<PAGE>   1
                                                                  Exhibit 4.12.5

THIS WARRANT HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED (THE "ACT") OR ANY STATE SECURITIES LAWS. NO SALE OR DISPOSITION MAY BE
EFFECTED WITHOUT (i) AN EFFECTIVE REGISTRATION STATEMENT RELATED THERETO, (ii)
AN OPINION OF COUNSEL OR OTHER EVIDENCE, REASONABLY SATISFACTORY TO THE COMPANY,
THAT SUCH REGISTRATIONS ARE NOT REQUIRED, (iii) RECEIPT OF A NO-ACTION LETTER
FROM THE APPROPRIATE GOVERNMENTAL AUTHORITIES, OR (iv) OTHERWISE COMPLYING WITH
THE PROVISIONS OF SECTION 7 OF THIS WARRANT.

                                 June 29, 1999

                         CONCENTRIC NETWORK CORPORATION

                         COMMON STOCK PURCHASE WARRANT

     THIS CERTIFIES THAT, for value received, Micosoft Corporation, is entitled
to subscribe for and purchase, subject to the provisions and upon the terms and
conditions hereinafter set forth, 500,000 fully paid and nonassessable shares of
Common Stock of Concentric Network Corporation, a Delaware corporation (the
"Company"). The shares (as defined below) shall be purchasable at a per share
purchase price equal to $33.27 (such price and such other price as shall result,
from time to time, from the adjustments specified in Section 4 hereof is herein
referred to as the "Warrant Price"). As used herein, (a) the term "Date of
Grant" shall mean June 29, 1999, and (b) the term "Shares" shall mean the Common
Stock to be issued by the Company hereunder and any stock into or for which any
such Common Stock may hereafter be converted or exchanged.

     1.   TERM.     The purchase right represented by this Warrant is
exercisable, in whole or in part, at any time and from time to time from the
Date of Grant through the earliest to occur of four years after the Date of
Grant. If during such term there is any consolidation or merger of the Company,
or any successor, with or into any entity, or any other corporate reorganization
in which the Company, or any successor, shall not be the continuing or surviving
entity of such consolidation, merger or reorganization or any transaction, any
series of related transactions in which in excess of 50% of the voting power of
Company, or any successor, is transferred, or any sale of all or substantially
all of the assets of the Company or any successor, adequate provision shall be
made as a part of such transaction for the assumption of this Warrant and the
full preservation of the economic benefits inherent in this Warrant. In the
event of any such transaction a replacement Warrant shall be promptly issued to
the then holder of this Warrant.

     2.   METHOD OF EXERCISE; PAYMENT; ISSUANCE OF NEW WARRANT.  Subject to
Section 1 hereof, the purchase right represented by this Warrant may be
exercised by the holder hereof, in whole or in part and from time to time, at
the election of the holder hereof, by (a) the surrender of this Warrant (with
the notice of exercise substantially in the form attached hereto as EXHIBIT A-1
duly completed and executed) at the principal office of the Company and by the
payment to the Company, by certified

<PAGE>   2
or bank check, or by Wire Transfer to an account designated by the Company (a
"Wire Transfer") of an amount equal to the then applicable Warrant Price
multiplied by the number of Shares then being purchased, or by net exercise in
accordance with Section 9, and (b) the delivery of a duly executed Investor
Representation Statement in the form attached hereto as Exhibit A-2. The person
or persons in whose name(s) any certificate(s) representing the Shares shall be
issuable upon exercise of this Warrant shall be deemed to have become the
holder(s) of record of, and shall be treated for all purposes as the record
holder(s) of, the shares represented thereby (and such shares shall be deemed to
have been issued) immediately prior to the close of business on the date or
dates upon which this Warrant is exercised. In the event of any exercise of the
rights represented by this Warrant, certificates for the shares of stock so
purchased shall be delivered to the holder hereof as soon as possible and in any
event within thirty (30) days after such exercise and, unless this Warrant has
been fully exercised or expired, a new Warrant representing the portion of the
Shares, if any, with respect to which this Warrant shall not then have been
exercised shall also be issued to the holder hereof as soon as possible and in
any event within such thirty-day period.

