to
telecommunications originating in one LATA and terminating in another LATA or
outside of a LATA.  "InterLATA" will have the same meaning as "InterLATA
Services" as used in Section 3(21) of the Communications Act of 1936, as amended
by the Act.

"Marketing Service Description or MSD" -means the most current version of the
Marketing Service Description written by SBC, as mutually agreed from time to
time, which is attached and incorporated to this Agreement as ATTACHMENT 1,
Marketing Service Description ("MSD").

"Order" - means a SBC document executed hereunder ordering s (hereinafter
defined) and/or Products (hereinafter defined) and will be deemed to incorporate
(i) the provisions of this Agreement (including the Attachments attached
hereto), as it may from time to time be amended, (ii) the specifications
applicable to such Order and (iii) any subordinate documents attached to or
referenced in this Agreement, such specifications or such Order.

"Product"- means any Service (hereinafter defined) or data Product and
associated Documentation, purchased, or ordered by SBC or SBC's customers
pursuant to Orders issued hereunder by SBC.

"SBC's Service Region" - means all geographical areas in which SBC offers
telecommunications services or internet services, whether by wire line or
wireless transmission, including without limitation, SBC's eight state region,
which includes California, Texas, Missouri, Oklahoma, Nevada, Arkansas, Kansas,
and Connecticut,  as such may be expanded by mergers and/ or acquisitions or
otherwise.


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -20-
<PAGE>

"Service"- means a data service deployed by SBC on CNC's network or in CNC's
hosting facilities for which SBC is requesting branding, provisioning,
maintenance and performance guarantees as detailed in this Agreement or its
attachments.

"Service Level Agreements or SLAs" - means specific levels of performance agreed
to be provided by CNC to either SBC customers, SBC or both for the performance
of CNC Products or Services where a credit  may attach for failure to meet
performance levels.

"Statements of Work" - means a document executed hereunder pursuant to which SBC
will order Consulting Services to be provided in connection therewith which will
be deemed to incorporate (i) the provisions of this Agreement as it may, from
time to time, be amended, (ii) the agreed upon specifications applicable to such
Statement of  Work and (iii) any subordinate documents attached to or referenced
in this Agreement, such specifications or such Statements of Work.

"Technical Service Description or TSD" -means the most current version of the
Technical Service Description written by SBC, as mutually agreed from time to
time, which is attached and incorporated to this contract as ATTACHMENT 2,
Technical Service Description ("TSD").

2.  BENEFIT OF AGREEMENT EXTENDING TO SBC AFFILIATES

SBC Affiliates may place Orders which incorporate the terms and conditions of
this Agreement, either directly or, in the event the Parties mutually determine
that a direct relationship between CNC and such SBC Affiliate would be
inappropriate, through one  or more SBC Affiliates designated by the Parties
from time to time to place Orders on behalf of other SBC Affiliates.  All
amounts payable to CNC pursuant to such SBC Affiliate Orders will be considered
Revenues for the purposes of  Part I, Section 5, SBC Revenue Commitment.   An
SBC Affiliate placing Orders under this Agreement will be responsible for its
own obligations, including but not limited to, all charges incurred in
connection with such Order.  The provisions of this Agreement will not be
construed as requiring SBC to indemnify CNC, or to otherwise be responsible, for
any acts or omissions of an Affiliate, nor will any provision of this Agreement
be construed as requiring an Affiliate to indemnify CNC, or to otherwise be
responsible, for the acts or omissions of SBC.


3.  NO INTERLATA  ACTIVITIES BY SBC

CNC is responsible for all InterLATA portions of the Products and Services.
Without limiting the foregoing, CNC will provide status and resolution
information on all trouble



                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -21-
<PAGE>

tickets and status and disposition of customer service activities, for the
InterLATA portion of the Products and Services. Nothing in this Agreement
authorizes SBC to act as a reseller of InterLATA Products and Services.
Notwithstanding anything to the contrary contained herein, SBC will not (i)
establish the criteria, (ii) establish prices, terms or conditions, (iii) serve
as the exclusive marketing agent or (iv) serve as the initial point of contact
for customer care issues, of the InterLATA Products and Services.


4.  REPRESENTATIONS AND WARRANTIES

     4.1  Authorization

          (a)  CNC warrants that it is a corporation duly existing in good
               standing in its state of incorporation, and is duly authorized to
               enter into and perform all of its obligations hereunder.

          (b)  SBC warrants that it is a corporation duly existing in good
               standing in its state of incorporation, and is duly authorized to
               enter into and perform all of its obligations hereunder.


     4.2  Compliance with Specifications

     CNC hereby represents and warrants to SBC that the Products and Services
     will perform in all material respects in the manner specified in
     ATTACHMENTS 1, Marketing Service Description,  2, Technical Service
     Description, and 9, Performance Commitments and Reports, or other
     applicable attachments.


     4.3  Year 2000

     CNC hereby represents and warrants to SBC that the hardware and software
     that comprise CNC's Products and Services provided pursuant to this
     Agreement will include Year 2000 Capabilities.  CNC will obtain warranties
     from its non-compliant third party software providers as soon as they are
     commercially available.  All warranties for hardware and software will
     survive inspection, payment, and use according to the terms hereof.
     Additionally, CNC currently has a program in place to address Year 2000
     compliance, such that CNC will operate Year 2000 compliant Products and
     Services prior to October 31, 1999.



                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -22-
<PAGE>

     4.4  Warranty Limits

     THE WARRANTIES SET FORTH IN THIS SECTION ARE THE SOLE AND EXCLUSIVE
     WARRANTIES MADE BY THE PARTIES. THE PARTIES MAKE NO OTHER WARRANTIES OF ANY
     KIND, EXPRESS OR IMPLIED, WITH RESPECT TO THE PRODUCTS AND SERVICES OR ANY
     RELATED  SERVICES OR SOFTWARE. WITH THE EXCEPTION OF THE WARRANTIES
     SPECIFIED IN THIS SECTION 4, THE PARTIES EXPRESSLY DISCLAIM ANY WARRANTIES:
     (A) OF DESIGN, MERCHANTABILITY, NONINFRINGEMENT OR FITNESS FOR A PARTICULAR
     PURPOSE, EVEN IF THEY HAVE BEEN INFORMED OF SUCH PURPOSE; OR (B) THAT THEIR
     SERVICES OR ANY RELATED SERVICES OR SOFTWARE WILL BE ERROR-FREE, SECURE OR
     WITHOUT INTERRUPTION, (EXCEPT AS SPECIFIED HEREIN).


5.  LIABILITY LIMITS

IN NO EVENT WILL EITHER PARTY BE LIABLE TO THE OTHER FOR ANY LOSS OF PROFITS OR
ANY INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES RESULTING FROM ANY
CLAIMS OR DEMANDS BROUGHT AGAINST THE OTHER PARTY, EVEN IF SUCH PARTY HAS BEEN
ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.  NOTWITHSTANDING ANY PROVISIONS
CONTAINED IN THIS AGREEMENT TO THE CONTRARY, CNC'S ENTIRE LIABILITY TO SBC
CONCERNING PERFORMANCE OR NONPERFORMANCE BY CNC OR IN ANY WAY RELATED TO THE
SUBJECT MATTER OF THIS AGREEMENT, AND REGARDLESS OF WHETHER A CLAIM IS BASED IN
CONTRACT, NEGLIGENCE OR IN TORT, WILL NOT EXCEED TWENTY-FIVE MILLION DOLLARS
($25,000,000) OR THE AMOUNT RECEIVED BY CNC FROM SBC DURING THE TERM, WHICHEVER
IS GREATER.  SBC'S ENTIRE LIABILITY TO CNC IS CAPPED AT THE MONETARY VALUE OF
THE AMOUNT RECEIVED BY CNC FROM SBC DURING THE TERM.  THE FOREGOING LIMITATION
WILL NOT REDUCE EITHER PARTIES OBLIGATIONS UNDER PART II, SECTION 6,
INDEMNIFICATION, OR  LIABLITY FOR PERSONAL INJURY.


6.  INDEMNIFICATION AND INFRINGEMENT

     6.1  Infringement



                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -23-
<PAGE>

     (a)  In the event of any claim or allegation against SBC for any
          infringement or misappropriation of any third party copyright, patent,
          trademark, trade secret  by reason of, and in the exercise by SBC of
          the rights and licenses granted herein with respect to Products and
          Services provided by CNC, hereunder, CNC will, at its expense,
          indemnify SBC and defend such claim, and pay any costs, expenses and
          finally awarded damages in connection therewith, including the
          reasonable fees and expenses of the attorneys for such defense
          provided that SBC will cooperate with CNC in connection therewith and
          notify CNC of such claim or action within a reasonable time.  The
          foregoing will not limit SBC's ability to participate in any such
          action at its own expense, and CNC will keep SBC informed of, and will
          consult with, any independent attorneys appointed by SBC, regarding
          such litigation.  CNC may not agree to any settlement that would or
          might have any effect  upon SBC or its affiliates without first
          obtaining SBC's consent in writing.

     (b)  If the permitted use by SBC of  Products and Services provided by CNC,
          has become, or in CNC's opinion is likely to become, the subject of
          any claim of such infringement, CNC may at its option and expense
          either (i) procure for SBC the right to continue using the Products
          and Services provided by CNC , (ii) replace or modify Products and
          Services provided by CNC to make them non-infringing, provided that
          such modified Products and Services are equal or superior to the
          unmodified Products and Services in all material respects, (iii)
          substitute an equivalent for Products and Services provided by CNC
          provided that such replacement Products and Services are equal or
          superior to the original Products and Services in all material
          respects . In the event CNC does not comply with the foregoing, SBC
          may terminate or modify this Agreement and without limiting SBC's
          other remedies, CNC will reimburse SBC for any damages related to
          replacement of infringing Products and Services.

     (c)  CNC will not have any liability to SBC if any allegation of
          infringement is based upon (a) the interconnection and/or modification
          and/or use of Products or Services, or associated documentation in
          combination with other devices not furnished by CNC and which have not
          been disclosed to CNC as part of SBC's proposed configuration where
          the Product, Service or associated documentation would not by itself
          be infringing, or (b) if the infringement arises out of compliance
          with SBC's specifications or designs or out of modifications made to
          the Product or Service or associated documentation, unless such
          modifications are made by CNC.



