<PAGE>   1
  AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON APRIL 30, 1997
                                               REGISTRATION NO. 333-____________
================================================================================
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
                              ____________________

                                    FORM S-3
                             REGISTRATION STATEMENT
                                     Under
                           The Securities Act of 1933
                              ____________________

                            NETWORK APPLIANCE, INC.
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)
                              ____________________

        CALIFORNIA                                            77-0307520
(STATE OR OTHER JURISDICTION OF                            (I.R.S. EMPLOYER  
 INCORPORATION OR ORGANIZATION)                         IDENTIFICATION NUMBER)
                              ____________________

                           2770 SAN TOMAS EXPRESSWAY
                         SANTA CLARA, CALIFORNIA 95051
                                 (408) 367-3000
  (ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER, INCLUDING AREA CODE, OF
                   REGISTRANT'S PRINCIPAL EXECUTIVE OFFICES)
                              ____________________

                             DANIEL J. WARMENHOVEN
                     PRESIDENT AND CHIEF EXECUTIVE OFFICER
                            NETWORK APPLIANCE, INC.
                           2770 SAN TOMAS EXPRESSWAY
                         SANTA CLARA, CALIFORNIA 95051
                                 (408) 367-3000
  (NAME AND ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER, INCLUDING AREA
                          CODE, OF AGENT FOR SERVICE)
                              ____________________

                                    Copy to:
                            EDWARD M. LEONARD, ESQ.
                        BROBECK, PHLEGER & HARRISON LLP
                             TWO EMBARCADERO PLACE
                                 2200 GENG ROAD
                          PALO ALTO, CALIFORNIA 94303
                                 (415) 424-0160
                              ____________________

        APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE TO THE PUBLIC:
     From time to time after this Registration Statement becomes effective.

         If the only securities being registered on this Form are being offered 
pursuant to dividend or interest reinvestment plans, please check the following
box. [ ]
         If any of the securities being registered on this Form are to be
offered on a delayed or continuous basis pursuant to Rule 415 under the
Securities Act of 1933, other than securities offered only in connection with
dividend or interest reinvestment plans, check the following box.  [X]
         If this Form is filed to register additional securities for an
offering pursuant to Rule 462(b) under the Securities Act, please check the
following box and list the Securities Act registration statement number of the
earlier effective registration statement for the same offering.  [ ]
         If this Form is a post-effective amendment filed pursuant to Rule
462(c) under the Securities Act, check the following box and list the
Securities Act registration statement number of the earlier effective
registration statement for the same offering. [ ]
         If delivery of the prospectus is expected to be made pursuant to 
Rule 434, please check the following box. [ ]
                              ____________________

<TABLE>
<CAPTION>
                         CALCULATION OF REGISTRATION FEE
===============================================================================================================================
         Title of Each                   Amount             Proposed Maximum         Proposed Maximum            Amount
      Class of Securities                 to Be                 Offering                Aggregate            of Registration
        to be Registered               Registered          Price Per Share(1)       Offering Price(1)              Fee
- -------------------------------------------------------------------------------------------------------------------------------
 <S>                                 <C>                         <C>                    <C>                     <C>
 Common Stock,                       172,372 Shares              $26.75                 $4,610,951              $1,397.26
 no par value per share
===============================================================================================================================
</TABLE>

(1) The price of $26.75, which was the average of the high and low prices of
the Common Stock on the Nasdaq National Market System on April 23, 1997, is set
forth solely for the purpose of computing the registration fee pursuant to Rule
457(c).                       _________________

THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR DATES
AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL FILE
A FURTHER AMENDMENT THAT SPECIFICALLY STATES THAT THIS REGISTRATION STATEMENT
SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(A) OF THE
SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME
EFFECTIVE ON SUCH DATE AS THE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(A),
MAY DETERMINE.
================================================================================


<PAGE>   2


                                 172,372 SHARES



                            NETWORK APPLIANCE, INC.
                                  COMMON STOCK

                              ____________________


         This Prospectus relates to the public offering, which is not being
underwritten, of 172,372 shares of Common Stock, no par value per share, of
Network Appliance, Inc. (the "Company" or the "Registrant").  All 172,372
shares (the "Shares") may be offered 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").  All of the Shares were originally
issued by the Company 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.  The Shares were issued pursuant to an exemption from the
registration requirements of the Securities Act of 1933, as amended (the
"Securities Act"), provided by Section 4(2) thereof.  The Shares are being
registered by the Company pursuant to section 1.10 of the Agreement and Plan of
Reorganization between the Company and IMC.