     3.   Stock Fully Paid: Reservation of Shares.  All shares that may be
issued upon the exercise of the rights represented by this Warrant will, upon
issuance pursuant to the terms and conditions herein, be fully paid and
nonassessable, and free from all taxes, liens, and charges with respect to the
issue thereof. During the period within which the rights represented by this
Warrant may be exercised, the Company will at all times have authorized, and
reserved for the purpose of the issue upon exercise of the purchase rights
evidenced by this Warrant, a sufficient number of shares of its capital stock to
provide for the exercise of the rights represented by this Warrant.

     4.   Adjustment of Warrant Price and Number of Shares. The number and kind
of securities purchasable upon the exercise of this Warrant and the Warrant
Price shall be subject to adjustment from time to time upon the occurrence of
certain events, as follows:

          (a)  Reclassification.  In case of any reclassification or change of
securities of the class issuable upon exercise of this Warrant (other than a
change in par value, or from par value to no par value, or from no par value to
par value, or as a result of a subdivision or combination), the Company shall
duly execute and deliver to the holder of this Warrant a new Warrant (in form
and substance reasonably satisfactory to the holder of this Warrant), so that
the holder of this Warrant shall have the right to receive, at a total purchase
price not to exceed that payable upon the exercise of the unexercised portion of
this Warrant, and in lieu of the Shares theretofore issuable upon exercise of
this Warrant, the kind and amount of shares of stock, other securities, money
and property receivable upon such reclassification or change by a holder of the
number of Shares then purchasable under this Warrant. Such new Warrant shall
provide for adjustments that shall be as nearly equivalent as may be practicable
to the adjustments provided for in this Section 4. The provisions of this
subparagraph (a) shall similarly apply to successive reclassifications or
changes.

          (b)  Subdivision or Combination of Shares.  If the Company at any time
while this Warrant remains outstanding and unexpired shall subdivide or combine
its outstanding shares of capital stock into which this Warrant is exercisable,
the Warrant Price shall be proportionately decreased in the case of a
subdivision or increased in the case of a combination, effective at the close of
business on the date the subdivision or combination becomes effective.



                                      -2-
<PAGE>   3
          (c)  Stock Dividends and Other Distributions. If the Company at any
time while this Warrant is outstanding and unexpired shall (i) pay a dividend
with respect to its common stock issuable hereunder payable in the same class or
series of capital stock, or (ii) make any other distribution of common stock
with respect to its common stock (except any distribution specifically provided
for in Sections 4(a) and 4(b)), then the Warrant Price shall be adjusted, from
and after the date of determination of shareholders entitled to receive such
dividend or distribution, to that price determined by multiplying the Warrant
Price in effect immediately prior to such date of determination by a fraction
(i) the numerator of which shall be the total number of shares of the applicable
class or series of capital stock outstanding immediately prior to such dividend
or distribution, and (ii) the denominator of which shall be the total number of
shares of such class or series of capital stock outstanding immediately after
such dividend or distribution.

          (d)  Adjustment of Number of Shares. Upon each adjustment in the
Warrant Price, the number of Shares purchasable hereunder shall be adjusted, to
the nearest whole share, to the product obtained by multiplying the number of
Shares purchasable immediately prior to such adjustment in the Warrant Price by
a fraction, the numerator of which shall be the Warrant Price immediately prior
to such adjustment and the denominator of which shall be the Warrant Price
immediately thereafter.

     5.   Notice of Adjustments. Whenever the Warrant Price or the number of
Shares purchasable hereunder shall be adjusted pursuant to Section 4 hereof,
the Company shall make a certificate signed by its chief financial officer
setting forth, in reasonable detail, the event requiring the adjustment, and
the Warrant Price and the number of Shares purchasable hereunder after giving
effect to such adjustment, and shall cause copies of such certificate to be
mailed (without regard to Section 12 hereof, by first class mail, postage
prepaid) to the holder of this Warrant at such holder's last known address.

     6.   Fractional Shares. No fractional Shares will be issued in connection
with any exercise hereunder, but in lieu of such fractional shares the Company
shall make a cash payment therefor based on the fair market value of the Shares
on the date of exercise as reasonably determined in good faith by the Company's
Board of Directors.

