<PAGE>   1
                   SEE REVERSE SIDE FOR RESTRICTIVE LEGEND(S)

                   INCORPORATED UNDER THE LAWS OF THE STATE OF
                                    DELAWARE
                        SEE RESTRICTIONS ON REVERSE SIDE

G-__                                                                    **___**
                         SERIES F CONVERTIBLE REDEEMABLE
                            PREFERRED STOCK DUE 2010

                         CONCENTRIC NETWORK CORPORATION

           FULLY PAID AND NON-ASSESSABLE SHARES OF THE PREFERRED STOCK
          $0.001 PAR VALUE PER SHARE OF CONCENTRIC NETWORK CORPORATION



       THIS IS TO CERTIFY THAT __________________________ is the record holder
of ________________________________ fully paid and non-assessable shares of the
Series C 7% Convertible Redeemable Preferred Stock due 2010 of Concentric
Network Corporation having a liquidation preference of $1,000.00 per share,
transferable only on the share register of the corporation, in person or by a
duly authorized attorney, upon surrender of this certificate properly endorsed
or assigned.

       A statement of all the rights, preferences, privileges and restrictions
granted to or imposed upon the respective classes and/or series of shares of
stock of the corporation and upon the holders thereof may be obtained by any
stockholder upon request and without charge, at the principal office of the
corporation, and the corporation with furnish any stockholder, upon request and
without charge, a copy of such statement.

       WITNESS the signatures of its duly authorized officers this __ day of
June 2000.




------------------------------         -------------------------------------
[name, title]                          [name, title]




<PAGE>   2


       For value received, _______________________ do hereby sell, assign and
transfer unto _______________________ shares represented by the within
certificate and do hereby irrevocably constitute and appoint ___________________
attorney to transfer the said shares on the share register of the within named
corporation with full power of substitution in the premises.

Dated,
      -----------------------------

IN PRESENCE OF
                  ------------------------     --------------------------
                        (Witness)                     (Stockholder)

                                               --------------------------
                                                      (Stockholder)



NOTICE: THE SIGNATURE ON THIS ASSIGNMENT MUST CORRESPOND WITH THE NAME AS
WRITTEN UPON THE FACE OF THIS CERTIFICATE, IN EVERY PARTICULAR, WITHOUT
ALTERNATION OR ENLARGEMENT, OR ANY CHANGE WHATSOEVER.


