CONFIDENTIAL  TREATMENT  HAS BEEN SOUGHT FOR PORTIONS OF THIS EXHIBIT WHICH HAVE
BEEN OMITTED  PURSUANT TO RULE 24B-2 UNDER THE SECURITIES  EXCHANGE ACT OF 1934,
AS AMENDED AND FILED SEPARATELY WITH THE COMMISSION.


                                    ADDENDUM


August 26, 1999

Mr. Sebastian E. Cassetta
Chairman, Chief Executive Officer

SmartServ Online, Inc.
One Station Place
Stamford, CT  06902

Dear Sam,

This letter,  upon  execution by SmartServ  Online,  Inc.  (SmartServ)  and Data
Transmission  Network  Corporation  (DTN) will  constitute  an  addendum  to the
Software License and Service Agreement (the Agreement) between SmartServ and DTN
dated May 1, 1998 and amended May 1, 1999.  All terms  defined in the  Agreement
have the same meanings when used herein.

For the  development,  enhancement and  maintenance of **** trading  application
software, it is hereby agreed:

(1)  Software Development Fee

     DTN  will pay  SmartServ  a total of **** to  develop  trading  application
     software for **** ; and, of this total, DTN will pay **** to SmartServ upon
     delivery  and proof of  performance  of the first beta  version of the ****
     trading application software and the remaining **** within thirty (30) days
     after the **** trading  application  software is installed and commercially
     used.

(2)  Software Enhancement and Maintenance Fees

     SmartServ will enhance and maintain the **** trading  Application  software
     in accordance with the terms and conditions of the Agreement.  DTN will pay
     SmartServ **** of revenue earned by the use of the **** trading application
     software each month within twenty (20) days after the end of the respective
     months to which they relate.


****  REPRESENTS  MATERIAL  REDACTED  PURSUANT  TO A  REQUEST  FOR  CONFIDENTIAL
TREATMENT  PURSUANT TO RULE 24B-2 UNDER THE SECURITIES  EXCHANGE ACT OF 1934, AS
AMENDED.


<PAGE>

Mr. Sebastian E. Cassetta
Agreement Addendum
Page 2


This addendum  shall remain in effect for so long as the Agreement  shall remain
in effect. Except as set forth herein, all terms and conditions of the Agreement
remain in effect.  This addendum,  together with the  Agreement,  constitute the
entire  understanding  of  SmartServ  and DTN and cannot be  modified  except in
writing signed by both SmartServ and DTN.

Please  signify  SmartServ's  concurrence  with the terms  written  hereabove by
signing both copies of this addendum  where  indicated  herebelow and return one
copy to me.

Sincerely,

/s/ Charles R. Wood

Charles R. Wood
Sr. Vice President

CRW: co

Accepted and agreed:

SmartServ Online, Inc.                     Data Transmission Network Corporation


By:  /s/ Sebastian E. Cassetta             By:  /s/ Roger W. Wallace
------------------------------------       ------------------------------------
           (signature)                               (signature)

      Sebastian E. Cassetta                         Roger W. Wallace
------------------------------------       ------------------------------------
                (type or print name)              (type or print name)

Title:   Chairman & CEO                     Title:  Senior Vice President

Date:    10/3/99                                      Date:    9/2/99


****  REPRESENTS  MATERIAL  REDACTED  PURSUANT  TO A  REQUEST  FOR  CONFIDENTIAL
TREATMENT  PURSUANT TO RULE 24B-2 UNDER THE SECURITIES  EXCHANGE ACT OF 1934, AS
AMENDED.