     7.   Compliance with Act; Disposition of Warrant or Shares.

          (a)  Compliance with Act. The holder of this Warrant, by acceptance
hereof, agrees that this Warrant, and the Shares to be issued upon exercise
hereof are being acquired for investment and that such holder will not offer,
sell or otherwise dispose of this Warrant, or any Shares except under
circumstances which will not result in a violation of the Securities Act of
1933 (the "Act") or any applicable state securities laws. Upon exercise of this
Warrant, unless the Shares being acquired are registered under the Act and any
applicable state securities laws or an exemption from such registration is
available, the holder hereof shall confirm in writing that the Shares so
purchased are being acquired for investment and not with a view toward
distribution or resale in violation of the Act and shall confirm such other
matters related thereto as may be reasonably requested by the Company. This
Warrant and all Shares issued upon exercise of this Warrant (unless registered
under

                                      -3-
<PAGE>   4
the Act and any applicable state securities laws) shall be stamped or imprinted
with a legend in substantially the following form:

     "THE SECURITIES EVIDENCED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
     SECURITIES ACT OF 1933, AS AMENDED, OR ANY STATE SECURITIES LAWS. NO SALE
     OR DISPOSITION MAY BE EFFECTED WITHOUT (i) AN EFFECTIVE REGISTRATION
     STATEMENT RELATED THERETO, (ii) AN OPINION OF COUNSEL OR OTHER EVIDENCE,
     REASONABLY SATISFACTORY TO THE COMPANY, THAT SUCH REGISTRATION ARE NOT
     REQUIRED, (iii) RECEIPT OF AN NO-ACTION LETTERS FROM THE APPROPRIATE
     GOVERNMENTAL AUTHORITIES, OR (iv) OTHERWISE COMPLYING WITH THE PROVISIONS
     OF SECTION 7 OF THE WARRANT UNDER WHICH THESE SECURITIES WERE ISSUED,
     DIRECTLY OR INDIRECTLY."

     Said legend shall be removed by the Company, upon the request of a holder,
at such time as the restrictions on the transfer of the applicable security
shall have terminated. In addition, in connection with the issuance of this
Warrant, the holder specifically represents to the Company by acceptance of
this Warrant as follows:

          (1)  The holder is aware of the Company's business affairs and
financial condition, and has acquired information about the Company sufficient
to reach an informed and knowledgeable decision to acquire this Warrant. The
holder is acquiring this Warrant for its own account for investment purposes
only and not with a view to, or for the resale in connection with, any
"distribution" thereof in violation of the Act.

          (2)  The holder understands that this Warrant has not been registered
under the Act in reliance upon a specific exemption therefrom, which exemption
depends upon, among other things, the bona fide nature of the holder's
investment intent as expressed herein.

          (3)  The holder further understands that this Warrant must be held
indefinitely unless subsequently registered under the Act and qualified under
any applicable state securities laws, or unless exemptions from registration
and qualification are otherwise available. The holder is aware of the
provisions of Rule 144, promulgated under the Act.

     (b)  Disposition of Warrant or Shares.  With respect to any offer, sale or
other disposition of this Warrant or any Shares acquired pursuant to the
exercise of this Warrant prior to registration of such Warrant or Shares, the
holder hereof agrees until the expiration applicable holding period as provided
for in Rule 144(k) to give written notice to the Company prior thereto,
describing briefly the manner thereof, together with a written opinion of such
holder's counsel, or other evidence, if reasonably requested by the Company, to
the effect that such offer, sale or other disposition may be effected without
registration or qualification (under the Act as then in effect or any federal or
state securities law then in effect) of this Warrant or the Shares and
indicating whether or not under the Act certificates for this Warrant or the
Shares to be sold or otherwise disposed of require any restrictive legend as to
applicable restrictions on transferability in order to ensure

