Exhibit 99.6

                          EXHIBIT D TO MERGER AGREEMENT

                                     FORM OF

                            RULE 145 AFFILIATE LETTER


Cogent Communications Group, Inc.
1015 31st Street, N.W
Washington, D.C.  20007


Ladies and Gentlemen:

     I have been advised that as of the date of this letter I may be deemed to
be an "affiliate" of Allied Riser Communications Corporation, a Delaware
corporation (the "COMPANY"), as the term "affiliate" is defined for purposes of
paragraphs (c) and (d) of Rule 145 of the rules and regulations (the "RULES AND
REGULATIONS") of the Securities and Exchange Commission (the "COMMISSION")
promulgated under the Securities Act of 1933, as amended (the "ACT"), although
nothing contained herein should be construed as an admission of such fact.
Pursuant to the Agreement and Plan of Merger, dated as of August 28, 2001 (the
"MERGER AGREEMENT"), by and among Cogent Communications Group, Inc., a Delaware
corporation ("Parent"), Augustus Caesar Merger Sub, Inc., a Delaware corporation
and a wholly owned subsidiary of Parent ("Merger Sub") and the Company, Merger
Sub will be merged with and into the Company (the "MERGER") and the Company will
be the surviving corporation (the "SURVIVING CORPORATION"). Capitalized terms
used herein but not defined shall have the meanings ascribed to such terms in
the Merger Agreement.

     As a result of the Merger, I will receive shares of Parent Common Stock in
exchange for shares of Company Common Stock owned by me.

     I represent, warrant to and covenant with Parent that in the event I
receive any Parent Common Stock as a result of the Merger:

          A. I shall not make any sale, transfer or other disposition (including
     any hedge or other arrangement to reduce the undersigned's risks relating
     to any Parent Common Stock received by the undersigned in the Merger) of
     Parent Common Stock in violation of the Act or the rules and regulations
     thereunder.

          B. I have carefully read this letter and discussed, to the extent I
     felt necessary, with my counsel or counsel for the Company applicable
     limitations upon my ability to sell, transfer or otherwise dispose of
     Parent Common Stock.

          C. I have been advised that the issuance of Parent Common Stock to me
     pursuant to the Merger will be registered with the Commission under the Act
     on a Registration Statement on Form S-4. However, I have also been advised
     that I may be deemed to have been an affiliate of the Company and the
     distribution by me of Parent Common Stock has not been registered under the
     Act, and therefore I may not sell, transfer or otherwise dispose of Parent
     Common Stock issued to me in the Merger unless (i) such sale, transfer or
     other disposition has been registered under the Act, (ii) such sale,
     transfer or disposition is made in conformity with Rule 145, or (iii) in
     the opinion of counsel reasonably acceptable to Parent, or pursuant to a
     "no action" letter obtained by the undersigned from the



     staff of the Commission, such sale, transfer or other disposition is
     otherwise exempt from registration under the Act.

          D. I understand that Parent is under no obligation to register the
     sale, transfer or disposition of Parent Common Stock by me or on my behalf
     under the Act or to take any other action necessary in order to make
     compliance with an exemption from such registration available; PROVIDED,
     HOWEVER, that Parent shall use its reasonable best efforts to meet the
     requirements of paragraph (c) of Rule 144 promulgated under the Act.

          E. I also understand that stop transfer instructions will be given to
     Parent's transfer agent with respect to Parent Common Stock and that there
     will be placed on the certificates for Parent Common Stock issued to me, or
     any substitutions therefor, a legend stating in substance:

          "THE SHARES REPRESENTED BY THIS CERTIFICATE WERE ISSUED IN A
          TRANSACTION TO WHICH RULE 145 PROMULGATED UNDER THE SECURITIES ACT OF
          1933 APPLIES. THE SHARES REPRESENTED BY THIS CERTIFICATE MAY ONLY BE
          TRANSFERRED IN COMPLIANCE WITH THE REQUIREMENTS OF RULE 145 OR
          PURSUANT TO A REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933
          OR AN EXEMPTION FROM SUCH REGISTRATION."

          F. I also understand that unless the transfer by me of my Parent
     Common Stock has been registered under the Act or is a sale made in
     conformity with the provisions of Rule 145, Parent reserves the right to
     put the following legend on the certificates issued to my transferee:

          "THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
          UNDER THE SECURITIES ACT OF 1933 AND WERE ACQUIRED FROM A PERSON WHO
          RECEIVED SUCH SHARES IN A TRANSACTION TO WHICH RULE 145 PROMULGATED
          UNDER THE SECURITIES ACT OF 1933 APPLIES. THE SHARES HAVE BEEN
          ACQUIRED BY THE HOLDER NOT WITH A VIEW TO, OR FOR RESALE IN CONNECTION
          WITH, ANY DISTRIBUTION THEREOF WITHIN THE MEANING OF THE SECURITIES
          ACT OF 1933 AND MAY NOT BE SOLD, PLEDGED OR OTHERWISE TRANSFERRED
          EXCEPT PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT OR IN
          ACCORDANCE WITH AN EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF THE
          SECURITIES ACT OF 1933."

     I also understood and agree that the legends set forth in paragraphs E and
F above shall be removed by delivery of substitute certificates without such
legend if such legend is not required for purposes of the Act or this letter
agreement. I also understand and agree that such legends and the stop orders
referred to above will be removed if two years shall have elapsed from the date
the undersigned acquired Parent Common Stock received in the Merger and the
provisions of Rule 145(d)(3) are then available to the undersigned, or Parent
has received either an opinion of counsel, which opinion and counsel shall be
reasonably satisfactory to Parent, or a


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"no action" letter obtained by the undersigned from the staff of the Commission,
to the effect that the restrictions imposed by Rule 145 under the Act no longer
apply to the undersigned. For so long as this agreement is applicable and to the
extent necessary to permit the undersigned to sell Parent Common Stock pursuant
to Rule 145 and, to the extent applicable, Rule 144, the Parent agrees to use
its reasonable best efforts to permit such sales pursuant to Rule 145 and Rule
144.

     EXECUTION OF THIS LETTER SHOULD NOT BE CONSIDERED AN ADMISSION ON MY PART
THAT I AM AN "AFFILIATE" OF THE COMPANY AS DESCRIBED IN THE FIRST PARAGRAPH OF
THIS LETTER OR AS A WAIVER OF ANY RIGHTS I MAY HAVE TO OBJECT TO ANY CLAIM THAT
I AM SUCH AN AFFILIATE ON OR AFTER THE DATE OF THIS LETTER.

                                                Very truly yours,


                                                -----------------------------
                                                Signature


                                                -----------------------------
                                                Print Name



Accepted this ___ day of ___________, 2001 by

COGENT COMMUNICATIONS GROUP, INC.


By:  ___________________________

Name:  _________________________

Title:  __________________________


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