                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -24-
<PAGE>

     (d)  SBC will, at its expense, indemnify CNC and defend claims of
          infringement, and pay any costs, expenses and finally awarded damages
          in connection with SBC provided third party software for which and to
          the extent that SBC has received indemnification from the applicable
          third party and SBC proprietary software utilized in connection with
          the Products and Services including the reasonable fees and expenses
          of CNC attorneys for such defense.


     6.2  Indemnification

     (a)  CNC will indemnify, defend and hold harmless SBC (including its
          agents, employees, officers, and directors) from and against any and
          all liability, loss, finally awarded damages, court cost, attorneys'
          fees or other expense of any kind which arises out of any claim,
          demand, suit for damages, injunction or other relief, on account of,
          but not limited to, (a) injury to or death of any person, (b) damage
          to any property, including theft, or (c) any lien, caused by,
          resulting from or attributable to the Products and Services provided
          by CNC hereunder or (d) the acts or omissions resulting from the
          negligence or misconduct of CNC (including any of its agents, or
          subcontractors but excepting the active negligence or willful
          misconduct solely of SBC or its employees, subcontractors or agents)
          in furnishing the Products and Services hereunder.

     (b)  CNC agrees to defend SBC, at no cost or expense to SBC, against any
          such liability, claim, demand, suit or legal proceeding.  SBC agrees
          to notify CNC within a reasonable time of any written claims or
          demands against SBC for which CNC is responsible under this clause.
          The foregoing will not limit SBC's ability to participate in any such
          action at its own expense, and CNC will keep SBC informed of, and will
          consult with, any independent attorneys appointed by SBC, regarding
          such litigation.

     (c)  CNC agrees not to implead or bring any action against SBC or SBC's
          employees based on any claim by any person for personal injury or
          death that occurs in the course or scope of employment of such person
          by CNC and that arises out of the Products and Services furnished
          under this Contract.


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -25-
<PAGE>

     (d)  SBC will indemnify, defend and hold harmless CNC (including its
          agents, employees, officers, and directors) from and against any and
          all liability, loss, damage, court cost, attorneys' fees or other
          expense of any kind which arises out of any claim, demand, suit for
          damages, injunction or other relief, on account of acts or omissions
          resulting from the gross negligence or willful misconduct of SBC
          employees, contractors, and agents. SBC agrees to defend CNC, at no
          cost or expense to CNC, against any such liability, claim, demand,
          suit or legal proceeding.


7.  INSURANCE

     7.1  With respect to performance hereunder, and in addition to CNC's
          obligation to indemnify, CNC agrees to maintain, at all times during
          the term of this Agreement, the following minimum insurance coverages
          and limits and any additional insurance and/or bonds required by law:

          (a)  Workers' Compensation insurance with benefits afforded under the
               laws of the state in which the Products and Services are to be
               performed and Employers Liability insurance with minimum limits
               of $100,000 for Bodily Injury-each accident, $500,000 for Bodily
               Injury by disease-policy limits and $100,000 for Bodily Injury by
               disease-each employee.

          (b)  Commercial General Liability insurance with minimum limits of:
               $2,000,000 General Aggregate limit; $1,000,000 each occurrence
               sub-limit for all bodily injury or property damage incurred in
               any one occurrence; $1,000,000 each occurrence sub-limit for
               Personal Injury and Advertising; $2,000,000 Products/Completed
               Operations Aggregate limit, with a $1,000,000 each occurrence
               sub-limit for Products/Completed Operations.  Fire Legal
               Liability sub-limits of $300,000 are required for lease
               agreements.  SBC and its Affiliates will be named as an
               Additional Insured on the Commercial General Liability policy.

          (c)  If use of a motor vehicle is required, Automobile Liability
               insurance with minimum limits of $1,000,000 combined single
               limits per occurrence for bodily injury and property damage,
               which coverage will extend to all owned, hired and non-owed
               vehicles.


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -26-
<PAGE>

     7.2  SBC and its Affiliates requires that companies affording insurance
          coverage have a B+ VII or better rating, as rated in the A.M. Best Key
          rating Guide for Property and Casualty Insurance Companies.

     7.3  A certificate of insurance stating the types of insurance and policy
          limits provided the CNC must be received prior to commencement of any
          work.

     7.4  CNC will also require all subcontractors who may enter upon the work
          site to maintain the same insurance requirements listed above.

     7.5  To the extent that SBC employees are on CNC's location or performing
          services hereunder, SBC will provide similar coverage by self
          insurance methods with additional insurance coverage beyond self
          insurance.


8.  CUSTOMER INFORMATION

     8.1  All SBC Customer Information is owned by SBC including any knowledge
          base and contents therein used to support the activities contemplated
          by this Agreement.  SBC Customer Information may be provided to CNC
          for day to day management of the business, but will remain the
          property of SBC.

     8.2  SBC Customer Information will be transmitted and maintained securely
          as confidential and guarded with methods and practices acceptable to
          SBC in accordance with the confidentiality provisions of this
          Agreement.  SBC Customer Information will not be shared with third
          parties or used by CNC, either during or after the Term, for marketing
          purposes unless approved by SBC in advance in writing. Notwithstanding
          the above, CNC may share aggregate CNC customer information that
          includes aggregate SBC Customer Information, so long as individual or
          aggregate SBC Customer Information is not separable from such
          aggregate CNC customer information.

     8.3  All Customer Information is to be returned to SBC upon termination of
          this Agreement and  CNC will not retain or have access to any SBC
          Customer Information unless agreed in advance by SBC in writing as
          part of a transition plan.


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -27-
<PAGE>

     8.4  It is acknowledged that CNC will have a direct relationship with
          Customers in order to provide CNC's InterLATA Products and Services
          associated with  the Products and Services offered under this
          Agreement, and such relationship may result in  CNC obtaining
          information regarding Customers equivalent to all or part of the SBC
          Customer Information with respect to such Customers ("CNC InterLATA
          Customer Information"). CNC agrees to use such CNC InterLATA Customer
          Information solely to provide Products and Services to Customers under
          this Agreement, and without limiting the foregoing, such CNC InterLATA
          Customer Information will not be shared with third parties or used by
          CNC, either during or after the Term, for marketing purposes unless
          approved by SBC in advance in writing.

9.  NON-SOLICITATION OF SBC CUSTOMERS

CNC agrees that during the term of this Agreement and for a period of three (3)
years from any expiration or termination of this Agreement, CNC will not solicit
business from any end user Customers of SBC who purchase Products or Services by
or including any means which targets or identifies such Customers as SBC
Customers. This provision is not meant to prevent CNC from soliciting such
customers incidentally as part of a broad marketing campaign not directed at
Customers, but is intended to prevent the targeting by CNC of Customers by
virtue of their status as Customers, even if such targeting is a subpart of a
broader marketing campaign, e.g. by means of including Customers as a group as
targets of a broader marketing campaign.


10.  INTELLECTUAL PROPERTY RIGHTS

     10.1  CNC SOFTWARE

     For the Term, CNC hereby grants to SBC and Customers under this Agreement,
     and SBC hereby accepts, a nonexclusive, nontransferable license to use and
     distribute CNC  software for use solely with providing Products and
     Services to Customers.

     (a)  CNC warrants that third party software is licensed in accordance with
          the license agreement supplied by the supplier of such software, and
          that SBC's use and distribution of such software will not violate such
          license. CNC will provide SBC applicable licensing information for any
          third party software provided, such that SBC may determine its ability
          to comply.  Upon acceptance of the third party software, SBC will
          comply


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -28-
<PAGE>

          with the terms of such licenses. SBC will not be permitted to
          translate, transform, decompile, reverse engineer, disassemble, or
          otherwise determine or attempt to determine source code from the
          executable code of any software provided by CNC pursuant to this
          Agreement or to permit or authorize a third party to do so.

     (b)  Notwithstanding the foregoing paragraph, SBC will be permitted to
          reproduce or modify software provided by CNC pursuant to this
          Agreement but only in a manner approved by CNC in advance.

     Title to CNC software, is and will remain the exclusive property of CNC
     and/or its suppliers.  All Customer contracts for Products or Services will
     be consistent with the licensing provisions of this Section 10.1.


     10.2  SBC SOFTWARE

     For the Term, SBC hereby grants to CNC, and CNC hereby accepts, a
     nonexclusive, nontransferable license to use and distribute SBC software
     for use solely with providing Products and Services to Customers.

          (a)  SBC warrants that third party software is licensed in accordance
               with the end user license agreement supplied by the supplier of
               such software. CNC will not be permitted to translate, transform,
               decompile, reverse engineer, disassemble, or otherwise determine
               or attempt to determine source code from the executable code of
               any software provided by SBC pursuant to this Agreement or to
               permit or authorize a third party to do so.

          (b)  Notwithstanding the foregoing, CNC will be permitted to reproduce
               or modify software provided by SBC pursuant to this Agreement but
               only in a manner approved by SBC in advance.

     Title to SBC Software is and will remain the exclusive property of SBC
     and/or its suppliers. All Customer contracts for Products or Services will
     be consistent with the licensing provisions of this Section 10.2.

     10.3  Trademarks

     During the term of this Agreement, each Party is authorized by the other
     Party to make reasonable use of the other Party's trademarks (including
     such Party's


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              not for disclosure without prior written approval.


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     licensors' trademarks), solely in connection with the marketing,
     advertisement and promotion of Products and Services in accordance with
     this Agreement, including without limitation ATTACHMENT 12, Branding,
     provided that each Party will have the right and opportunity to disapprove
     any materials which make use of the trademarks of such Party or such
     Party's licensors that the other Party uses or proposes to use. Each Party
     will use such trademark notices as the other Party may designate from time
     to time in connection with any use of such other Party's or other Party's
     licensors' trademarks as permitted hereunder. Upon expiration or
     termination of this Agreement, each Party's rights to use any of the other
     Party's' trademarks will terminate, except in connection with activities
     permitted pursuant to Part II, Section 19, Customer Transition Provisions,
     above. Nothing contained in this Agreement will give either Party any
     interest in any of the other Party's or the other Party's licensors'
     trademarks, logos, trade names or designations other than the limited
     rights expressly granted in this Section 10.3.

