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                                                                   EXHIBIT 10.43
 
                        CONCENTRIC NETWORK CORPORATION

                        COMMON STOCK PURCHASE AGREEMENT

                                 JULY 30, 1997
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                               TABLE OF CONTENTS

                                                                Page
                                                                ----

1.   Agreement To Sell and Purchase..........................    1

     1.1   Authorization Of Shares...........................    1
     1.2   Sale And Purchase Of Common Stock.................    1
     1.3   Hart-Scott-Rodino Compliance......................    1

2.   Closing, Delivery And Payment...........................    1

3.   Representations and Warranties Of The Company...........    2

     3.1   Organization, Good Standing And Qualification.....    2
     3.2   Authorization; Binding Obligations................    3
     3.3   Compliance With Other Instruments.................    3
     3.4   Securities Exemption..............................    3
     3.5   Full Disclosure...................................    3
     3.6   Valid Issuance of Shares..........................    4
     3.7   Litigation, Etc...................................    4
     3.8   Governmental Consent, etc.........................    4
     3.9   Termination of Employment.........................    5
     3.10  Intellectual Property Rights......................    5
     3.11  Proprietary Information Agreements................    5
     3.12  Lock-up Agreements................................    6

4.   Representations And Warranties Of Purchaser.............    6

     4.1   Requisite Power And Authority.....................    6
     4.2   Consents..........................................    6
     4.3   Investment Representations........................    6
     4.4   Legends...........................................    7
     4.5   Removal of Legend and Transfer Restrictions.......    8

5.   Conditions To Closing...................................    8

     5.1   Conditions To Purchaser's Obligations At
            The Closing......................................    8
     5.2   Conditions To Obligations Of The Company..........    9

6.   Rule 144 Reporting......................................    9

                                      -i-
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                               TABLE OF CONTENTS
                                  (continued)

                                                                Page
                                                                ----

7.   Miscellaneous..........................................     10

     7.1   Governing Law....................................     10
     7.2   Survival.........................................     10
     7.3   Successors And Assigns...........................     10
     7.4   Separability.....................................     10
     7.5   Amendment And Waiver.............................     11
     7.6   Notices..........................................     11
     7.7   Expenses.........................................     11
     7.8   Attorneys' Fees..................................     11
     7.9   Titles And Subtitles.............................     11
     7.10  Counterparts.....................................     11
     7.11  Broker's Fees....................................     12
     7.12  Termination......................................     12
     7.13  Subsequent, Consents, Permits and Waivers........     12
 
List of Exhibits:

Exhibit A   -  Preliminary Prospectus
Exhibit B   -  Legal Opinion

                                     -ii-
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                         CONCENTRIC NETWORK CORPORATION

                        COMMON STOCK PURCHASE AGREEMENT


     THIS COMMON STOCK PURCHASE AGREEMENT (THE "AGREEMENT") IS ENTERED INTO AS
OF JULY 30, 1997, BY AND BETWEEN CONCENTRIC NETWORK CORPORATION, A FLORIDA
CORPORATION (THE "COMPANY") AND BAY NETWORKS, INC., A DELAWARE CORPORATION
("PURCHASER").


                                    RECITALS

     WHEREAS, the Company intends to reincorporate in Delaware by merger into
Concentric Network Corporation, a Delaware corporation that was formed for the
purpose of the reincorporation and that will be the surviving entity; the
"Company" as used in this Agreement refers to, prior to such merger, the Florida
corporation and, after such merger, the Delaware Corporation; and

     WHEREAS, Purchaser desires to purchase shares of the Company's  common
stock ("Common Stock"), such purchase to be made in a private placement to close
concurrently with, but not before, the closing of the initial public offering of
the Company having an aggregate price to the public of $40 million (the "IPO")
pursuant to the registration statement filed on Form S-1 (File No. 333-27241)
with the Securities and Exchange Commission (the "Commission") (such
registration statement, as amended, shall be referred to herein as the
"Registration Statement"); and

     WHEREAS, the Company desires to issue and sell the Shares (as defined
below) to Purchaser on the terms and conditions set forth herein.

