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[LETTERHEAD]

May 13, 1999

F5 Networks, Inc.
2-- First Avenue West, Suite 500
Seattle, WA 98119

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection 
with the filing by F5 Networks, Inc. (the "Company") of a Registration 
Statement on Form S-1 (the "Registration Statement") with the Securities and 
Exchange Commission (the "Commission") covering an underwritten public 
offering of up to three million four hundred fifty thousand (3,450,000) 
shares of Common Stock (the "Common Stock").

In connection with this opinion, we have (i) examined and relied upon the 
Registration Statement and related Prospectus, the Company's Amended and 
Restated Articles of Incorporation and Bylaws, as currently in effect, and 
the originals or copies certified to our satisfaction of such records, 
documents, certificates, memoranda and other instruments as in our judgment 
are necessary or appropriate to enable us to render the opinion expressed 
below; (ii) assumed that the Second Amended and Restated Articles of 
Incorporation, as set forth in Exhibit 3.2 of the Registration Statement, 
shall have been duly approved and filed with the office of the Washington 
Secretary of State; and (iii) that the shares of Common Stock will be sold 
by the Underwriters at a price established by the Pricing Committee of the 
Board of Directors of the Company.

On the basis of the foregoing, and in reliance thereon, we are of the opinion 
that the Common Stock, when sold and issued in accordance with the 
Registration Statement and related Prospectus, will be validly issued, fully 
paid and non-assessable.

We consent to the reference to our firm under the caption "Legal Matters" in 
the Prospectus included on the Registration Statement and to the filing of 
this opinion as an exhibit to the Registration Statement.

Very truly yours, 

COOLEY GODWARD LLP


By: /s/ PATRICK A. POHLEN
   ------------------------------------
    Patrick A. Pohlen