     10.4  New Developments

          (a)  During the Term of this Agreement and in connection with its
               obligations hereunder, CNC may develop new technologies, products
               and services which incorporate intellectual property rights ("New
               Developments").  It is the intention of the Parties that prior to
               undertaking any work on a New Development, CNC and SBC will agree
               in a separate written Statement of Work, as defined in Attachment
               20, Consulting Services as to the Parties' respective rights in
               and with respect to such New Development.  If any such New
               Development is developed in the absence of such a Statement of
               Work, then within thirty (30) days of completion of such
               development, CNC will specify  in writing to SBC that the
               intellectual property rights fall within one of these four (4)
               categories :

               (i)  CNC Developments - proprietary development owned by CNC and
                    licensed from CNC by SBC upon the terms set forth in this
                    Part II, Section 10.1, CNC Software.

               (ii) SBC Custom Developments - Developments owned and financed by
                    SBC and developed by CNC . Any custom application or
                    technology designed by CNC at SBC's request and financed by
                    SBC, which will be owned by


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                     SBC and licensed to CNC in accordance with Part II, Section
                     10.2, SBC Software.

               (iii) Jointly Financed Developments - Developments jointly owned
                     and financed by CNC and SBC, owned by both Parties in equal
                     undivided shares, and subject to worldwide nonexclusive use
                     by either Party without any obligation to account to the
                     other Party.

               (iv)  Modified or Enhanced Developments - CNC or SBC software
                     enhanced at SBC's request and financed by SBC. With regard
                     to such Developments, SBC will own such modifications or
                     enhancements in accordance with Part II, Section 10.2, SBC
                     Software, while SBC and CNC retain ownership of their
                     respective intellectual property underlying such
                     modifications or enhancements.

          (b)  Within thirty (30) days of receipt of CNC's notice, SBC will
               notify CNC if it disagrees as to the classification of the New
               Development contained in such notice from CNC, and, if the
               Parties cannot resolve such disagreement informally, the dispute
               resolution provisions of Part II, Section 16, Arbitration,  will
               apply. If SBC fails to respond to any notice from CNC under this
               Section 9, then there will be a rebuttable presumption that the
               contents of such are correct.  If CNC fails to notify SBC as
               specified hereunder, then SBC may notify CNC of its understanding
               of the status of such New Development, and CNC will have thirty
               (30) days to respond to such notification as set forth above. If
               neither Party notifies the other regarding such New Development,
               then SBC will be automatically  granted a worldwide, perpetual,
               non-exclusive right to use the New Development in its business,
               and to sublicense its rights in such New Development to its
               Affiliates and Customers.

     10.5  General

     Except as expressly provided to the contrary above in this Section 10, all
     intellectual property rights of each Party are expressly reserved by such
     Party, and no license, express or implied, is created by this Agreement.



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              not for disclosure without prior written approval.


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11.  CONFIDENTIAL INFORMATION

     11.1  SBC's Information

     No Confidential Information obtained by CNC from SBC under, or in
     contemplation of, this Agreement, including the existence or substance of
     the discussions regarding the contemplated transactions, will become CNC's
     property.  All copies of such information in written, graphic or other
     tangible form will be returned to SBC upon request.  Unless such
     information was or is (i) previously known to CNC free of any obligation to
     keep it confidential, or (ii) subsequently made public by SBC or (iii)
     independently developed by CNC without the use of SBC Confidential
     Information, or (iv) rightfully received by CNC from a source other than
     SBC without violation of any obligation of confidentiality, or (v) required
     to be disclosed under a valid order created by a court or government agency
     (provided that CNC provides prior written notice to SBC of such obligation
     and the opportunity to oppose such disclosure), CNC will keep it
     confidential, will use it only in performing hereunder, and may use it for
     other purposes only upon such terms as may be agreed upon in writing by
     SBC.  SBC will have the right to review and approve CNC's procedures for
     handling such information and may make such inspections, as SBC deems
     necessary to assure that such information is being properly protected.  In
     such event, CNC will fully cooperate with SBC in ensuring such information
     is being properly protected.


     11.2  CNC's Information

     No Confidential Information obtained by SBC from CNC under, or in
     contemplation of, this Agreement, including the existence or substance of
     the discussions regarding the contemplated transactions will become SBC's
     property.  All copies of such information in written, graphic or other
     tangible form will be returned to CNC upon request.  Unless such
     Information was or is (i) previously known to SBC free of any obligation to
     keep it confidential, or (ii) subsequently made public by CNC, or (iii)
     independently developed by SBC without the use of CNC Confidential
     Information, or (iv) rightfully received by SBC from a source other than
     CNC without violation of any obligation of confidentiality, or (v) required
     to be disclosed under a valid order created by a court or government agency
     (provided that SBC provides prior written notice to CNC of such obligation
     and the opportunity to oppose such disclosure), SBC will keep it
     confidential, will use it only in performing hereunder, and may use it for
     other purposes only upon such terms as may be agreed upon in writing by



                            PROPRIETARY INFORMATION
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              not for disclosure without prior written approval.


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



     CNC.  CNC will have the right to review and approve SBC's procedures for
     handling such information and may make such inspections, as CNC deems
     necessary to assure that such information is being properly protected.  In
     such event, SBC will fully cooperate with CNC in ensuring such Information
     is being properly protected.

The provisions of this Section 11 are deemed retroactive to May 13, 1998 and
supercede Nondisclosure Agreement No. 980200, signed by both Parties on May 13,
1998.


12.  CNC PARTITIONING

[*]

13.  RECORDS AND AUDITS

CNC will maintain accurate records of all matters which relate to CNC's
obligations hereunder in accordance with generally accepted accounting
principles and practices uniformly and consistently applied in a format that
will permit audit.  CNC will retain such records for a period of two (2) years
from the date of expiration or termination of the Agreement. , SBC may appoint a
nationally recognized accounting firm as an independent third party auditor to
audit CNC's records, directly relevant to this Agreement, no more than once per
year, upon twenty (20) business days notice [*]. Such audits will be solely at
SBC's expense, will not unreasonably interfere with CNC's normal operations, and
will be subject to CNC's reasonable security and confidentiality requirements.
Notwithstanding the foregoing,


                            PROPRIETARY INFORMATION
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              not for disclosure without prior written approval.


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<PAGE>

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



the audit procedures set forth in Part I, Section 8.2, [*], govern all audits
concerning [*].


14.  CURE

The Parties will not be deemed to be in default under any of the terms of this
Agreement, and  the non-defaulting Party may not seek or attempt to enforce any
remedy for any claimed default, unless defaulting Party  fails to cure or
correct same within thirty (30) days or such longer period as mutually agreed,
following receipt of written notice thereof from the non-defaulting Party.


15.  DISPUTE ESCALATION

Any disputes, controversies or claims arising from this Agreement  will first be
addressed by the Steering  Committee.  Either Party may call a meeting of the
Steering Committee upon ten (10) days prior written notice.  If  the Steering
Committee is unable to resolve such dispute or conflict within ten (10) calendar
days following its first meeting discussing such dispute, either Party may
invoke the provisions of the Part II, Section 16, Arbitration.


16.  ARBITRATION

     16.1 Any dispute, controversy or claim which relates in any way to this
          Agreement and which has not been resolved by the Parties using the
          Dispute Escalation process described in the Part II, Section 15,
          Dispute Escalation, will be submitted to binding arbitration before an
          arbitration panel established under the rules of the American
          Arbitration Association.

     16.2 The arbitration panel will consist of three arbitrators; one selected
          by each Party, and the third selected by the arbitrators selected by
          the Parties.  Resolution of disputes, controversies or claims will be
          determined by a majority vote of the arbitration panel.

     16.3 The arbitration will be held in San Francisco, California unless the
          Parties otherwise mutually agree in writing.  The Parties agree to
          cooperate in order to bring a reasonable and prompt conclusion to such
          arbitration, not



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              not for disclosure without prior written approval.


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          to exceed 75 days. The arbitration panel will allow reasonable
          discovery, and may require the transcription of testimony.

     16.4 The Parties will equally bear the fees, costs and expenses of the
          arbitration, including the costs or fees of any expert or witness;
          provided, however, that the arbitration panel may award to the
          prevailing Party damages which compensate it for any such costs and
          fees, including legal costs, associated with the arbitration.  Any
          payments owed by either Party related to the subject matter so
          arbitrated may be suspended, and the interest or other late charges
          due thereon may be tolled, at the discretion of the paying Party for
          the duration of the arbitration.


17.  GOVERNING LAW

This Agreement will be construed in accordance with the laws of the State of
California, without regard to that State's choice of law provisions.


18.  RELATIONSHIP OF PARTIES

This Agreement is not intended to constitute, create, give effect to or
otherwise recognize a joint venture, partnership or formal business organization
of any kind between the Parties.  The rights and obligations of the Parties will
be only those expressly set forth herein.


19.  REMEDIES

All remedies specified in this Agreement are cumulative, and are not intended to
be exclusive of any other remedies to which the injured Party may be entitled at
law or equity. Notwithstanding the foregoing, if Damages for Delay, as
prescribed in Part I, Section 3.2, are triggered, those damages will be
exclusive for the time period covered.  It the occurrence continues beyond the
time period for which damages were selected, the remedies described under this
Agreement will be available to SBC.   The use of one or more available remedies
will not bar the use of any other available remedy for the purpose of enforcing
the provisions of this Agreement.

20.  AMENDMENT AND WAIVER



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              not for disclosure without prior written approval.


                                      -35-
<PAGE>

No amendment or waiver of any provisions of this Agreement will be effective
unless the same will be in writing and signed by a duly authorized
representative on behalf of the Party against whom such amendment, waiver or
consent is claimed.  In addition, no course of dealing or failure of any Party
to enforce strictly any term, right or condition of this Agreement will be
construed as a waiver of such term, right or condition. Waiver by either Party
of any default by the other Party will not be deemed a waiver by the non-
defaulting Party of any other default.