     NOW, THEREFORE, in consideration of the foregoing recitals and the mutual
promises hereinafter set forth, the parties hereto agree as follows:

1.   AGREEMENT TO SELL AND PURCHASE.

     1.1   AUTHORIZATION OF SHARES.  On or prior to the Closing (as defined in
Section 2 below), the Company shall have authorized the sale and issuance to
Purchaser of the Shares. Prior to the Closing the Company will have adopted and
filed a Certificate of Incorporation (the "Certificate of Incorporation") with
the Secretary of State of the State of Delaware authorizing sufficient shares of
Common Stock to cover the sale and issuance of the Shares to be purchased
hereunder.

     1.2   SALE AND PURCHASE OF COMMON STOCK.  Subject to the terms and
conditions hereof, the Company hereby agrees to issue and sell to Purchaser and
Purchaser agrees to purchase from the Company, at the Closing, a number of
shares of Common Stock of the Company equal to the Purchase Price (as hereafter
defined) divided by the Price to Public per share set forth on the cover 
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page of the final Prospectus (as defined in Section 3.5 below) at an aggregate
purchase price of Three Million Dollars ($3,000,000) (the "Purchase Price"). The
shares of Common Stock to be purchased hereunder are referred to as the
"Shares." For purposes of this Agreement, the $40 million aggregate price to the
public of the IPO shall include: (i) the $3,000,000 to be invested by Purchaser
as provided in this Agreement, and (ii) an aggregate investment of approximately
$17 million to be made in the Company by Williams Communications Group, Inc.
("Williams").

     1.3   HART-SCOTT-RODINO COMPLIANCE.  Notwithstanding anything else in this
Agreement, if the sale and issuance of the Shares is subject to the premerger
notification requirements of the Hart-Scott-Rodino Antitrust Improvements Act of
1976, as amended (the "HSR Act"), it shall be a condition to the Closing that
any waiting period under the HSR Act applicable to the purchase of the Shares
shall have expired or been terminated and any approvals required thereunder
shall have been obtained, and the parties shall cooperate in promptly filing
premerger reports and in taking all steps reasonably necessary to obtain early
termination of any applicable HSR Act waiting periods.  If any such waiting
period shall not have expired or been subject to early termination on or before
the date ninety (90) days from the date of this Agreement, either party may
terminate this Agreement by giving written notice to the other.

2.   CLOSING, DELIVERY AND PAYMENT.
 
     Subject to the terms of Section 5, the closing of the sale and purchase of
the Shares under this Agreement (the "Closing") shall take place concurrently
with, but not before, the closing of the IPO at the offices of Wilson Sonsini
Goodrich & Rosati, Professional Corporation, 650 Page Mill Road, Palo Alto,
California 94304.  The date of the Closing is referred to as the "Closing Date."
At the Closing, subject to the terms and conditions hereof, the Company will
deliver to Purchaser a certificate representing the number of Shares to be
purchased at the Closing against payment by or on behalf of Purchaser of the
Purchase Price therefor by cash, wire transfer, or by such other means as shall
be mutually agreeable to Purchaser and the Company.

3.   REPRESENTATIONS AND WARRANTIES OF THE COMPANY.

     Except as set forth in the Schedule of Exceptions attached hereto as Annex
1 or as otherwise disclosed in the Registration Statement on Form S-1 (File No.
333-27241) filed with the Securities and Exchange Commission (the "Registration
Statement"), the Company hereby represents and warrants to Purchaser as
following:

     3.1    ORGANIZATION, GOOD STANDING AND QUALIFICATION.  The Company is a
corporation duly organized, validly existing and in good standing under the laws
of the State of Florida, and will, as of the Closing Date, be a corporation duly
organized, validly existing and in good standing under the laws of the State of
Delaware.  The Company has full power and authority to own and operate its
properties and assets, and to carry on its business as presently conducted.  The
Company is duly qualified, is authorized to do business and is in good standing
as a foreign corporation in all jurisdictions in which the nature of its
activities and of its properties (both owned and leased) makes 

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such qualification necessary, except for those jurisdictions, in the aggregate,
in which failure to do so would not have a material adverse effect on the
Company or its business.