         The Shares may be offered by the Selling Shareholders from time to
time in transactions in the over-the-counter market, in negotiated
transactions, or a combination of such methods of sale, at fixed prices which
may be changed, at market prices prevailing at the time of sale, at prices
related to prevailing market prices or at negotiated prices.  The Selling
Shareholders may effect such transactions by selling the Shares to or through
broker-dealers, and such broker-dealers may receive compensation in the form of
discounts, concessions or commissions from the Selling Shareholders and/or the
purchasers of the Shares for whom such broker-dealers may act as agents or to
whom they sell as principals, or both (which compensation as to a particular
broker-dealer might be in excess of customary commissions).  See "Plan of
Distribution."

         The Company will not receive any of the proceeds from the sale of the
Shares.  The Company has agreed to bear certain expenses in connection with the
registration of the Shares being offered and sold by the Selling Shareholders.

         The Company's Common Stock is quoted on the Nasdaq National Market
under the symbol "NTAP."  On April 23, 1997 the average of the high and low
price for the Common Stock was $26.75.
                        _______________________________

         The Selling Shareholders and any broker-dealers or agents that
participate with the Selling Shareholders in the distribution of the Shares may
be deemed to be "underwriters" within the meaning of Section 2(11) of the
Securities Act, and any commissions received by them and any profit on the
resale of the Shares purchased by them may be deemed to be underwriting
commissions or discounts under the Securities Act.

                                   __________

  THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES AND
    EXCHANGE  COMMISSION  OR  ANY  STATE  SECURITIES  COMMISSION NOR HAS THE
     SECURITIES AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION
          PASSED UPON THE ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY
             REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.

================================================================================

                 The date of this Prospectus is April 30, 1997






<PAGE>   3
         No person has been authorized to give any information or to make any
representations other than those contained in this Prospectus in connection
with the offering made hereby, and if given or made, such information or
representations must not be relied upon as having been authorized by the
Company, any Selling Shareholder or by any other person.  Neither the delivery
of this Prospectus nor any sale made hereunder shall, under any circumstances,
create any implication that information herein is correct as of any time
subsequent to the date hereof.  This Prospectus does not constitute an offer to
sell or a solicitation of an offer to buy any security other than the
securities covered by this Prospectus, nor does it constitute an offer to or
solicitation of any person in any jurisdiction in which such offer or
solicitation may not lawfully be made.

                             AVAILABLE INFORMATION

         The Company is subject to the informational requirements of the
Securities Exchange Act of 1934, as amended (the "Exchange Act"), and in
accordance therewith files reports, proxy statements, information statements
and other information with the Securities and Exchange Commission (the
"Commission").  Reports, proxy statements and other information filed by the
Company may be inspected and copied at the public reference facilities
maintained by the Commission at Room 1024, 450 Fifth Street, N.W., Washington,
D.C. 20549, and at the Commission's Regional Offices located at 75 Park Place,
New York, New York 10007 and 500 West Madison Street, Suite 1400, Chicago,
Illinois 60661-2511.  Copies of such material can be obtained by mail from the
Public Reference Branch of the Commission at Room 1024, 450 Fifth Street, N.W.,
Washington, D.C. 20549 at prescribed rates.  The Common Stock of the Company is
quoted on the Nasdaq National Market, and such material may also be inspected
at the offices of Nasdaq Operations, 1735 K Street N.W. Washington, D.C. 20006.
The Commission maintains a World Wide Web site that contains reports, proxy and
information statements and other information regarding registrants that file
electronically with the Commission.  The address of the Commission's web site
is http://www.sec.gov.

         The Company has filed with the Commission a registration statement on
Form S-3 (herein, together with all amendments and exhibits, referred to as the
"Registration Statement") under the Securities Act with respect to the Common
Stock offered hereby.  This Prospectus does not contain all of the information
set forth in the Registration Statement, certain parts of which are omitted in
accordance with the rules and regulations of the Commission.  For further
information regarding the Company and the Common Stock offered hereby,
reference is hereby made to the Registration Statement and to the exhibits and
schedules filed therewith.  The Registration Statement, including the exhibits
and schedules thereto, may be inspected at the public reference facilities
maintained by the Commission at Room 1024, 450 Fifth Street, N.W., Washington,
D.C. 20549 and copies of all or any part thereof may be obtained from such
office upon payment of the prescribed fees.

                INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

         The following documents filed with the Commission (File No. 0-27130)
pursuant to the Exchange Act are incorporated herein by reference:

         1.      The Company's Annual Report on Form 10-K for the fiscal year
ended April 30, 1996, filed July 25, 1996, as amended on Form 10-K/A filed on 
October 29, 1996;

         2.      The Company's Quarterly Reports on Form 10-Q for the quarters
ended July 26, 1996, October 25, 1996 and January 24, 1997;

         3.      The Company's Current Reports on Form 8-K filed on September
9, 1996 and March 28, 1997, as amended on Form 8-K/A filed on April 16, 1997;

         4.      Definitive Proxy Statement dated October 1, 1996, filed in
connection with the Company's 





                                       2.