                                      -4-
<PAGE>   5
compliance with such law. Promptly upon receiving such written notice and
reasonably satisfactory opinion or other evidence, if so requested, the
Company, as promptly as practicable but no later than fifteen (15) days after
receipt of the written notice, shall notify such holder that such holder may
sell or otherwise dispose of this Warrant or such Shares, all in accordance
with the terms of the notice delivered to the Company. If a determination has
been made pursuant to this Section 7(b) that the opinion of counsel for the
holder or other evidence is not reasonably satisfactory to the Company, the
Company shall so notify the holder promptly with details thereof after such
determination has been made. Notwithstanding the foregoing, this Warrant or
such Shares may, as to such federal laws, be offered, sold or otherwise
disposed of in accordance with Rule 144 or 144A under the Act, provided that
the Company shall have been furnished with such information as the Company may
reasonably request to provide a reasonable assurance that the provisions of
Rule 144 or 144A have been satisfied. Each certificate representing this
Warrant or the Shares thus transferred (except a transfer pursuant to Rule 144
or 144A) shall bear a legend as to the applicable restrictions on
transferability in order to ensure compliance with such laws, unless in the
aforesaid opinion of counsel for the holder, such legend is not required in
order to ensure compliance with such laws. The Company may issue stop transfer
instructions to its transfer agent in connection with such restrictions.

          (e)  Applicability of Restrictions. Neither any restrictions of any
legend described in this Warrant nor the requirements of Section 7(b) above
shall apply to any transfer or grant of a security interest in this Warrant (or
the Shares obtainable upon exercise thereof) or any part hereof (i) to a
partner of the holder if the holder is a partnership, or (ii) to a partnership
of which the holder is a partner; provided however, in any such transfer, if
applicable, the transferee shall agree in writing to be bound by the terms of
this Warrant as if an original signatory hereto.

     8.   Rights as Shareholders; Information. No holder of this Warrant, as
such, shall be entitled to vote or receive dividends or be deemed the holder of
Shares, nor shall anything contained herein be construed to confer upon the
holder of this Warrant, as such, any of the rights of a shareholder of the
Company or any right to vote for the election of directors or upon any matter
submitted to shareholders at any meeting thereof, or to receive notice of
meetings, or to receive dividends or subscription rights or otherwise until
this Warrant shall have been exercised and the Shares purchasable upon the
exercise hereof shall have become deliverable, as provided herein.
Notwithstanding the foregoing, the Company will transmit to the holder of this
Warrant such information, documents and reports as are generally distributed to
the holders of any class or series of the securities of the Company
concurrently with the distribution thereof to the shareholders.

     9.   Right to Convert Warrant into Stock Net Issuance. The registered
holder hereof shall have the right to convert this Warrant, in whole, at any
time at or prior to the expiration of the Warrant pursuant to Section 1 hereof,
by the surrender of this Warrant and the notice of exercise form attached
hereto duly executed to the principal office of the Company, into Shares as
provided in this Section 9. Upon exercise of this conversion right, the holder
hereof shall be entitled to receive that number of Shares of the Company (y)
equal to the quotient obtained by dividing ((A - B)(X)) by (A), where:

          A.   =    the Fair Market Value (as defined below) of one Share on the
                    date of conversion of this Warrant.



                                      -5-
<PAGE>   6
          B = the Warrant Price for one Share under this Warrant.

          X = the number of Shares as to which this Warrant is being converted.

     If the above calculation results in a number equal to or less than zero,
then no Shares shall be issued or issuable upon conversion of this Warrant.

     "Fair Market Value" of a Share shall mean:

          (a)  The average closing price of the Company's common stock quoted on
the Nasdaq National Market or, if the Company's common stock is not quoted on
the Nasdaq National Market, the average closing price quoted on any exchange on
which the Company's common stock is listed, whichever is applicable, for the
last twenty-five (25) trading days prior to the date of delivery of the notice
pursuant to this Section 9; or

          (b)  If the Company's common stock is not traded on an exchange, the
Fair Market Value shall be determined in good faith by the Company's Board of
Directors based on the Company's most recent sale of common or preferred stock.

     Upon conversion of this Warrant in accordance with this Section 9, the
registered holder hereof shall be entitled to receive a certificate for the
number of Shares determined in accordance with the foregoing.

     10.  Representations and Warranties.  The Company represents and warrants
to the holder of this Warrant as follows:

          (a)  This Warrant has been duly authorized and executed by the Company
and is a valid and binding obligation of the Company enforceable in accordance
with its terms, subject to laws of general application relating to bankruptcy,
insolvency and the relief of debtors and the rules of law or principles at
equity governing specific performance, injunctive relief and other equitable
remedies;

          (b)  The Shares have been, or will be duly authorized and reserved for
issuance by the Company and, when issued in accordance with the terms hereof
will be validly issued, fully paid and non-assessable.