     3.2    AUTHORIZATION; BINDING OBLIGATIONS.  All corporate action on the 
part of the Company, its officers, directors and shareholders necessary for the
authorization, execution and delivery of this Agreement and the Certificate of
Incorporation, for the sale and issuance of the Shares pursuant hereto and for
the performance of the Company's obligations hereunder has been taken or will be
taken prior to the Closing. This Agreement, when executed and delivered, will be
a valid and binding obligation of the Company enforceable in accordance with its
terms. The sale of the Shares is not and will not be subject to any preemptive
rights or rights of first refusal that have not been properly waived or complied
with. When issued in compliance with the provisions of this Agreement and the
Certificate of Incorporation, the Shares will be validly issued, fully paid and
nonassessable, and will be free of any liens or encumbrances; provided, however,
that the Shares may be subject to restrictions on transfer under state and/or
federal securities laws as set forth herein or as otherwise required by such
laws at the time a transfer is proposed.

     3.3    COMPLIANCE WITH OTHER INSTRUMENTS.  The execution, delivery and
performance of and compliance with this Agreement and the issuance and sale of
the Shares pursuant hereto will not (i) materially conflict with, or result in a
material breach or violation of, or constitute a material default under, or
result in the creation or imposition of any material lien, (ii) violate,
conflict with or result in the breach of any material terms of, or result in the
material modification of, any material contract or otherwise give any other
contracting party the right to terminate a material contract, or constitute (or
with notice or lapse of time both constitute) a material default under any
material contract to which the Company is a party or by or to which it or any of
its assets or properties may be bound or subject or (iii) result in any
violation, or be in conflict with or constitute a default under any term, of its
charter or bylaws.

     3.4    SECURITIES EXEMPTION.  Assuming the accuracy of the representations
and warranties of the Purchaser contained in Section 4.3 hereof, the offer, sale
and issuance of the Shares will be exempt from the registration requirements of
the Securities Act of 1933, as amended (the "Securities Act"), and will have
been registered or qualified (or are exempt from registration and qualification)
under the registration, permit or qualification requirements of all applicable
state securities laws.

     3.5    FULL DISCLOSURE.

            (a)   The Company has delivered to Purchaser a draft of the
Registration Statement substantially in the form filed with the Commission.

            (b)   The Company shall deliver the finalized text of the 
preliminary prospectus included in any pre-effective amendment to the
Registration Statement that is to be printed by the Company for distribution to
prospective purchasers (the "Preliminary Prospectus"). When so delivered, the
Preliminary Prospectus shall be appended to this Agreement as Exhibit A and
                                                              ---------
incorporated herein as if delivered at the time this Agreement was executed. The
Preliminary

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Prospectus will contain all statements that are required to be stated therein in
accordance with, and will comply as to form in all material respects with, the
Securities Act and will not include any untrue statement of a material fact or
omit to state any material fact required to be stated therein or necessary to
make the statements therein not misleading.

            (c)   When the Registration Statement becomes effective, and at all
times subsequent thereto, up to the Closing Date, (1) the Registration Statement
and the Prospectus (as hereinafter defined) and any amendments or supplements
thereto will contain substantially all statements that are required to be stated
therein in accordance with the Securities Act and will comply as to form in all
material respects with to the requirements of the Securities Act, and (2)
neither the Registration Statement nor the Prospectus, nor any amendment or
supplement thereto, will include any untrue statement of a material fact or will
omit to state any material fact required to be stated therein or necessary to
make the statements therein not misleading.

