
                                                                     EXHIBIT 5.1

                                                            Main (206) 447-0900
                                                             Fax (206) 447-0849
                                                                     25931-0002


                                January 14, 2005



The Board of Directors
F5 Networks, Inc.
501 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 715,000  shares (the  "Shares")  of
common stock, no par value (the "Common  Stock"),  issuable by the Company under
the MagniFire  Acquisition  Equity Incentive Plan and NonQualified  Stock Option
Agreement with Karl Triebes (collectively, the "Plans").

         We have  reviewed,  among  other  things,  the  Company's  Articles  of
Incorporation and Bylaws, each as amended,  the Plans 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 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 Plans has been
duly  authorized  and,  when  issued  pursuant  to the  Plans  and  any  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



