F5 (FFIV) 10-K 2012 FY Annual 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 Annual Report of F5 Networks, Inc. (the “Company”) on Form 10-K for the period ended September 30, 2012 as filed with the Securities and Exchange Commission on the date hereof (the “Report”), we, John McAdam, President and Chief Executive Officer and Andy Reinland, Executive Vice President and Chief Financial 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: November 21, 2012

/s/ JOHN MCADAM

John McAdam

Chief Executive Officer and President

/s/ ANDY REINLAND

Andy Reinland

Executive Vice President, Chief Financial 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.