Exhibit 32.1

                            CERTIFICATION PURSUANT TO
                             18 U.S.C. SECTION 1350,
                             AS ADOPTED PURSUANT TO
                  SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

In  connection  with the Annual Report of F5 Networks,  Inc. (the  "Company") on
Form 10-K/A for the period ended  September 30, 2005 as filed with the
Securities and  Exchange  Commission  on the date hereof (the  "Report"),  we,
John McAdam, President and Chief Executive Officer and John Rodriguez,  Senior
Vice President and Chief  Accounting  Officer  (principal  financial  officer)
of the  Company, certify,  pursuant to 18 U.S.C. Section 1350, as adopted
pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that:

          (1) The Report fully complies with the  requirements  of Section 13(a)
or 15(d) of the Securities Exchange Act of 1934; and

          (2) The information  contained in the Report fairly  presents,  in all
material  respects,  the  financial  condition  and result of  operations of the
Company.

Date: April 3, 2007


                  /s/ John Mcadam
           ------------------------------
                  John McAdam


                  /s/ JOHN RODRIGUEZ
           ------------------------------
                  John Rodriguez


A signed  original of this written  statement  required by Section 906, or other
document authenticating, acknowledging, or otherwise adopting the signature that
appears in typed form within the  electronic  version of this written  statement
required by Section 906, has been  provided to F5  Networks,  Inc.,  and will be
retained by F5 Networks,  Inc.,  and  furnished to the  Securities  and Exchange
Commission or its staff upon request.