
<PAGE>

                [DAVIS WRIGHT TREMAINE LLP LETTERHEAD]

June 1, 1999


F5 Networks, Inc.
Attention:  Joann Reiter
200 First Avenue West, Suite 500
Seattle, Washington 98119

Re: 401(k) Profit Sharing Plan and Trust

Dear Ladies and Gentlemen:

We have acted as counsel to F5 Networks, Inc., a Washington corporation (the
"Corporation"), in connection with its Registration Statement on Form S-8
(the "Registration Statement").  Capitalized terms used herein that are not
otherwise defined have the meanings ascribed thereto as set forth in the
Registration Statement and the exhibits thereto.

We have examined such documents, papers, statutes and authorities as we have
deemed necessary to form a basis for the opinions hereinafter expressed.  We
have assumed the genuineness of all signatures, the authenticity of
documents, certificates and records submitted to us as originals, the
conformity to the originals of all documents, certificates and records
submitted to us as copies, the legal capacity of all natural persons
executing documents, certificates and records, and the completeness and
accuracy as of the date of this opinion letter of the information contained
in such documents, certificates and records.

Based upon the foregoing, we are of the opinion that:

      1.  The Corporation is duly formed and validly existing under the
          laws of the State of Washington.

      2.  The Plan and the Shares have been duly authorized and, after
          receipt of the consideration for such shares as specified in the
          Underwriting Agreement and when appropriate certificates have been
          duly executed by the proper officers of the Corporation, the Shares
          will be validly issued, fully paid and nonassessable.

                                                               Page 95 of 97
                                                        Exhibit Index Page 7
<PAGE>

This opinion is limited to the laws of the State of Washington and the
federal laws of the United States of the type typically applicable to
transactions contemplated by the Registration Statement.  We express no
opinion with respect to the laws of any other country, state or jurisdiction.

This opinion letter is limited to the matters stated herein and no opinion is
implied or may be inferred beyond the matters expressly stated.  This letter
speaks only as of the date hereof and is limited to present statutes,
regulations and administrative and judicial interpretations.  We undertake no
responsibility to update or supplement this letter after the date hereof.

We consent to being named in the Registration Statement as counsel who are
passing upon the validity of the shares of common stock to be issued pursuant
to the Registration Statement.  Subject to the foregoing, this opinion letter
may be relied upon by you only in connection with the Offering and may not be
used or relied upon by you for any other purpose or by any other person for
any purpose whatsoever without, in each instance, our prior written consent.

Sincerely,

Davis Wright Tremaine LLP





                                                               Page 96 of 97
                                                        Exhibit Index Page 7

