F5 (FFIV) 10-Q 2008 Q3 Quarterly report

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 ended June 30, 2008 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: August 7, 2008

/s/ JOHN MCADAMJohn McAdam

Chief Executive Officer and President

/s/ JOHN RODRIGUEZJohn Rodriguez

Senior Vice President and Chief Accounting

Officer (principal financial officer)

     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.