21.  RELEASES VOID

Neither Party will require waivers or releases of any personal rights from
representatives of the other in connection with visits to each other's
respective premises.


22.  FORCE MAJEURE

     22.1 Neither Party hereto will be held responsible for any delay or
          failure in performance of any part of this Agreement to the extent
          that such delay or failure is caused by fire, earthquake, tornado,
          hurricane, flood, explosion, war, strike, embargo, government
          requirement, civil or military authorities, Act of God or by the
          public enemy, transportation facilities, acts or omissions of carriers
          or other causes beyond the control of CNC or SBC.  If any force
          majeure condition occurs, the Party delayed or unable to perform will
          give prompt notice thereof to the other Party hereto and the Party
          affected by the other's inability to perform may elect to:

          (a)  Terminate this Agreement or any Order or part of either as to
               Products not already shipped or Products and Services not already
               performed if such delay or failure continues for more than thirty
               (30) days.

          (b)  Suspend this Agreement for the duration of the force majeure
               condition, buy or sell elsewhere Products or Services to be
               bought or sold hereunder, and deduct from any commitment the
               quantity bought or sold or for which such commitments have been
               made elsewhere.

          (c)  Resume performance hereunder once the force majeure condition
               ceases with an option in the affected Party to extend the period
               of


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                                      -36-
<PAGE>

               this Agreement up to the length of time the force majeure
               condition endured.

     22.2 Unless written notice to the contrary is given within thirty (30)
          days after such affected Party is notified of the force majeure
          condition, option (b) above will be deemed selected.


23.  ASSIGNMENT

     23.1 Neither Party hereto may assign, subcontract or otherwise transfer
          its rights or obligations under this Agreement except with the prior
          written consent of the other Party hereto, which consent will not be
          unreasonably withheld or delayed; provided, however, (i) SBC will have
          the right to assign this Agreement to any present or future Affiliate
          of SBC without securing the consent of CNC and may grant to any such
          assignee the same rights and privileges SBC enjoys hereunder and (ii)
          CNC will have the right to assign this Agreement to any purchaser of
          all or substantially all its assets if such purchaser has a net worth
          equivalent to or greater than CNC as represented by the most recent
          financial statements of CNC and such purchaser, provided that no such
          assignment or transfer will relieve SBC or CNC of any obligation to
          perform hereunder until same will have been performed by the assignee
          or transferee.

     23.2 Any attempted assignment not consented to in the manner prescribed
          herein, except an assignment confined solely to monies due or to
          become due, will be void.  It is expressly agreed that any assignment
          of monies will be void if (i) CNC fails to give SBC at least thirty
          (30) days prior written notice of such assignment or (ii) such
          assignment attempts to impose upon SBC obligations to the assignee
          additional to the payment of such moneys or preclude SBC from dealing
          solely and directly with CNC in all matters pertaining to this
          Agreement, including the negotiation of amendments or settlements of
          charges due.


24.  CONFLICT OF INTEREST

CNC represents and warrants that no officer, director, employee or agent of SBC
has been or will be employed, retained or paid a fee, or otherwise has received
or will receive any personal compensation or consideration, by or from CNC or
any of CNC's officers, directors, employees or agents in connection with the
obtaining, arranging or



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              not for disclosure without prior written approval.


                                      -37-
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negotiation of this Agreement or other documents or agreements entered into or
executed in connection herewith.


25.  LIENS PROHIBITED

All Products and Services furnished to SBC by CNC hereunder will be free from
any claims, third party liens or encumbrances.  Should CNC as a general
contractor of Products and Services subcontract such Product and Service
delivery to subcontractors that have mechanics lien rights under applicable law,
SBC may require satisfactory proof that all parties furnishing labor and/or
materials for the Products and Services have been paid before making payment to
CNC.


26.  SEVERABILITY

With the exception of regulatory events governed by Part II, Section 28,
Compliance with Laws/Regulatory Contingency if any provision or any part of a
provision of this Agreement will be invalid or unenforceable, such invalidity or
non-enforceability will not invalidate or render unenforceable any other portion
of this Agreement.  The entire Agreement will be construed as if it did not
contain the particular invalid or unenforceable provision(s) and the rights and
obligations of the CNC and SBC will be construed and enforced accordingly.


27.  SURVIVAL

Each Party's respective obligations under this Agreement which by their terms
would continue beyond the termination, or expiration hereof, including but not
limited to, those in the clauses entitled COMPLIANCE WITH LAWS, INTELLECTUAL
PROPERTY, INDEMNIFICATION, INSURANCE, LIABILITY, PUBLICITY, RELEASES VOID,
INVOICING AND PAYMENT, CONFIDENTIAL INFORMATION and WARRANTY, will survive the
termination or expiration of this Agreement.


28.  COMPLIANCE WITH LAWS/REGULATORY CONTINGENCY

     28.1  Compliance With Laws

     Each Party agrees to comply with the provisions of the Fair Labor Standards
     Act, the Occupational Safety and Health Act, and all other applicable
     federal, state,



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              not for disclosure without prior written approval.


                                      -38-
<PAGE>

     county and local laws, ordinances, regulations and codes (including the
     identification and procurement of required permits, certificates, approvals
     and inspections) in the performance of this Agreement. CNC further agrees,
     during the term hereof, to comply with all applicable Executive and Federal
     regulations as set forth in Attachment 19, Executive Orders and Associated
     Regulations.

     28.2  Regulatory Contingency

     It is the intent of the Parties that nothing contained in this Agreement be
     construed so as to make the activities of SBC constitute electronic
     publishing (as such term is defined in Section 274 of the Act), or the
     provision of interLATA services (as such term is defined in Section 272 of
     the Act).  In the event that SBC determines in good faith that regulations
     issued or interpreted by a governmental entity subsequent to this Agreement
     have such a result or otherwise cause SBC or any Affiliate, including SBC
     Communications Inc. or one of its Affiliates, to be a provider of
     electronic publishing services or interLATA services or to be in violation
     of the Act, the Parties agree to work together in good faith to renegotiate
     the objectionable terms or conditions so as to render the activities of SBC
     consistent with the Act, order or regulation and the Parties intent
     hereunder. If an agreement to modify the terms of this Agreement cannot be
     reached following such good faith negotiations, either Party may
     immediately terminate this Agreement without further liability. In the
     event that SBC determines in good faith that regulations issued or
     interpreted by a governmental entity subsequent to this Agreement that
     require SBC to add carriers other than CNC, or cause any other material
     change in circumstances which makes this Agreement between the Parties
     infeasible, the Parties agree to work together in good faith to negotiate a
     mutually acceptable accommodation. If the Parties are unable to negotiate a
     mutually acceptable accommodation, and the Steering Committee is unable to
     fashion an acceptable resolution, then this Agreement will terminate
     without consequence to either Party.  If either of such terminations
     occurs, SBC may, at its option, discontinue its revenue commitments to CNC
     without further liability, as stated in Part II, Section 5, SBC's Revenue
     Commitments to CNC.


29.  TAXES

     29.1 CNC agrees to pay and hold SBC and its affiliates harmless from and
          against any penalty, interest, additional tax or other charge that may
          be levied or assessed as a result of the delay or the failure of CNC
          for any



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                                      -39-
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          reason to pay any tax or file any return or information required by
          law, rule or regulation or by this Agreement to be paid or filed by
          CNC.

     29.2 Upon request by SBC, the Parties will consult with respect to the
          basis and rates upon which CNC will pay any taxes for which SBC is
          obligated to reimburse CNC under this Agreement.  If SBC determines
          that in its opinion any such taxes are not payable or should be paid
          on a basis less than the full price or at rates less than the full tax
          rates, or that no tax is due or is required to be withheld from SBC
          because SBC intends to sell the property in substantially the same
          condition as received from CNC, CNC will comply with such
          determination.  If the taxing authority seeks collection for a greater
          amount of taxes than that so determined by SBC, CNC will promptly
          notify SBC.  If SBC desires to contest such collection, SBC will
          promptly notify CNC.  Although CNC will cooperate with and provide
          reasonable assistance to SBC, SBC will direct the conduct of any
          proceedings, hearings or litigation involved in any contest with
          respect to taxes for which SBC is obligated to reimburse CNC under
          this Agreement.  SBC will reimburse CNC for any taxes, interest,
          penalties or out of pocket expenses of witnesses appearing in such
          proceedings, which CNC may be required to pay because of CNC's
          complying with SBC' determinations with respect to the payment or
          contesting any such taxes.

     29.3 If any taxing authority advises CNC that it intends to audit CNC with
          respect to any taxes for which SBC is obligated to reimburse CNC under
          this Agreement, CNC will (a) promptly so notify SBC, (b) afford SBC an
          opportunity to participate on an equal basis with CNC in such audit
          with respect to such taxes, and (c) keep SBC fully informed as to the
          progress of such audit. Each Party will bear its own expenses with
          respect to any such audit and the responsibility for any additional
          tax resulting from such audit will be determined in accordance with
          the applicable provisions of this Section


30.  ACCESS TO PREMISES

Each Party will, at no charge, permit the other reasonable access to its
premises in connection with the performance of this Agreement or any Order.
Employees and representatives of each Party will, while on the other's premises,
comply with all reasonable site rules and regulations.



                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -40-
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31.  USE OF SBC SYSTEMS

SBC reserves the right to determine how CNC's personnel provided to perform
Products and Services hereunder will access SBC's network remotely and locally.
CNC agrees and promises to use the applicable SBC computer systems on which the
Services are provided in a professional and effective manner, and only for the
purposes of the performance of this Agreement.  Any other intentional use or
misuse of such computer systems will constitute a breach of this Agreement.  CNC
acknowledges that any person CNC selects to work under this Agreement will be
expected to fully comply with the known requirements, conditions, or
restrictions applicable to the use of SBC's computers or telephone network or
applicable to accessing SBC's database, network or information systems.  CNC
will indemnify and hold SBC harmless for any failure (whether negligent or
intentional) to meet these known requirements, conditions, or restrictions by
any of these persons.  CNC further agrees to cooperate fully in any SBC inquiry
into alleged violations by CNC's personnel with respect to SBC's computers,
property or Confidential Information.