     For purposes of this Agreement, "Prospectus" means the prospectus, as
amended, on file with the Commission at the time the Registration Statement
becomes effective, including the information deemed to be part of the
Registration Statement at the time of effectiveness pursuant to Rule 430A, if
applicable, except that if the Prospectus filed by the Company pursuant to Rule
424(b) differs from the prospectus on file at the time the Registration
Statement becomes effective, the term "Prospectus" shall refer to the Rule
424(b) Prospectus from and after the time it was filed with the Commission or
transmitted to the Commission for filing.

     3.6    VALID ISSUANCE OF SHARES.  The Shares which will be purchased by
Purchaser hereunder, when issued, sold and delivered in accordance with the
terms hereof for the consideration expressed herein, will be duly and validly
authorized and issued, fully paid and nonassessable.

     3.7    LITIGATION, ETC.  There is no action, suit, proceeding nor, to the
best of the Company's knowledge, any investigation pending or currently
threatened against the Company, that questions the validity of this Agreement or
the right of the Company to enter into such agreements, or which might result,
either individually or in the aggregate, in any material adverse change in the
assets, condition, affairs or prospects of the Company, financial or otherwise.

     3.8    GOVERNMENTAL CONSENT, ETC.  No consent, approval or authorization 
of, or designation, declaration or filing with, any governmental authority on
the part of the Company is required in connection with the valid execution,
delivery, and performance of this Agreement or the offer, sale or issuance of
the Shares, or the consummation of any other transaction contemplated by this
Agreement except certain filings as may be required under the Securities Act and
state securities laws and regulations, which filings will be made timely in
accordance with the applicable law or regulation.

     3.9    TERMINATION OF EMPLOYMENT.  The Company is not aware that any 
officer or key employee, or that any group of key employees, intends to
terminate their employment with the Company, nor does the Company have a present
intention to terminate the employment of any of the 

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foregoing. The employment of each officer and employee of the Company is
terminable at the will of the Company.

     3.10   INTELLECTUAL PROPERTY RIGHTS.  The Company (a) owns or has the right
to use, free and clear of all rights, liens, claims and restrictions, all
patents, trademarks, service marks, trade names, copyrights and other intangible
or intellectual property rights (and licenses with respect to the foregoing)
needed for or used in the conduct of its business as now conducted and as
proposed to be conducted without infringing upon or otherwise acting adversely
to the right or claimed right of any person under or with respect to any of the
foregoing and (b) does not and will not infringe any patents, copyrights,
trademarks or other intellectual property rights (including trade secrets),
privacy or similar rights of any third party, nor has any material claim
(whether embodied in an action, past or present) of such infringement been, to
the Company's knowledge, threatened or asserted nor is such a claim pending
against the Company.  The Company owns or has the unrestricted right to use all
trade secrets, including know-how, inventions, designs, processes, and technical
data required for or incident to the development, manufacture, operation and
sale of all products and services sold or proposed to be sold by the Company and
all of the patents, trademarks, service marks, trade names, copyrights and trade
secrets of the Company are held by the Company free and clear of any rights,
liens or claims of others, including, without limitation, current and former
employees, former employers of all current and former employees, consultants,
officers, directors and shareholders of the Company.  The Company has not
received any communications alleging that the Company has violated or, by
conducting its business as now conducted and as proposed to be conducted would
violate any of the patents, trademarks, service marks, trade names, copyrights
or trade secrets or other proprietary rights of any other person or entity.

     3.11   PROPRIETARY INFORMATION AGREEMENTS.  All employees and consultants
whom the Company considers to be key technical employees or consultants are
parties to a written agreement ("Proprietary Information and Inventions
Agreement") under which each such employee or consultant (i) is obligated to
disclose and transfer to the Company, without the receipt by such person of any
additional value therefor (other than normal salary or fees for consulting
services), all inventions, developments and discoveries which, during the period
of employment with or performance of services for the Company, he or she makes
or conceives of either solely or jointly with others, that relate to any subject
matter which his or her work for the Company may be concerned, or relate to  or
are connected with the business, products or projects of the Company, or involve
the use of the time, material or facilities of the Company, and (ii) is
obligated to maintain the confidentiality of proprietary information of the
Company.  To the Company's knowledge, no Company employee or consultant is in
violation of the Proprietary Information and Inventions Agreement to which such
employee or consultant is a party.  To the Company's knowledge, no Company
employee or consultant is obligated under any contract (including licenses,
covenants or commitments of any nature) or other agreement, or subject to any
judgment, decree or order of any court or administrative agency, that would
conflict with their obligation to use their best efforts to promote the
interests of the Company or that would conflict with the Company's business as
conducted or as proposed to be conducted.  Neither the execution nor delivery of
this Agreement nor the carrying on of the Company's business by the employees
and consultants of the Company, nor the conduct of the Company's business as
proposed, will, to the Company's knowledge, conflict with 

