<PAGE>   1
                                        Filed Pursuant to Rule 424(b)(3) and (c)
                                                           File Number 333-26163

                              PROSPECTUS SUPPLEMENT
                                       to
                          Prospectus Dated May 9, 1997

                                 172,372 SHARES

                             NETWORK APPLIANCE, INC.

                                  COMMON STOCK
                            (NO PAR VALUE PER SHARE)

         This Prospectus Supplement supplements the Prospectus dated May 9, 1997
(the "Prospectus") of Network Appliance, Inc. (the "Company") relating to the
public offering, which is not being underwritten, and sale by certain
shareholders of the Company or by pledgees, donees, transferees or other
successors in interest that receive such shares as a gift, partnership
distribution or other non-sale related transfer (the "Selling Shareholders") of
172,372 shares of Common Stock, no par value, of the Company (the "Common
Stock") who received such shares in connection with the acquisition by statutory
merger of Internet Middleware Corporation ("IMC"), by and through a merger of
IMC with and into the Company. This Prospectus Supplement should be read in
conjunction with the Prospectus, and this Prospectus Supplement is qualified by
reference to the Prospectus except to the extent that the information herein
contained supersedes the information contained in the Prospectus. Capitalized
terms used in this Prospectus Summary and not otherwise defined herein have the
meanings specified in the Prospectus.

                              SELLING SHAREHOLDERS

         The following table sets forth the number of shares of Common Stock
owned by shareholders of the Company who were not specifically identified in the
Prospectus as Selling Shareholders. The table of Selling Shareholders in the
Prospectus is hereby amended to include the following shareholders as Selling
Shareholders:


<TABLE>
<CAPTION>
                                                                    Number of
                                                                     Shares
                              Number of Shares     Percent of    Registered for
                                Beneficially       Outstanding        Sale
Name of Selling Shareholder         Owned            Shares        Hereby (1)
- ---------------------------         -----            ------        ----------

<S>                         <C>                 <C>   
Danzig/Thomas Charitable           50,000               *            50,000
Remainder Trust
- -----------
*  less than one percent
</TABLE>


(1) This Registration Statement shall also cover any additional shares of Common
Stock which become issuable in connection with the shares registered for sale
hereby by reason of any stock dividend, stock split, recapitalization or other
similar transaction effected without the receipt of consideration which results
in an increase in the number of the Selling Shareholders' outstanding shares of
Common Stock.
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