EXHIBIT 5.1


                [HELLER EHRMAN WHITE & MCAULIFFE LLP LETTERHEAD]

                                   May 6, 2002



The Board of Directors
F5 Networks, Inc.
401 Elliott Avenue West
Seattle, Washington 98119

         Re:      Registration Statement on Form S-8

Ladies and Gentlemen:

          This opinion is furnished to F5 Networks, Inc., a Washington
corporation (the "Company") in connection with the filing of a Registration
Statement on Form S-8 (the "Registration Statement") with the Securities and
Exchange Commission under the Securities Act of 1933, as amended, relating to
the proposed sale by the Company of up to 1,500,000 shares (the "Shares") of
common stock, no par value (the "Common Stock"), issuable by the Company under
the 2000 Employee Equity Incentive Plan, as amended (the "Plan").

          We have reviewed, among other things, the Company's Articles of
Incorporation and Bylaws, each as amended, the Plan and related agreements and
records of corporate proceedings and other actions taken or proposed to be taken
by the Company in connection with the authorization, issuance and sale of the
Shares pursuant to awards made under the Plan. We have made such other factual
inquiries as we deemed necessary to render this opinion.

          Based upon the foregoing and in reliance thereon, it is our opinion
that the reservation for issuance of the Shares pursuant to the Plan has been
duly authorized and, when issued pursuant to awards granted and exercised in
accordance with the Plan and related agreements, the Shares will be validly
issued, fully paid and non-assessable.

          We express no opinion herein as to the laws of any state or
jurisdiction other than the State of Washington and the federal laws of the
United States.

         We hereby authorize and consent to the use of this opinion as an
exhibit to the Registration Statement and to all references to us in the
Registration Statement and any amendments thereto.

                                      Very truly yours,

                                     /S/ HELLER EHRMAN WHITE & McAULIFFE LLP