                                       5
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or result in a breach of the terms, conditions or provisions of, or constitute a
default under, any contract, covenant or instrument under which any of such
employees and consultants is now obligated. The Company does not believe it is
or will be necessary to utilize any inventions of any of its employees and
consultants (or people it currently intends to hire) made prior to their
employment by the Company.

     3.12   LOCK-UP AGREEMENTS.  GS Capital Partners, L.P., Kleiner Perkins
Caufield & Byers VII, KPCB VII Founders Fund, KPCB Information Sciences Zaibatsu
Fund II and TMI Telemedia International Ltd. and the executive officers of the
Company have entered into 360 day lock-up agreements with the Company and/or the
underwriters of the IPO in substantially similar form to that entered into by
the shareholders at the request of the Company's underwriters, and Racal
Datacom, Inc. will be, prior to the effectiveness of the Registration Statement,
subject to a 180 day lock-up restriction.

4.   REPRESENTATIONS AND WARRANTIES OF PURCHASER

     Purchaser hereby represents and warrants to the Company as follows (such
representations and warranties do not lessen or obviate the representations and
warranties of the Company set forth in this Agreement):

     4.1    REQUISITE POWER AND AUTHORITY.  Purchaser has all necessary power 
and authority under all applicable provisions of law to execute and deliver this
Agreement and to carry out the provisions of this Agreement. All action on
Purchaser's part required for the lawful execution and delivery of this
Agreement has been or will be effectively taken prior to the Closing. This
Agreement, when executed and delivered, will be a valid and binding obligation
of Purchaser, enforceable in accordance with its terms, except (i) as limited by
applicable bankruptcy, insolvency, reorganization, moratorium or other laws of
general application affecting enforcement of creditors' rights and (ii) general
principles of equity that restrict the availability of equitable remedies.

     4.2    CONSENTS.  All consents, approvals, orders, authorizations,
registrations, qualifications, designations, declarations or filings with any
governmental or banking authority on the part of Purchaser required in
connection with the consummation of the transactions contemplated in this
Agreement have been or shall have been obtained prior to and be effective as of
the Closing.

     4.3    INVESTMENT REPRESENTATIONS.  Purchaser understands that the Shares
have not been registered under the Securities Act.  Purchaser also understands
that the Shares are being offered and sold pursuant to an exemption from
registration contained in the Securities Act based in part upon Purchaser's
representations contained in the Agreement.  Purchaser hereby represents and
warrants as follows:

            (a)   PURCHASER IS AN ACCREDITED INVESTOR.  Purchaser represents 
that Purchaser is an Accredited Investor within the meaning of Rule 501(a) of
Regulation D under the Securities Act.

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            (b)   PURCHASER BEARS ECONOMIC RISK.  Purchaser must bear the 
economic risk of this investment indefinitely unless the Shares are registered
pursuant to the Securities Act, or an exemption from registration is available.
Purchaser understands that it has no registration rights with respect to the
Shares. Purchaser also understands that there is no assurance that any exemption
from registration under the Securities Act will be available and that, even if
available, such exemption may not allow Purchaser to transfer all or any portion
of the Shares under the circumstances, in the amounts or at the times Purchaser
might propose.

            (c)   ACQUISITION FOR OWN ACCOUNT.  Purchaser is acquiring the 
Shares for Purchaser's own account for investment only, and not with a view
towards their distribution within the meaning of the Securities Act.

