EXHIBIT 5.1

                                January 20, 2004



F5 Networks, Inc.
400 Elliot Ave. W.
Seattle, Washington 98119

Re:      Registration Statement on Form S-8

Ladies and Gentlemen:

     This  opinion  is  furnished  to  F5  Networks,  Inc.  (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 225,000  shares (the  "Shares")  of common  stock,  no par value (the "Common
Stock"),  issuable  by the  Company  pursuant to a  Non-Qualified  Stock  Option
Agreement  dated  October 20,  2003  between the Company and M. Thomas Hull (the
"Agreement").

     We have  based  our  opinion  upon our  review  of the  following  records,
documents, instruments and certificates:

     a) the Articles of Incorporation of the Company;

     b) the Bylaws of the Company;

     c) records certified to us by an officer of the Company as constituting all
records of proceedings and of actions of the Board of Directors and shareholders
relating to the approval of the Agreement and the  authorization of the issuance
of the Shares pursuant to the Agreement; and

     d) the Agreement.

     In connection with this opinion,  we have,  with your consent,  assumed the
authenticity  of all  records,  documents  and  instruments  submitted  to us as
originals,  the  genuineness  of all  signatures,  the legal capacity of natural
persons and the  authenticity  and  conformity  to the originals of all records,
documents and instruments submitted to us as copies.

     This opinion is limited to the laws of the State of Washington. We disclaim
any  opinion as to any  statute,  rule,  regulation,  ordinance,  order or other
promulgation  of any  other  jurisdiction  or any  federal,  regional  or  local
governmental body.

     Based upon the foregoing and our examination of such questions of law as we
have  deemed  necessary  or  appropriate  for the purpose of this  opinion,  and
subject  to the  assumptions  and  qualifications  expressed  herein,  it is our
opinion that the issuance of the Shares  pursuant to the Agreement has been duly
authorized  and, upon issuance and delivery of the Shares  pursuant to the terms
of  the  Agreement,   the  Shares  will  be  validly  issued,   fully  paid  and
non-assessable.

     We expressly  disclaim any obligation to advise you of any  developments in
areas covered by this opinion that occur after the date of this opinion.

     We hereby  authorize  and consent to the use and filing of this  opinion as
Exhibit 5.1 to the Registration Statement.

                                        Very truly yours,



                                       /s/ Heller Ehrman White & McAuliffe LLP