THE SERIES C PREFERRED STOCK EVIDENCED HEREBY HAS NOT BEEN REGISTERED UNDER THE
UNITED STATES SECUIRITIES ACT OF 1933, AS AMENDED (THE "SECURITIES ACT"), OR ANY
STATE SECURITIES LAWS, AND, ACCORDINGLY, MAY NOT BE OFFERED OR SOLD WITHIN THE
UNITED STATES OR TO, OR FOR THE ACCOUNT OR BENEFIT OF, UNITED STATES PERSONS
EXCEPT AS SET FORTH N THE FOLLOWING SENTENCE. BY ITS ACQUISITION HEREOF, THE
HOLDER (1) REPRESENTS THAT (A) IT IS AN INSTITUTIONAL "ACCREDITED INVESTOR" (AS
DEFINED IN RULE 501(A)(1), (2), (3) OR (7) UNDER THE SECURITIES ACT)
("INSTITUTIONAL ACCREDITED INVESTOR") OR (B) IT IS NOT A UNITED STATES PERSON
AND IS ACQUIRING THE SERIES C PREFERRED STOCK EVIDENCED HEREBY IN AN OFFSHORE
TRANSACTION; (2) AGREES THAT IT WILL NOT, PRIOR TO THE EXPIRATION OF THE HOLDING
PERIOD APPLICABLE TO SALES OF THE SERIES C PREFERRED STOCK EVIDENCED HEREBY
UNDER RULE 144(K) UNDER THE SECURITIES ACT (OR ANY SUCCESSOR PROVISION), RESLL
OR OTHERWISE TRANSFER THE SERIES C PREFERRED STOCK EVIDENCED HEREBY OR THE
COMMON STOCK ISSUABLE UPON CONVERSION OF SUCH THE SERIES C PREFERRED STOCK
WITHIN THE UNITED STATES OR TO, OR FOR THE ACCOUNT OR BENEFIT OF, U.S. PERSONS
EXCEPT (A) TO CONCENTRIC NETWORK CORPORATION OR ANY SUBSIDIARY THEREOF, (B) TO
AN INSTITUTIONAL ACCREDITED INVESTOR THAT, PRIOR TO SUCH TRANSFER, FURNISHES TO
CONCENTRIC NETWORK CORPORATION, A SIGNED LETTER CONTAINING CERTAIN
REPRESENTATIONS AND AGREEMENTS RELATING TO THE RESTRICTIONS ON TRANSFER OF THE
SERIES C PREFERRED STOCK EVIDENCED HEREBY, (C) PURSUANT TO THE EXEMPTION FROM
REGISTRATION PROVIDED BY RULE 144 UNDER THE SECURITIES ACT (IF AVAILABLE) OR (D)
PURSUANT TO A REGISTRATION STATEMENT WHICH HAS BEEN DECLARED EFFECTIVE UNDER THE
SECURITIES ACT (AND WHICH CONTINUES TO BE EFFECTIVE AT THE TIME OF SUCH
TRANSFER); (3) AGREES THAT PRIOR TO SUCH TRANSFER (OTHER THAN A TRANSFER PURSUAN
TO CLAUSE 2(D) ABOVE), IT WILL FURNISH CONCENTRIC NETWORK CORPORATION, SUCH
CERTIFICATIONS, LEGAL OPINIONS OR OTHER INFORMATION AS IT MAY REASONABLY REQUIRE
TO CONFIRM THAT SUCH TRANSFER IS BEING MADE PURSUANT TO AN EXEMPTION FROM, OR IN
A TRANSACTION NO SUBJECT TO, THE REGISTRATION REQUIREMENTS OF THE SECURITIES ACT
AND (4) AGREES THAT IT WILL DELIVER TO EACH PERSON TO WHOM THE SERIES C
PREFERRED STOCK EVIDENCED HEREBY IS TRANSFERRED A NOTICE SUBSTANTIALLY TO THE
EFFECT OF THIS LEGEND. IN CONNNECTION WITH THE TRANSFER OF THE SERIES C
PREFERRED STOCK EVIDENCED HEREBY PRIOR TO THE EXPIRATION OF THE HOLDING PERIOD
APPLICABLE TO SALES OF THE SERIES C PREFERRED STOCK EVIDENCED HEREBY UNDER RULE
144 (K) UNDER THE

<PAGE>   3



SECURITIES ACT (OR ANY SUCCESSOR PROVISION), THE HOLDER MUST CHECK THE
APPROPRIATE BOX SET FORTH ON THE REVERSE HEREOF RELATING TO THE MANNER OF SUCH
TRANSFER AND SUBMIT THIS CERTIFICATE TO CONCENTRIC NETWORK CORPORATION. IF THE
PROPOSED TRANSFEREE IS A PURCHASER WHO IS NOT A UNITED STATES PERSON OR AN
INSTITUTIONAL ACCREDITED INVESTOR, THE HOLDER MUST, PRIOR TO SUCH TRANSFER,
FURNISH TO CONCENTRIC NETWORK CORPORATION, SUCH CERTIFICATIONS, LEGAL OPINIONS
OR OTHER INFORMATION AS IT MAY REASONABLY REQUIRE TO CONFIRM THAT SUCH TRANSFER
IS BEING MADE PURSUANT TO AN EXEMPTION FROM, OR IN A TRANSACTION NO SUBJECT TO,
THE REGISTRATION REQUIREMENTS OF THE SECURITIES ACT. THIS LEGEND WILL BE REMOVED
UPON THE EARLIER OF THE TRANSFER OF THE SERIES C PREFERRED STOCK EVIDENCED
HEREBY PURSUANT TO CLAUSE 2(D) ABOVE OR UPON ANY TRANSFER OF THE SERIES C
PREFERRED STOCK EVIDENCED HEREBY UNDER RULE 144(K) UNDER THE SECURITIES ACT (OR
ANY SUCCESSOR PROVISION). THE SHARES OF SERIES C PREFERRED STOCK EVIDENCED
HEREBY MAY NOT BE SOLD, DISPOSED OF OR OTHERWISE TRANSFERRED TO ANY PERSON OTHER
THAN TO MICROSOFT CORPORATION OF ANY OF ITS AFFILIATES PRIOR TO DECEMBER 18,
2000.