            (d)   PURCHASER CAN PROTECT ITS INTEREST.  Purchaser represents 
that by reason of its, or of its management's, business or financial experience,
Purchaser has the capacity to protect its own interests in connection with the
transactions contemplated in this Agreement. Purchaser is not a corporation,
trust or partnership specifically formed for the purpose of consummating these
transactions.

            (e)   COMPANY INFORMATION.  Purchaser has had an opportunity to 
discuss the Company's business, management and financial affairs with directors,
officers and management of the Company and has had the opportunity to review the
Company's operations and facilities. Purchaser has also had the opportunity to
ask questions of and receive answers from, the Company and its management
regarding the terms and conditions of this investment.

     4.4    LEGENDS.  Each certificate representing the Shares may be endorsed
with the following legends:

            "THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
            REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE "ACT"),
            AND ARE "RESTRICTED SECURITIES" AS DEFINED IN RULE 144 PROMULGATED
            UNDER THE ACT. THE SECURITIES MAY NOT BE SOLD OR OFFERED FOR SALE OR
            OTHERWISE DISTRIBUTED EXCEPT (I) IN CONJUNCTION WITH AN EFFECTIVE
            REGISTRATION STATEMENT FOR THE SHARES UNDER THE ACT, OR (II) IN
            COMPLIANCE WITH RULE 144 OR (III) PURSUANT TO AN OPINION OF COUNSEL
            TO THE CORPORATION THAT SUCH REGISTRATION OR COMPLIANCE IS NOT
            REQUIRED AS TO SUCH SALE, OFFER OR DISTRIBUTION."

                                       7
<PAGE>
 
The Company need not register a transfer of any Shares, and may also instruct
its transfer agent not to register the transfer of the Shares, unless the
conditions specified in the foregoing legends are satisfied.

     4.5    REMOVAL OF LEGEND AND TRANSFER RESTRICTIONS.  Any legend endorsed 
on a certificate pursuant to subsection 4.4 and the stop transfer instructions
with respect to such Shares shall be removed and the Company shall issue a
certificate without such legend to the holder thereof if such legend may be
properly removed under the terms of Rule 144 promulgated under the Securities
Act or if such holder provides the Company with an opinion of counsel for such
holder, reasonably satisfactory to legal counsel for the Company, to the effect
that a sale, transfer or assignment of such Shares may be made without
registration.
 
5.   CONDITIONS TO CLOSING.

     5.1    CONDITIONS TO PURCHASER'S OBLIGATIONS AT THE CLOSING.  Purchaser's
obligation to purchase the Shares identified in Section 1.2 of the Agreement at
the Closing are subject to the satisfaction, at or prior to the Closing, of the
following conditions:

            (a)   CONCURRENT CLOSING OF IPO.  The Closing shall occur 
concurrently with, and not before, the closing of the IPO.

            (b)   OTHER INVESTOR PARTICIPATION.  The Company will have a binding
commitment by Williams or other investors willing to purchase an additional
$11.95 million of the Company's Common Stock in connection with the investment
by the Purchaser hereunder, pursuant to terms and conditions no more favorable
to such investors (other than the warrant coverage, board seat and acquisition
notices provided to Williams) than that offered to Purchaser.

            (c)   REPRESENTATIONS AND WARRANTIES TRUE; PERFORMANCE OF 
OBLIGATIONS. The representations and warranties made by the Company in Section 3
(excepting Section 3.5(b)) hereof shall be true and correct in all material
respects as of the Closing with the same force and effect as if they had been
made as of the Closing, and the Company shall have performed and complied with
all obligations and conditions herein required to be performed or complied with
by it on or prior to the Closing.

            (d)   LEGAL INVESTMENT.  At the time of the Closing, the sale and
issuance of the Shares shall be legally permitted by all laws and regulations to
which Purchaser and the Company are subject.

