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                                                                     EXHIBIT 5.1

                 [LETTERHEAD OF BROBECK, PHLEGER & HARRISON LLP]

                                 July 13, 2000


Network Appliance, Inc.
495 East Java Drive
Sunnyvale, CA 94089

        Re: Network Appliance, Inc. Registration Statement of Form S-3 for
            Resale of 549,457 Shares of Common Stock

Ladies and Gentlemen:

        We have acted as special counsel to Network Appliance, Inc., a
California corporation (the "Company"), in connection with the registration
pursuant to the Company's Registration Statement on Form S-3 (the "Registration
Statement") filed with the Securities and Exchange Commission under the
Securities Act of 1933, as amended (the "Securities Act"), of 549,457 shares of
the Company's common stock, without par value (the "Shares").

        This opinion is being furnished in accordance with the requirements of
Item 16 of Form S-3 and Item 601(b)(5)(i) of Regulation S-K.

        We have reviewed the Company's charter documents and the corporate
proceedings taken by the Company in connection with the issuance and sale of the
Shares. Based on such review, we are of the opinion that the Shares are duly
authorized, legally issued, fully paid and nonassessable.

        We consent to the filing of this opinion letter as Exhibit 5.1 to the
Registration Statement and to the reference to this firm under the caption
"Legal Matters" in the prospectus which is part of the Registration Statement.
In giving this consent, we do not thereby admit that we are within the category
of persons whose consent is required under Section 7 of the Securities Act, the
rules and regulations of the Securities and Exchange Commission promulgated
thereunder or Item 509 of Regulation S-K.

        This opinion letter is rendered as of the date first written above and
we disclaim any obligation to advise you of facts, circumstances, events or
developments which hereafter may be brought to our attention and which may
alter, affect or modify the opinion expressed herein. Our opinion is expressly
limited to the matters set forth above and we render no opinion, whether by
implication or otherwise, as to any other matters relating to the Company or the
Shares.

                                            Very truly yours,

                                            /s/ BROBECK, PHLEGER & HARRISON LLP

                                            BROBECK, PHLEGER & HARRISON LLP