     11.  Covenant Not To Transfer Warrant or Shares.  The holder of this
Warrant agrees not to offer, pledge, sell, contract to sell, sell any option or
contract to purchase, purchase any option or contract to sell, grant any option,
right or warrant to purchase, or otherwise transfer or dispose of, directly or
indirectly the Warrant or any of the Shares or any securities convertible into
or exercisable or exchangeable for the Warrant or the Shares (whether such
shares or any such securities are now owned by the undersigned or are hereafter
acquired), for 545 days following the date hereof, provided that the holder of
this Warrant may sell the Warrant or the Share or any other securities of the
Company without regard to the provisions of this Section 11 if (i) a tender or
exchange offer is made (as evidenced by the filing with the SEC of a Schedule
14D-1 (if applicable, or any successor form promulgated or adopted for such
purpose by the SEC) and the actual dissemination of tender offer materials to
security holders) by another person, entity or group to purchase or exchange for

                                      -6-
<PAGE>   7
cash or other consideration any shares of Common Stock or rights, warrants or
options therefor which, if successful, would result in such person, entity or
group beneficially owning or having the right to acquire more than a majority
of the voting shares of the Company; (ii) in the event of a sale of
substantially all assets of the Company or a merger, consolidation or other
transaction, as a result of which the stockholders of the Company as of
immediately prior to such transaction hold less than 50% of the voting
securities of the surviving entity following such transaction (collectively, a
"Transaction"), upon the earlier of (x) the public announcement or disclosure
or (y) consummation of a Transaction or (iii) such sale is to a person or
entity that directly, or indirectly, through one or more intermediaries,
controls, or is controlled by, or is under common control with, the holder of
this Warrant. Notwithstanding the foregoing or anything else to the contrary in
this Warrant, the holder may enter into bona fide transactions through a
nationally recognized investment banking firm which constitute a hedge against
changes in the market price of the Common Stock, provided, however, no public
disclosure is made with respect to such hedge transactions, except in an
initial filing pursuant to Section 13 of the Exchange Act, if required, the text
of which is reasonably satisfactory to the Company, or if in the opinion of
counsel to holder such disclosure is required as a matter of law.

     12.  Modification and Waiver. This Warrant and any provision hereof may be
changed, waived, discharged or terminated only by an instrument in writing
signed by the party against which enforcement of the same is sought.

     13. Notices. Any notice, request, communication or other document required
or permitted to be given or delivered to the holder hereof or the Company shall
be delivered, or shall be sent by certified or registered mail, postage
prepaid, to each such holder at its address as shown on the books of the
Company or to the Company at the address indicated therefor on the signature
page of this Warrant.

     14. Lost Warrants or Stock Certificates. The Company covenants to the
holder hereof that, upon receipt of evidence reasonably satisfactory to the
Company of the loss, theft, destruction or mutilation of this Warrant or any
stock certificate and, in the case of any such loss, theft or destruction, upon
receipt of an indemnity reasonably satisfactory to the Company, or in the case
of any such mutilation upon surrender and cancellation of such Warrant or stock
certificate, the Company will make and deliver a new Warrant or stock
certificate, of like tenor, in lieu of the lost, stolen, destroyed or mutilated
Warrant or stock certificate.

     15. Descriptive Headings. The descriptive headings of the several
paragraphs of this Warrant are inserted for convenience only and do not
constitute a part of this Warrant. The language in this Warrant shall be
construed as to its fair meaning without regard to which party drafted this
Warrant.

     16. Governing Law. This Warrant shall be construed and enforced in
accordance with, and the rights of the parties shall be governed by, the laws
of the State of Delaware.

     17. Severability. The invalidity or unenforceability of any provision of
this Warrant in any jurisdiction shall not affect the validity or
enforceability of such provision in any other jurisdiction, or affect any other
provision of this Warrant, which shall remain in full force and effect.


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<PAGE>   8
     18. Entire Agreement: Modification. This Warrant constitutes the entire
agreement between the parties pertaining to the subject matter contained in it
and supersedes all prior and contemporaneous agreements, representations, and
undertakings of the parties, whether oral or written, with respect to such
subject matter.