32.  WORK DONE BY OTHERS

If any part of the Products and Services is dependent upon work done by others,
CNC will inspect and promptly report to SBC any defect that renders such other
work unsuitable for CNC's proper performance. CNC's silence will constitute
approval of such other work as fit, proper and suitable for CNC's performance of
the Products and Services.


33.  SECURITY

The Parties have established internal security methods and procedures as
specified in ATTACHMENT 15, Network Management and Monitoring.  These security
methods and procedures pertain, but are not limited to, physical plant (network
operations center and points of presence); security between nodes (routers);
computer access; authentication; access controls (which personnel gain access to
certain applications); use of firewalls, tunneling, encryption; and network
security.


34.  INSPECTIONS


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -41-
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SBC may inspect or test, during normal business hours at all reasonable times
and places, upon five (5) days notice and request, any Products or Services
pertaining to any Order.  The exercise of, or failure to exercise, this right
will not relieve CNC of its obligation to furnish all Products and Services
materially in compliance with this Agreement and the applicable Order.  If,
prior to acceptance by SBC, any Products and/or Services are found not compliant
in all material respects with ATTACHMENT 2, Technical Service Description, SBC
will have the right to reject the same, and at no charge to SBC, to require (i)
that such Products be repaired or replaced promptly with Products that are
compliant in all material respects with ATTACHMENT 2, Technical Service
Description, at CNC's risk and expense (including freight charges), and (ii) the
performance or re-performance of such Products and Services.  Detailed
inspection records are to be maintained by CNC and made available to SBC upon
request.


35.  INDEPENDENT CONTRACTOR/ SBC AGENT

     35.1 CNC represents that it is engaged in an independent business and will
          perform its obligations under this Agreement as an independent
          contractor and not as the agent or employee of SBC, except as
          specified in Section 35.2, below.  Each Party is solely responsible
          for its acts and those of its agents, employees, and subcontractors
          (also collectively referred to as "Personnel" hereunder).  All CNC
          Personnel performing Products and Services hereunder are solely the
          employees of CNC and are not the agents or employees of SBC.  CNC has
          and retains the right to exercise full control of and supervision over
          the employment, compensation and discharge of all Personnel assisting
          in the performance of its obligations under this Agreement.  Each
          Party is solely responsible for all matters relating to the payment of
          compensation to its Personnel and will comply with all worker's
          compensation, unemployment, disability insurance, social security,
          withholding, and all other applicable federal, state and local Laws
          and Regulations governing such matters.

     35.2 For the purpose of placing orders for Customer circuits with
          Southwestern Bell Telephone, Pacific Bell, Southern New England
          Telephone and if and when appropriate, Ameritech, (collectively the
          "Telco") and only in this instance as specified hereunder, to the
          extent that CNC will be an agent of SBC Internet Services ("SBCIS").
          Such agency will be accomplished through a letter agreement between
          SBCIS and Customers, such that a letter of agency will authorize SBCIS
          to order Telco services on behalf of the Customer and permit CNC to
          place orders as SBCIS' agent.



                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


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36.  DIVERSION OF EMPLOYMENT

SBC and CNC agree that during the Term they will not knowingly solicit for hire
any person who is employed by such other Party and who has been engaged in the
performance of activities related to this Agreement, without the express written
permission of the original employing Party.


37.  UNIVERSAL DESIGN

     37.1 CNC advocates, supports, and encourages its suppliers to advocate and
          support the manufacturing and provision of products which embrace the
          concept of "universal design."  CNC will use commercially reasonable
          efforts to manufacture and provide Products and Services, including
          future versions of Products and Services, which are accessible to the
          widest range of consumers including those with disabilities.

     37.2 CNC will reasonably cooperate with SBC in addressing disability
          access issues, including but not limited to hearing aid interference,
          that may arise in connection with a Customer's use of  CNC's Products
          and Services furnished hereunder.  Specifically,  CNC agrees:

          (a)  to ensure that its equipment is designed, developed and
               fabricated to be accessible to and usable by people with
               disabilities, and

          (b)  to ensure that the service is accessible to and usable to people
               with disabilities, or

          (c)  to ensure that the equipment or service is compatible with
               existing peripheral devices or specialized customer premises
               equipment commonly used by individuals with disabilities to
               achieve access, if the requirements of subsection (a) and (b) are
               not readily achievable.


38.  HEADINGS

The headings of the clauses in each Section are inserted for convenience only
and are not intended to affect the meaning or interpretation of this Agreement.



                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -43-
<PAGE>

39.  NOTICES

Except as otherwise provided in this Agreement, all notices or other
communications hereunder will be deemed to have been duly given when received in
writing and either (i) delivered in person, (ii) delivered to an agent, such as
an overnight or similar delivery services, or (iii) delivered by the United
States Mail and addressed as follows:


To SBC Operations:                         To CNC:

Pacific Bell Internet Services &           Concentric Network Corporation, Inc.
Southwestern Bell Internet Services        1400 Parkmoor Avenue
303 Second St., North Tower, Suite 650     San Jose, CA 95126
San Francisco, California, 94107           Phone:  (408) 817-2800
Phone:  (415) 442-4900                     Fax: (408) 817-2875
Fax:   (415)  357-1659                     Attention: Manager-Contracts
Attention: Senior Attorney

With a copy (which will not be             With a copy (which will not be
considered formal notice) to:              considered formal notice) to:

SBC Operations, Inc.                       Concentric Network Corporation, Inc.
Suite 150, 111 Soledad Street, Rm 10Q5     1400 Parkmoor Avenue
San Antonio, TX  78205                     San Jose, CA 95126
Phone: (210) 351-3660                      Phone: (408) 817-2800
Fax:  (210) 370-1953                       Fax:  (408) 817-2875
Attention:  Director, Strategic Alliances  Attn.:  Senior Vice President and
                                           General Manager of the Network
                                           Services Division

The address to which notices or communications may be given by either Party may
be changed by written notice given by such Party to the other pursuant to this
Section 38.


40.  ENTIRE AGREEMENT/RELATIONSHIP AMONG DOCUMENTS

     40.1 Neither Party is bound by the provisions of this Agreement until it
          is executed in writing by authorized individuals from each Party.  The
          terms contained in this Agreement, including all exhibits,
          attachments, and schedules, supercede all previous agreements or
          documents entered into


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -44-
<PAGE>

          between the Parties. Notwithstanding the foregoing, the Trial
          Agreement, dated October 19, 1998 will remain in effect until the
          completion of the Product and Service trial and the Web Hosting
          Agreement, dated October 9, 1998 and the Stock Purchase Agreement
          dated October 19, 1998 are unaffected by this Agreement.

     40.2 The order of precedence between the Parts and Sections of this
          Agreement are as follows in descending order:  (i)  Part I, of this
          Agreement, (ii) Part II, of this Agreement, (iii) Attachments to this
          Agreement, and (iv)  any Statement of Work or Order arising out of
          this Agreement.  Notwithstanding the foregoing, any qualifying
          superceding statement referencing a specific provision of this
          Agreement, which is contained within an Attachment to this Agreement,
          is governed by that provision.



                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -45-
<PAGE>

41.  SIGNATURES

     This Agreement shall be effective as of the Effective Date specified above.


Concentric Network Corporation, Inc.     SBC Operations, Inc.

By:                                      By:
   ------------------------------------     ------------------------------------
Print Name:   Henry Nothhaft             Print Name:   Al Porta
           ----------------------------             ----------------------------
Title:   Chairman, President and CEO     Title:   VP, Technology and Development
      ---------------------------------        ---------------------------------
Date:                                    Date:
     ----------------------------------       ----------------------------------


                                         SBC Internet Services

                                         By:
                                            ------------------------------------
                                         Print Name:   Steven Hubbard
                                                    ----------------------------
                                         Title:    President and CEO
                                               ---------------------------------
                                         Date:
                                              ----------------------------------


                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -46-
<PAGE>

 ATTACHMENTS
------------

     1.   MARKETING SERVICE DESCRIPTION

     2.   TECHNICAL SERVICE DESCRIPTION

     3.   DEPLOYMENT AND DELIVERY

     4.   USE OF SBC TRANSPORT

     5.   CNC'S EXISTING STRATEGIC RELATIONSHIPS

     6.   BILLING

     7.   SYSTEMS

     8.   PRICING

     9.   PERFORMANCE COMMITMENTS AND REPORTS

     10.  SALES CHANNEL CONFLICTS

     11.  FORECASTING

     12.  BRANDING

     13.  CUSTOMER CARE AND TECHNICAL SUPPORT

     14.  SALES TRAINING AND SUPPORT

     15.  NETWORK MANAGEMENT AND MONITORING

     16.  PROVISIONING

     17.  SERVICE LEVEL AGREEMENTS

     18.  WHOLESALE DIAL ACCESS

     19.  EXECUTIVE ORDERS AND ASSOCIATED REGULATIONS

     20.  CONSULTING SERVICES

     21.  ACCEPTANCE TEST PLAN AND PROCEDURES




                            PROPRIETARY INFORMATION
   The Information contained herein is for the use of SBC Operations, Inc.
(and its Affiliated Companies) and Concentric Network Corporation only and is
              not for disclosure without prior written approval.


                                      -47-
<PAGE>

                                Attachment 1A
                                -------------
                         Marketing Service Description
                         -----------------------------
                          Package and Feature Matrices
                          ----------------------------

The following Package/Feature Matrices for Online Office Packages and EVPN
define the components that are included and those that are optional for each
package.

Attachment 1B provides additional technical detail and description of offered
services. Attachment 1C provides component hardware configurations for all
Online Office and EVPN customer sites.