            (e)   CONSENTS, PERMITS, AND WAIVERS.  The Company shall have 
obtained any and all authorizations, approvals, consents, permits and waivers
necessary or appropriate for consummation of the transactions contemplated by
this Agreement (except for such as may be properly obtained subsequent to the
Closing, and such items shall be effective on and as of the Closing).

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            (f)   OUTSTANDING PREFERRED STOCK.  The shares of Preferred Stock of
the Company, issued and outstanding as of the date of this Agreement, will
convert into shares of Common Stock in connection with the IPO.

            (g)   COPIES.  The Company shall have delivered to Purchaser a true 
and complete copy of the Registration Statement and any amendments thereto, of
each exhibit filed therewith and of the Preliminary Prospectus and final form of
Prospectus.

            (h)   LEGAL OPINION.  The Company shall have delivered an opinion of
counsel to the Purchaser in substantially the form attached hereto as Exhibit B.
                                                                      --------- 

     5.2    CONDITIONS TO OBLIGATIONS OF THE COMPANY.  The Company's obligation 
to issue and sell the Shares at the Closing is subject to the satisfaction, on
or prior to the Closing. of the following conditions:

            (a)   CONCURRENT CLOSING OF IPO.  The closing of the IPO shall occur
concurrently with, but not before,  the Closing.

            (b)   REPRESENTATIONS AND WARRANTIES TRUE.  The representations and
warranties made by Purchaser in Section 4 hereof shall be true and correct in
all material respects at the date of the Closing, with the same force and effect
as if they had been made on and as of said date.

            (c)   PERFORMANCE OF OBLIGATIONS.  Purchaser shall have performed 
and complied with all agreements and conditions herein required to be performed
or complied with by Purchaser on or before the Closing.

6.   RULE 144 REPORTING.

     With a view to making available to Purchaser the benefits of certain rules
and regulations of the SEC which may permit the sale of the Shares to the public
without registration, the Company agrees at all times after ninety (90) days
after the effective date of the Registration Statement to:

            (a)   make and keep public information available, as those terms are
understood and defined in SEC Rule 144.

            (b)   file with the SEC in a timely manner all reports and other
documents required of the Company under the Securities Act and the Securities
Exchange Act of 1934, as amended (the "Exchange Act").

                                       9
<PAGE>
 
            (c)   so long as Purchaser owns any Shares, to furnish to Purchaser
within a reasonable time upon a written request by Purchaser, a written
statement by the Company as to its compliance with the reporting requirements of
said Rule 144 (at any time after ninety (90) days after the effective date of
the first registration statement filed by the Company for an offering of its
securities to the general public) and of the Exchange Act (at any time after it
has become subject to such reporting requirements), a copy of the most recent
annual or quarterly report of the Company, and such other reports and documents
so filed by the Company as Purchaser may reasonably request in complying with
any rule or regulation of the SEC allowing Purchaser to sell any such securities
without registration.

7.   MISCELLANEOUS

     7.1    GOVERNING LAW.  This Agreement shall be governed in all respects by
the laws of the State of California.

     7.2    SURVIVAL.  The representations, warranties, covenants and agreements
made herein shall survive any investigation made by Purchaser and the closing of
the transactions contemplated hereby.  All statements as to factual matters
contained in any certificate or other instrument delivered by or on behalf of
the Company pursuant hereto in connection with the transactions contemplated
hereby shall be deemed to be representations and warranties by the Company
hereunder solely as of the date of such certificate or instrument, except as
expressly provided otherwise in such certificate or instrument.

     7.3    SUCCESSORS AND ASSIGNS.  Except as otherwise expressly provided
herein, the provisions hereof shall inure to the benefit of, and be binding
upon, the successors, assigns, heirs, executors and administrators of the
parties hereto and shall inure to the benefit of and be enforceable by each
person who shall be a holder of the Shares from time to time; provided, however,
that prior to the receipt by the Company of adequate written notice of the
transfer of any Shares specifying the full name and address of the transferee,
the Company may deem and treat the person listed as the holder of such Shares in
its records as the absolute owner and holder of such Shares for all purposes,
the payment of any dividends or any redemption price.

