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 Quarterly Report of F5 Networks, Inc. (the
"Company") on Form 10-Q for the period ending June 30, 2004 as filed with the
Securities and Exchange Commission on the date hereof (the "Report"), we, John
McAdam, President and Chief Executive Officer and Steven Coburn, Senior Vice
President and Chief 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: August 12, 2004

      /s/ JOHN MCADAM
   --------------------
      John McAdam

      /s/ STEVEN B. COBURN
   -------------------------
      Steven B. Coburn

      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.