[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 5 pages from
Exhibit 1A to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                           Proprietary Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.
<PAGE>

                                Attachment 1B
                                -------------
                         Marketing Service Description
                         -----------------------------
             Concentric Network's House of IP (HoIP) Services v1.0
             -----------------------------------------------------



[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 6 pages from
Exhibit B to the Agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                   Page 1
<PAGE>

                                Attachment 1C
                                -------------
                         Marketing Service Description
                         -----------------------------
                     Access Configurations for House of IP
                     -------------------------------------

Following are the Access Configurations for the House of IP products.  These
configurations are split into four groups.


[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 8 pages from
Exhibit 1C to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.


                                      1
<PAGE>

                                  Attachment 2
                                  ------------

                                  House of IP
                                  -----------


[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 179 pages from
Exhibit 2 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.
<PAGE>

                                Attachment 3
                                ------------

                            Deployment and Delivery
                            -----------------------

1.1  Broad Deployment Phase 1 Launch Schedule

     CNC will provide Products and Services to SBC as outlined in ATTACHMENT 1,
     Marketing Service Description, of the Agreement, or another appropriate
     attachment to the Agreement, according to the following launch schedule
     such that services are available for customer turn up on the dates listed
     below:

     [*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 4 pages from
Exhibit 3 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and Concentric
            Network Corporation without prior written agreement.

                                      1
<PAGE>

                                ATTACHMENT 4
                                ------------

                     CNC Use of SBC Transport and Services

I. Dedicated Access

   A.  Where available, (i) SBC services and facilities, and (ii) Affiliate or
       third party services and facilities designated by SBC from time to time
       subject to the approval of CNC, such approval not to be unreasonably
       withheld, will be used exclusively for connections and transport
       between the Customer and CNC's POP in connection with Products and
       Services made available pursuant to this Agreement; provided, however,
       that the foregoing will in no event be construed as giving SBC any
       right to determine CNC's activities with respect to InterLATA services
       within the SBC Service Region.

[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 2 pages from
Exhibit 4 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                      1
<PAGE>

                                Attachment 5

                     CNC's Existing Strategic Relationships
                     --------------------------------------


[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 3 pages from
Exhibit 5 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

<PAGE>

                                Attachment 6
                                ------------

                            Customer Billing/Finance
                            ------------------------



Process Overview

SBC will initiate a request for service from CNC with the submission of a
service order, statement of work, and purchase order.  The purchase order will
be the source of financial information exchanged with CNC and shall reflect any
service requested through the service order and statement of work.  The SBC
purchase order will contain a flag denoting it as a "New", "Change",
"Disconnect" or "Cancel" order and will include a target date for CNC
provisioning.  CNC will use the service order, statement of work, and the
purchase order to provision services.

Upon completion of provisioning of a customer's service CNC will generate an
invoice to SBC.  The invoice will address charges associated with the completed
purchase order.  The initial completed purchase order will include all services
and equipment associated with the installation of the customer's service.
Subsequent invoices will include the recurring charges and will be generated on
a monthly basis.

CNC will bill SBC for recurring charges in advance and usage charges in arrears.
Invoices will be categorized by SBIS, PBIS, and Nevada Bell and will list
charges by purchase order number.  The preferred media for information exchange
is electronic. All electronic exchanges will be controlled via a mutually agreed
upon control mechanism.




[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 3 pages from
Exhibit 6 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                      1
<PAGE>

                                  Attachment 7
                                  ------------

                                    Systems
                                    -------

Scope

Attachment 7 "Systems" defines the desired working relationship that will exist
between the systems organizations of SBC and CNC.  Members from both
organizations will meet on a regular basis to determine and coordinate systems
development efforts.

The House of IP Market Launch OSS Planning Scope Document will be developed by
the Parties.  This document will describe the scope of the Operations Support
Systems development work required to support the House of IP product set.  CNC
will be an integral member of the development effort.

[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 4 pages from
Exhibit 7 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                 Page 1 of 1
<PAGE>

                                  Attachment 8
                                  ------------

                                    Pricing
                                    -------


[*]



[*] Certain information from this Exhibit has been omitted and filed separately
with the Securities Exchange Commission.  A total of 9 pages from Exhibit 8 to
the agreement have been omitted.  Confidential treatment has been requested with
respect to the omitted portions.
<PAGE>

                                Attachment 9
                                ------------

                      Performance Commitments and Reports
                      -----------------------------------

The Definitive Agreement describes a variety of areas where the Parties have
agreed to performance commitments including Warranties in the DA at Part II,
Section 4 and Service Level Agreements contained in the Marketing Service
Description and Attachment 17.  This Attachment 9 provides further definition of
performance in addition to Warranties and SLAs, specifically performance
commitments and performance objectives.


[*]




[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 6 pages from
Exhibit 9 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                      1
<PAGE>

                                Attachment 10
                                -------------

                            Sales Channel Conflicts
                            -----------------------

Since CNC provides its services to a variety of resellers, wholesalers, and
those it categorizes as OEMs, such as SBC and its affiliates, sales conflicts
will naturally arise between where multiple providers, all of whom use CNC as
part of their network solution.

[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 1 page from
Exhibit 10 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                 Page 1 of 1
<PAGE>

                                Attachment 11
                                -------------

                                  Forecasting
                                  -----------


A.  General.  The Parties recognize that CNC requires visibility into data about
SBC's order flow process to be able to meet capacity requirements as well as
performance requirements.  The Parties will use reasonable efforts to design
processes such that CNC receives information in sufficient time to meet SBC
requirements.

[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 2 pages from
Exhibit 11 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

<PAGE>

                                Attachment 12
                                -------------

                                  Branding
                                  --------

Use of Trademarks
-----------------

The Parties will comply with Part II, Section 9.3 titled Trademarks in the use
of each other's trademarks.

Approval for Use Process
------------------------

Such use of either Party's proprietary marks shall be subject to the approval
requirements recited below:

CNC agrees that all artwork or copy to be used by CNC or its licensee(s) on
merchandising, advertising, or promotional materials which includes SBC or an
affiliate's ("SBC") name or SBC's logo shall be submitted to SBC or its
designated representative for approval at least ten (10) business days prior to
the production or manufacture of such materials, which approval shall not be
unreasonably withheld.  Should SBC wish to comment upon or disapprove such
materials, SBC shall provide written commentary or notice of disapproval
(including the basis therefore) with ten (10) business days following SBC's
receipt thereof.  If such notice is not received with the ten (10) business day
period, such artwork or copy shall be deemed approved.  Once artwork or copy has
been approved, it may be reused without any need to secure additional approvals
if the artwork or copy is reused without material alteration.

SBC agrees that all artwork or copy to be used by SBC or its licensee(s) on
merchandising, advertising, or promotional materials which includes CNC's name
or CNC's logo shall be submitted to CNC or its designated representative for
approval at least ten (10) business days prior to the production or manufacture
of such materials, which approval shall not be unreasonably withheld.  Should
CNC wish to comment upon or disapprove such materials, CNC shall provide written
commentary or notice of disapproval (including the basis therefor) with ten (10)
business days following CNC's receipt thereof.  If such notice is not received
with the ten (10) business day period, such artwork or copy shall be deemed
approved.  Once artwork or copy has been approved, it may be reused without any
need to secure additional approvals if the artwork or copy is reused without
material alteration.

The Parties will work toward creating uniform documentation which details the
general principles for use of each other's logos and trademark as appropriate.
None the less, each new instance of logo or trademark will follow the process
detailed above.


                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                     -1-
<PAGE>

CNC shall exercise best efforts to determine that all licensed third party use
of SBC's name and proprietary marks shall be consistent with usage(s) approved
by SBC as provided above.

Neither of the parties shall (i) file any application for any mark or obtain or
attempt to obtain ownership of any mark or trade name, in any country of the
world, which refers to or is suggestive of the names comprising any
distinguishing part of the proprietary rights of the other party, or (ii) use
the proprietary rights of the other party or any part of it in its trading name
or in the name of any of its products or services.  Both parties shall ensure
that their respective Affiliates will comply with the restrictions set forth.
Should SBC or CNC, as the case may be, use its own proprietary rights in or on
materials also bearing or incorporating proprietary rights of the other party,
such use shall be such as to avoid the creation or appearance of joint rights or
a joint trademark by suitable spatial separation one from the other and
distinctive script and type-face.



                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                     -2-
<PAGE>

                                Attachment 13
                                -------------
                       Customer Care / Technical Support
                       ---------------------------------

THIS ATACHMENT GOVERNS AND REFERS ONLY TO SUPPORT OF INTRALATA PRODUCTS AND
SERVICES. CNC WILL BE SOLELY RESPONSIBLE FOR SUPPORT OF INTERLATA PRODUCTS AND
SERVICES, AND ANY CUSTOMER INQUIRIES RECEIVED BY SBC PERTAINING TO THE INTERLATA
PORTION OF PRODUCTS AND SERVICES WILL BE IMMEDIATELY REFERRED TO CNC.

1.1  Objectives:

     .   Determining customer information needs across all processes and
         providing that information in the most appropriate delivery method,
         including product documentation, self-help online resources,
         email/fax, and live phone support. All processes will be designed to
         minimize live support needs.

     .   Providing customer service within performance targets provided herein
         and in as part of the Definitive Agreement, on a consistent basis

     .   Mutually determining processes and procedures that will reduce
         customer care cost. These processes and procedures will include but
         not be limited to motivational metrics that create a desire to reduce
         cost, and self-help solutions

     .   Continuously reducing handle time and average number of calls per
         subscriber

     .   Improving service and reducing expenses via technology driven
         processes and enablers; shift access from voice to self-help media

     .   Retaining account base

1.2  Technical Support Responsibilities

Technical support encompasses providing all information within the technical
support boundaries required to maintain a customer's service.