                                   CONCENTRIC NETWORK CORPORATION
                                   a Delaware corporation

                                   By: /s/ Michael Anthofer
                                   Name: Michael Anthofer
                                   Title: SVP & CFO
                                   Address: 1400 Parkmoor Ave., San Jose 95126












                                       8

<PAGE>   9
                                  EXHIBIT A-1

                               NOTICE OF EXERCISE

     To: CONCENTRIC NETWORK CORPORATION (the "Company")

     1.   The undersigned hereby:

          [ ]  elects to purchase            shares of Common Stock of the
               Company pursuant to the terms of the attached Warrant, and
               tenders herewith payment of the purchase price of such shares in
               full; or

          [ ]  elects to exercise its net issuance rights pursuant to Section 9
               of the attached Warrant with respect to            shares of
               Common Stock.

     2.   Please issue a certificate or certificates representing said shares
in the name of the undersigned or in such other name or names as are specified
below:


                                        ----------------------------------------
                                        (Name)


                                        ----------------------------------------
                                        (Address)


                                        ----------------------------------------
                                        (Address)

     3.   The undersigned represents that the aforesaid shares being acquired
for the account of the undersigned for investment and not with a view to, or for
resale in connection with, the distribution thereof and that the undersigned has
no present intention of distributing or reselling such shares, all except as in
compliance with applicable securities laws.

------------
   (Date)

                                        ----------------------------------------
                                                      (Signature)
<PAGE>   10
                                 EXHIBIT A-2

                     INVESTMENT REPRESENTATION STATEMENT

PURCHASER:



COMPANY:

SECURITY:

AMOUNT:

DATE:

     In connection with the purchase of the above-listed securities (the
"Securities"), I, the Purchaser, represent to the Company the following:

          (a)  I am aware of the Company's business affairs and financial
condition, and have acquired sufficient information about the Company to reach
an informed and knowledgeable decision to acquire the Securities. I am
purchasing these Securities for my own account for investment purposes only and
not with a view to, or for the resale in connection with, any "distribution"
thereof for purposes of the Securities Act of 1933 ("Securities Act").

          (b)  I understand that the Securities have not been registered under
the Securities Act in reliance upon a specific exemption therefrom, which
exemption depends upon, among other things, the bona fide nature of my
investment intent as expressed herein. In this connection, I understand that,
in the view of the Securities and Exchange Commission ("SEC"), the statutory
basis for such exemption may be unavailable if my representation was predicated
solely upon a present intention to hold these Securities for the minimum
capital gains period specified under tax statutes, for a deferred sale, for or
until an increase or decrease in the market price of the Securities, or for a
period of one year or any other fixed period in the future.

          (c)  I further understand that the Securities must be held
indefinitely unless subsequently registered under the Securities Act or unless
an exemption from registration is otherwise available. Moreover, I understand
that the Company is under no obligation to register the Securities. In addition,
I understand that the certificate evidencing the Securities will be imprinted
with a legend which prohibits the transfer of the Securities unless they are
registered or such registration is not required in the opinion of counsel for
the Company.

          (d)  I am aware of the provisions of Rule 144, promulgated under the
Securities Act, which, in substance, permits limited public resale of
"restricted securities" acquired, directly or

<PAGE>   11
indirectly, from the issuer thereof (or from an affiliate of such issuer), in a
non-public offering subject to the satisfaction of certain conditions.

   (e)  I further understand that at the time I wish to sell the Securities,
there may be no public market upon which to make such a sale.

   (f)  I further understand that in the event all of the requirements of Rule
144 are not satisfied, registration under the Securities Act, compliance with
Regulation A, or some other registration exemption will be required; and that,
notwithstanding the fact that Rule 144 is not exclusive, the Staff of the SEC
has expressed its opinion that persons proposing to sell private placement
securities other than in a registered offering and otherwise than pursuant to
Rule 144 will have a substantial burden of proof in establishing that an
exemption from registration is available for such offers or sales, and that such
persons and their respective brokers who participate in such transactions do so
at their own risk.


                                       -----------------------------------------
                                       Name of Purchaser


                                       -----------------------------------------
                                       Signature of Authorized Signatory


                                       -----------------------------------------
                                       Print Name and Title


                                       -----------------------------------------
                                       Date


                                      -2-