     7.4    SEPARABILITY.  In case any provision of the Agreement shall be
invalid, illegal or unenforceable, such provision shall, to the extent
practicable, be modified so as to make it valid, legal and enforceable and to
maintain as nearly as practicable the intent of the parties, and the validity,
legality and enforceability of the remaining provisions shall not in any way be
affected or impaired thereby.

     7.5    AMENDMENT AND WAIVER.

            (a)   This  Agreement may be amended or modified only upon the 
written consent of the parties hereto.

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<PAGE>
 
            (b)   The obligations of the Company and the rights of the holder of
the Shares under this Agreement may be waived only with the written consent of
the parties hereto.

            (c)   Except to the extent provided in this Section 7.5, neither 
this Agreement nor any provision hereof may be changed, waived, discharged or
terminated, except by a statement in writing signed by the party against which
enforcement of the change, waiver, discharge or termination is sought.

            (d)   Any amendment or waiver effected in accordance with this 
Section 7.5 shall be binding upon any future holder of some or all of the
Shares.

     7.6    NOTICES.  All notices and other communications required or permitted
hereunder shall be in writing and shall be deemed effectively given and received
(a) upon personal delivery, (b) on the fifth day following mailing sent by
registered or certified mail, return receipt requested, postage prepaid, (c)
upon confirmed delivery by means of a nationally recognized overnight courier
service or (d) upon confirmed transmission of facsimile addressed:  (i) if to
Purchaser, at Purchaser's address as set forth on the Company's records, or at
such other address as Purchaser shall have furnished to the Company in writing
or (ii) if to the Company, at its address as set forth at the end of this
Agreement, or at such other address as the Company shall have furnished to
Purchaser in writing.

     7.7    EXPENSES.  The Company shall pay all costs and expenses that it 
incurs with respect to the negotiation, execution, delivery and performance of
the Agreement, and Purchaser shall pay all costs and expenses that it incurs
with respect to the negotiation, execution, delivery and performance of this
Agreement.

     7.8    ATTORNEYS' FEES.  If legal action is brought to enforce or interpret
this Agreement, the prevailing party shall be entitled to recover its reasonable
attorneys' fees and legal costs in connection therewith.

     7.9    TITLES AND SUBTITLES.  The titles of the paragraphs and 
subparagraphs of the Agreement are for convenience of reference only and are not
to be considered in construing this Agreement.

     7.10   COUNTERPARTS.  This Agreement may be executed in any number of
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one instrument.

     7.11   BROKER'S FEES.  Each party hereto represents and warrants that no
agent, broker, investment banker, person or firm acting on behalf of or under
the authority of such party hereto is or will be entitled to any broker's or
finder's fee or any other commission directly or indirectly in connection with
the transactions contemplated herein.  Each party hereto further agrees to
indemnify each other party for any claims, losses or expenses incurred by such
other party as a result of the representation in this Section 7.11 being untrue.

                                      11
<PAGE>
 
     7.12   TERMINATION.  If the Registration Statement for the IPO has not 
become effective by October 31, 1997, Purchaser in its discretion may elect to
terminate this Agreement by providing written notice to the Company at any time
thereafter.

     7.13   SUBSEQUENT, CONSENTS, PERMITS AND WAIVERS.  The Company shall obtain
promptly after the Closing all authorizations, approvals, consents, permits and
waivers that are necessary or applicable for consummation of the transactions
contemplated by this Agreement and that were not obtained prior to the Closing
because they may be properly obtained subsequent to the Closing.

                                      12
<PAGE>
 
     IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the date set forth in the first paragraph hereof.

COMPANY:

CONCENTRIC NETWORK CORPORATION
10590 N. Tantau Avenue
Cupertino, CA 95014


By:  /s/ Henry R. Nothhaft
     -----------------------------
     Henry R. Nothhaft
     President and Chief Executive Officer


PURCHASER:

BAY NETWORKS, INC.


By:  /s/ David Rynne
     -----------------------------
     (Signature)

Its  CFO
     -----------------------------

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