[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 4 pages from
Exhibit 13 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                 Page 1 of 1
<PAGE>

                                Attachment 14
                                -------------

                           Sales Training and Support
                           --------------------------

Training in General
-------------------

a)  If requested by SBC, CNC shall provide, upon mutually agreed upon fees,
    terms, and conditions, full and complete training courses taught by
    qualified and knowledgeable instructors for all of the Products and
    Services covered by this Agreement, as deemed necessary for the Term of
    this Agreement. The training shall be available to SBC, at SBC's option,
    as formal courses conducted at CNC's site and/or, if available, as
    computer based training and/or "suitcased" courses at SBC's sites. At the
    same time, SBC is aware that CNC does not have a training division and
    that requests for training will take into account the resource
    availability of each Party.

b)  Prior to providing training, CNC shall issue a training quote to the SBC
    Project Lead or other similar contact describing the content of the class,
    location, scheduled dates for the training, required experience level,
    cost per attendee or total cost of the course, plus an estimate of travel
    expenses. A CNC representative shall coordinate with the SBC Project Lead
    to meet the needs of the employees requesting training. Once the quote is
    accepted, CNC may finalize plans for training. SBC may cancel a scheduled
    training with 20 days notice at no cost to SBC. In addition SBC may cancel
    a scheduled training class with 48 hours notice, but shall reimburse CNC
    for any unavoidable out-of-pocket costs incurred by CNC.

c)  Each Service deployed by CNC and branded for SBC will include necessary
    training for SBC employees to implement and deploy the Service. At a
    minimum, CNC will provide two (2) seats at no charge per major release to
    SBC. Initial training will normally be provided prior to or concurrent
    with the installation date of the product at each applicable installation
    site.

d)  At SBC's request, CNC will train SBC's trainers to provide such training
    to SBC's employees. Compensation, if any, will be determined prior to
    commencement of the training.

e)  CNC training will consist of the following:

    1.  Sales Training - Designed to communicate basic information about the
    --  --------------
        Product, sales strategies, customer solution selling, etc.


[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 1 page from
Exhibit 14 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                 Page 1 of 1
<PAGE>

    2.  Customer Service Training - Designed to communicate the basic skills and
    --  -------------------------
        knowledge of the applicable industry, products, and solution problem
        solving to support SBC's operation of the Product

    3.  SBC will provide CNC personnel sales training on jointly marketed
        products and services if applicable.

f)  To the extent that CNC has the right to do so, CNC agrees to provide a
    master copy of all training materials and documentation with permission
    for SBCIS to copy for the purposes of providing training to SBC's
    employees. SBC agrees to duplicate and abide by all copyright and
    proprietary notices which appear in the original. Unless CNC is
    contractually restricted from granting such a right, SBC may modify CNC's
    course materials and documentation for SBC's internal use or may request
    that CNC modify materials and documentation at an agreed upon price.

g)   If requested by SBC, CNC shall develop and provide at a cost to be
     mutually agreed by the Parties, the license and courseware interactive
     video for all the Products and Services covered by this Agreement for
     delivery to SBC's employees by either SBC or CNC instructors at SBC's
     discretion.

f)   To the extent requested by SBC in its request for a quotation, CNC shall
     provide training on weekends or holidays.

g)   Reasonable travel and living expenses for CNC's employees associated with
     courses provided on SBC's premises shall be reimbursed.

[*]


[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 1 page from
Exhibit 14 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                 Page 2 of 1
<PAGE>

                                Attachment 15
                                -------------

                       Network Management and Monitoring
                       ---------------------------------

*  SBC requires the capability for its customer care and operations groups to
   have tools to proactively monitor the health, availability, and performance
   of the SBC services rendered by CNC in order to provide the highest level
   of support for SBC's customers and report on the performance and
   availability requirements documented in the Definitive Agreement . This
   section describes the initial Monitoring Service capabilities to be
   provided to SBC by CNC to monitor the services rendered to SBC by CNC,
   including systems, network & customer premises equipment & services.

[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 4 pages from
Exhibit 15 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                      1
<PAGE>

                               Attachement 16
                               --------------

                                Provisioning
                                ------------


SBC and CNC agree to cooperate in the following areas:  Customer Service,
Reporting, and Performance Measurement.  This will be accomplished as outlined
below:

[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 1 page from
Exhibit 16 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                 Page 1 of 1
<PAGE>

                                Attachment 17
                                -------------

                            Service Level Agreements
                            ------------------------

The following are Service Level Agreements which CNC has made to SBC for
purchase of IntraLATA services.

Latency

CNC will guarantee that the average round trip packet transmission time for all
network traffic will not exceed 80 milliseconds edge-to-edge within the CNC
Network, as measured over a calendar month. For these guarantees, the CNC
Network is defined as the CNC backbone, as bounded by its nationally deployed
SuperPOPs, as well as the 400+ MCI points of presence (POPs) used by CNC to
supplement its local presence, and the backhaul connections between the
SuperPOPs and the MCI POPs. Excluded from the definition at this time are any
other provider networks (including, but not restricted to, Frame Relay and DSL
providers), as well as any private peering, public peering, and NNI connections.
In the event that CNC fails to meet this latency guarantee, CNC will provide a
credit of 1% for every millisecond of average packet latency above the target of
80ms. Any excess latency caused by force majeure, scheduled maintenance, or by
the customer or the customer's equipment will be excluded from the calculations.
The credit will be calculated by applying the appropriate percentage to the SBC
wholesale costs for the month's CNC network charges for all eligible
connections. Eligible connections for this guarantee are all T1, T3 and Frame
Relay connections provided to EVPN and Online Office Business Class customers.
ISDN lines are excluded from SLAs at this time.  SBC can receive a maximum
credit of 100% of the IntraLATA monthly recurring charges owed by SBC to CNC for
eligible lines when the latency guarantee is not met under this section. Latency
statistics will be gathered utilizing tools developed and utilized by CNC's
Network Operations Center. For informational purposes, daily average latency
statistics is expected to  be available for SBC review via a Web site by April
30, 1999, but compliance or non-compliance with the guarantee will be based on
the average round-trip latency values over the entire calendar month.

Network Availability guarantee of 100% edge-to-edge

CNC will guarantee that the Concentric Backbone Network will be available to
customers 100% of the time during a calendar month. For these guarantees, the
CNC Network is defined as the CNC backbone, as bounded by its nationally
deployed SuperPOPs, as well as the 400+ MCI points of presence (POPs) used by
CNC to supplement its local presence, and the backhaul connections between the
SuperPOPs and the MCI POPs. Also included is the NNI circuit(s) between the CNC
Network and the SBC network. Excluded from the definition at this time are any
other provider networks (including, but not restricted to, Frame Relay and DSL
providers), as well as any private peering, public peering, and NNI connections
to carriers other than SBC. In the event that CNC fails to

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                      1
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meet this guarantee, CNC will provide a credit of 1/30 of the monthly service
charges for every hour, or fraction thereof, of network downtime experienced
by an end user. The amount of downtime will be calculated by determining the
total number of minutes of premise-to-premise downtime (backbone plus local
loop) for every eligible connection. Eligible connections for this guarantee
are all T1, T3 and Frame Relay connections provided to EVPN and Online Office
Business Class customers. The amount of edge-to-edge downtime experienced by
the eligible customer will then be calculated by subtracting any downtime due
to local loop or customer premise router failures, and then subtracting any
backbone outages due to force majeure, scheduled maintenance, and any downtime
caused by the customer or the customer's equipment, as well as any downtime
caused by problems with the SBC-managed router(s) terminating the CNC-SBC
NNI(s). The result is the amount of downtime experienced by the eligible
customer caused by edge-to-edge network availability failures. The credits
will be calculated by applying the guarantee against the SBC wholesale costs
for the month's CNC network charges for all eligible connections. SBC can
receive a maximum credit of 100% of the monthly wholesale costs to eligible
lines when the network availability guarantee is not met under this section.
Network availability statistics will be gathered utilizing tools developed and
utilized by CNC's Network Operations Center, as well as tools such as HP
Openview and CNC's trouble ticketing system. For informational purposes, daily
network availability statistics is expected to be available for SBC review via
a Web site by April 30, 1999, but compliance or non-compliance with the
guarantee will be based on the availability values calculated at month's end.

24 Hour Call Success Rate

CNC will guarantee that the average call success rate for all dial network
traffic will not fall below 95% or the industry average for 24 hour call success
rate, whichever is higher, as measured monthly by Inverse Technologies, a third
party rating service. In the event that CNC fails to meet this guarantee, CNC
will provide a credit of 10% for every letter grade (as defined by Inverse)
below target. The credit will be calculated by applying the appropriate
percentage to the SBC wholesale costs for the month's CNC dial network charges
for all eligible connections. Eligible connections for this guarantee are all
secure analog dial customers. SBC can receive a maximum credit of 100% of the
monthly wholesale costs for eligible dial users when the guarantee is not met
under this section. Inverse Technology places thousands of calls into 40+ key
locations around the country and reports the results on a monthly basis to CNC.

Initial Modem Connect Speed Guarantee

CNC will guarantee that the average initial modem connect speed for all dial
network traffic will not fall below 27.5kbps or the industry average, as
measured monthly by Inverse Technologies, a third party rating service. In the
event that CNC fails to meet this guarantee, CNC will provide a credit of 10%
for every letter grade (as defined by Inverse) below target. The credit will be
calculated by applying the appropriate percentage to the SBC wholesale costs for
the month's CNC dial network charges for all eligible


                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                      2
<PAGE>

connections. Eligible connections for this guarantee are all secure analog
dial customers. SBC can receive a maximum credit of 100% of the monthly
wholesale costs for eligible dial users when the guarantee is not met under
this section. Inverse Technology places thousands of calls into 40+ key
locations around the country and reports the results on a monthly basis to
CNC. For this metric, Inverse measures using 33.6kbps modems.

Packet Loss

CNC will guarantee that the average round trip packet loss for all network
traffic will not exceed 1% edge-to-edge within the CNC Network, as measured over
a calendar month. For these guarantees, the CNC Network is defined as the CNC
backbone, as bounded by its nationally deployed SuperPOPs, as well as the 400+
MCI points of presence (POPs) used by CNC to supplement its local presence, and
the backhaul connections between the SuperPOPs and the MCI POPs. Excluded from
the definition at this time are any other provider networks (including, but not
restricted to, Frame Relay and DSL providers), as well as any private peering,
public peering, and NNI connections. In the event that CNC fails to meet this
packet loss guarantee, CNC will provide a credit of 1% for every tenth of a
percent of packet loss above the target of 1%. Any excess packet loss caused by
force majeure, scheduled maintenance, or by the customer or the customer's
equipment will be excluded from the calculations. The credit will be calculated
by applying the appropriate percentage to the SBC wholesale costs for the
month's CNC network charges for all eligible connections. Eligible connections
for this guarantee are all T1, T3 and Frame Relay connections provided to EVPN
and Online Office Business Class customers with. SBC can receive a maximum
credit of 100% of the IntraLATA monthly recurring charges owed by SBC to CNC for
eligible lines when the latency guarantee is not met under this section. Packet
loss statistics will be gathered utilizing tools developed and utilized by CNC's
Network Operations Center. For informational purposes, daily average packet loss
statistics is expected to be available for SBC review via a Web site by April
30, 1999, but compliance or non-compliance with the guarantee will be based on
the average round-trip packet loss values over the entire calendar month.

SLAs Across Network Boundaries
------------------------------

For SLAs across network boundaries, the Parties agree to put processes in place
to provide SLAs across network boundaries prior to product launch under
Attachment 3, Deployment and Delivery.   In addition, CNC commits to extending
the SLAs contained in this Attachment 17 available on DSL no later than 90 days
following execution of this Agreement.  During the initial 90 day period, CNC
will provide latency, network availability and packet SLAs as defined above for
DSL; however, the measurement of these SLAs will not extend to CNC equipment
unique to DSL.

Credit Procedures
-----------------

Until an electronic procedure is established, CNC will provide a manually
produced spreadsheet which details the SLA failures and Customers which qualify
for a credit. This spreadsheet will be delivered 5 days after the end of the
month each month.   Each SBC


                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                      3
<PAGE>

customer will be listed by eligible site and customer name separately on the
spreadsheet. In addition, the CNC spreadsheet will indicate where some but not
all customer site are eligible for a credit. At the same time, CNC will either
deliver a check equal to the credit due to SBC for the SLAs or a Product
credit at SBC's option.

Lastly, CNC's metrics and data used to determine credits for SLAs will be made
available for SBC's reasonable inspection to confirm the accuracy of the SLA
calculations provided by CNC.


                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.


                                      4
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                                Attachment 18
                                -------------

                         Wholesale Dial Access Services
                         ------------------------------

In the near future, subject to legal and regulatory requirements, SBC or its
Affiliates will be offering a wholesale dial access/modem pooling service which
will allow CNC to aggregate its dial access traffic on a LATA by LATA basis
("Dial Access [*]




[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 2 pages from
Exhibit 18 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                      1
<PAGE>

                                Attachment 19
                                -------------

                  Executive Orders and Associated Regulations
                  -------------------------------------------

Work under this contract may be subject to the provisions of certain Executive
Orders, federal laws, state laws, and associated regulations governing
performance of this contract including, but not limited to: Executive Order
11246, Executive Order 11625, Executive Order 11701, and Executive Order 12138,
Section 503 of the Rehabilitation Act of 1973 as amended and the Vietnam Era
Veteran's Readjustment Assistance Act of 1974.  To the extent that such
Executive Orders, federal laws, state laws, and associated regulations apply to
the work under this contract, and only to that extent, Contractor agrees to
comply with the provisions of all such Executive Orders, federal laws, state
laws, and associated regulations, as now in force or as may be amended in the
future, including, but not limited to the following:

1. EQUAL EMPLOYMENT OPPORTUNITY DUTIES AND PROVISIONS OF GOVERNMENT CONTRACTORS

   In accordance with 41 C.F.R.(S)60-1.4(a), the parties incorporate herein by
this reference the regulations and contract clauses required by that section,
including but not limited to, Contractor's agreement that it will not
discriminate against any employee or applicant for employment because of race,
color, religion, sex, or national origin.  The Contractor will take affirmative
action to ensure that applicants are employed, and that employees are treated
during employment, without regard to their race, color, religion, sex, or
national origin.

2. AGREEMENT OF NON SEGREGATED FACILITIES

   In accordance with 41 C.F.R.(S)60-1.8, Contractor agrees that it does not
and will not maintain or provide for its employees any facilities segregated
on the basis of race, color, religion, sex, or national origin at any of its
establishments, and that it does not and will not permit its employees to
perform their services at any location, under its control, where such
segregated facilities are maintained. The term "facilities" as used herein
means waiting rooms, work areas, restaurants and other eating areas, time
clocks, rest rooms, wash rooms, locker rooms and other storage or dressing
areas, parking lots, drinking fountains, recreation or entertainment areas,
transportation, and housing facilities provided for employees; provided, that
separate or single-user restroom and necessary dressing or sleeping areas
shall be provided to assure privacy between the sexes.

3.  AGREEMENT OF AFFIRMATIVE ACTION PROGRAM

    Contractor agrees that it has developed and is maintaining an Affirmative
Action Plan as required by 41 C.F.R.(S)60-1.4(b).

4.  AGREEMENT OF FILING

    Contractor agrees that it will file, per current instructions, complete and
accurate reports on Standard Form 100 (EE0-1), or such other forms as may be
required under 41 C.F.R.(S)60-1.7(a).

5.  AFFIRMATIVE ACTION FOR HANDICAPPED PERSONS AND DISABLED VETERANS, VETERANS
    OF THE VIETNAM ERA.

    In accordance with  41 C.F.R.(S)60-250.20, and 41 C.F.R.(S)60-741.20, the
parties incorporate herein by this reference the regulations and contract
clauses required by those provisions to be made a part of government contracts
and subcontracts.

6.  UTILIZATION OF SMALL, SMALL DISADVANTAGED AND WOMEN-OWNED SMALL BUSINESS
    CONCERNS

    As prescribed in 48 C.F.R., Ch. 1, 19.708(a):

    (a) It is the policy of the United states that small business concerns,
small business concerns owned and controlled by socially and economically
disadvantaged individuals and small business concerns owned and controlled by
women shall have the maximum practicable opportunity to participate in
performing contracts let by any Federal agency, including contracts and sub-
contracts for systems, assemblies, components, and related services for major
systems. It is further the policy of the United States that its prime
contractors establish procedures to ensure the timely payment amounts due
pursuant to the terms of the subcontracts with small business concerns, small
business concerns owned and controlled by socially and economically
disadvantaged individuals and small business concerns owned and controlled by
women.


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  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
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                                     1
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  (b)  The Contractor hereby agrees to carry out this policy in the awarding of
subcontracts to the fullest extent consistent with efficient contract
performance.  The Contractor further agrees to cooperate in any studies or
surveys as may be conducted by the United States Small Business Administration
or the awarding agency of the United States as may be necessary to determine the
extent of the Contractor's compliance with this clause.

  (c)  As used in this contract, the term small business concern shall mean a
small business as defined pursuant to section 3 of the Small Business Act and
relevant regulations promulgated pursuant thereto.  The term small business
concern owned and controlled by socially and economically disadvantaged
individuals shall mean a small business concern which is at least 51 percent
unconditionally owned by one or more socially and economically disadvantaged
individuals; or, in the case of any publicly owned business, at least 51 per
centum of the stock of which is unconditionally owned by one or more socially
and economically disadvantaged individuals; and (2) whose management and daily
business operations are controlled by one or more such individuals.  This term
also means small business concern that is at least 51 percent unconditionally
owned by an economically disadvantaged Indian tribe or Native Hawaiian
Organization, or a publicly owned business having at least 51 percent of its
stock unconditionally owned by one of these entities which has its management
and daily business controlled by members of an economically disadvantaged Indian
tribe or Native Hawaiian Organization, and which meets the requirements of 13
CRF part 124.  The Contractor shall presume that socially and economically
disadvantaged individual include Black Americans, Hispanic Americans, Native
Americans, Asian-Pacific Americans, Subcontinent Asian Americans, and other
minorities, or any other individual found to be disadvantaged by the
Administration pursuant to section 8(a) of the Small business Act.  The
Contractor shall presume that socially and economically disadvantaged entities
also include Indian Tribes and Native Hawaiian Organizations.

  (d)  The term "small business concern owned and controlled by women" shall
mean a small business concern (i) which is at least 51 percent owned by one or
more women, or, in the case of any publicly owned business, at least 51 percent
of the stock of which is owned by one or more women, and (ii) whose management
and daily business operations are controlled by one or more women; and

  (e)  Contractors acting in good faith may rely on written representations by
their sub-contractors regarding their status as a small business concern, a
small business concern owned and controlled by socially and economically
disadvantage individuals or a small business concern owned and controlled by
women.

7.   SMALL, SMALL DISADVANTAGED AND WOMEN-OWNED SMALL BUSINESS SUB-CONTRACTING
PLAN.   The sub-contractor will adopt a plan similar to the plan required by 48
CFR Ch. 1 at 52.219-9.



                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                      2
<PAGE>

                                 Attachment 20
                                 -------------

                              CONSULTING SERVICES
                              -------------------

In addition to the applicable terms of the Definitive Agreement to which this
Exhibit is attached, the terms of this Exhibit H will govern the terms of the
provision of Consulting Services, as described in an executed Statement of Work,
to SBC by CNC.  All capitalized terms used and not otherwise defined herein will
have the meanings ascribed thereto in the Definitive Agreement.

[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 9 pages from
Exhibit 20 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                            Confidential Information
    Not for disclosure outside of SBC Operations Inc. and its Affiliates or
        Concentric Network Corporation without prior written agreement.

                                      1
<PAGE>

                                Attachment 21
                                -------------

                      Acceptance Test Plan and Procedures
                      -----------------------------------


The Parties have agreed to the test plan included in this attachment for the
initial Product acceptance for the May 7, 1999 launch in the initial cities
which include Austin, in Texas and San Francisco, Oakland, San Jose, Los
Angeles, and Orange County in California.

[*]



[*] Certain information from this Exhibit has been omitted and filed
separately with the Securities Exchange Commission. A total of 2 pages from
Exhibit 21 to the agreement have been omitted. Confidential treatment has been
requested with respect to the omitted portions.

                          Confidential Information
  Not for disclosure outside of SBC Operations Inc. and its Affiliates or
       Concentric Network Corporation without prior written agreement.

                                      1