<PAGE>   4
1996 Annual Meeting of Shareholders;

         5.      The description of the Company's Common Stock, no par value
per share, contained in its Registration Statement on Form 8-A filed on
November 1, 1995, including any amendment or report filed for the purpose of
updating such description; and

         6.      All reports and other documents filed by the Company pursuant
to Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act subsequent to the
date of this Prospectus and prior to the termination of the offering.

         Any statement contained in a document incorporated by reference herein
shall be deemed to be incorporated by reference in this Prospectus and to be
part hereof from the date of filing of such documents.  Any statement modified
or superseded shall not be deemed, except as so modified or superseded, to
constitute a part of this Prospectus.  The Company will provide without charge
to each person to whom this Prospectus is delivered a copy of any or all of
such documents which are incorporated herein by reference (other than exhibits
to such documents unless such exhibits are specifically incorporated by
reference into the documents that this Prospectus incorporates).  Written
requests for copies should be directed to Jeffry R. Allen, Vice President,
Finance and Operations and Chief Financial Officer, at the principal executive
offices of Network Appliance, Inc., 2770 San Tomas Expressway, Santa Clara,
California 95051.  The Company's telephone number is (408) 367-3000.





                                       3.


<PAGE>   5
                                  THE COMPANY

         The principal executive offices of the Company are located at 2770 San
Tomas Expressway, Santa Clara, California 95051.  The Company's telephone
number is (408) 367-3000.

                              PLAN OF DISTRIBUTION

         The Company will receive no proceeds from this offering.  The Shares
offered hereby may be sold by the Selling Shareholders from time to time in
transactions in the over-the-counter market, in negotiated transactions, or a
combination of such methods of sale, at fixed prices which may be changed, at
market prices prevailing at the time of sale, at prices related to prevailing
market prices or at negotiated prices.  The Selling Shareholders may effect
such transactions by selling the Shares to or through broker-dealers, and such
broker-dealers may receive compensation in the form of discounts, concessions
or commissions from the Selling Shareholders and/or the purchasers of the
Shares for whom such broker-dealers may act as agents or to whom they sell as
principals, or both (which compensation as to a particular broker-dealer might
be in excess of customary commissions).

         In order to comply with the securities laws of certain states, if
applicable, the Shares will be sold in such jurisdictions only through
registered or licensed brokers or dealers.  In addition, in certain states the
Shares may not be sold unless they have been registered or qualified for sale
in the applicable state or an exemption from the registration or qualification
requirement is available and is complied with.

         The Selling Shareholders and any broker-dealers or agents that
participate with the Selling Shareholders in the distribution of the Shares may
be deemed to be "underwriters" within the meaning of the Securities Act, and
any commissions received by them and any profit on the resale of the Shares
purchased by them may be deemed to be underwriting commissions or discounts
under the Securities Act.

         Under applicable rules and regulations under the Exchange Act, any
person engaged in the distribution of the Shares may not simultaneously engage
in market making activities with respect to the Common Stock of the Company for
a period of two business days prior to the commencement of such distribution.
In addition and without limiting the foregoing, each Selling Shareholder will
be subject to applicable provisions of the Exchange Act and the rules and
regulations thereunder, including, without limitation, Rules 10b-6 and 10b-7,
which provisions may limit the timing of purchases and sales of shares of the
Company's Common Stock by the Selling Shareholders.





                                       4.


<PAGE>   6
                              SELLING SHAREHOLDERS

         The following table sets forth the number of shares of Common Stock
owned by each of the Selling Shareholders. Except as indicated, none of the
Selling Shareholders has had a material relationship with the Company within
the past three years other than as a result of the ownership of the Shares or
other securities of the Company. Because the Selling Shareholders may offer all
or some of the Shares which they hold pursuant to the offering contemplated by
this Prospectus, and because there are currently no agreements, arrangements or
understandings with respect to the sale of any of the Shares, no estimate can
be given as to the amount of Shares that will be held by the Selling
Shareholders after completion of this offering.  The Shares offered by this
Prospectus may be offered from time to time by the Selling Shareholders named
below.


<TABLE>
<CAPTION>
                                                  Number of                                      Number of
                                                    Shares              Percent of                 Shares
                                                 Beneficially           Outstanding            Registered for
Name of Selling Shareholder                          Owned                 Shares              Sale Hereby(1)  
- ---------------------------                      ------------           -----------            --------------
<S>                                                <C>                       <C>                   <C>
Peter Danzig                                        93,222                   *                      93,222
Len Rand                                            43,158                   *                      43,158
Audrey MacLean                                      25,895                   *                      25,895
Anawat Chankhunthod                                  8,631                   *                       8,631
John Schuster                                          345                   *                         345
Grant Goodale                                           86                   *                          86
Brian Quirion                                          345                   *                         345
Brian Wink                                             345                   *                         345
Peter Kent                                             207                   *                         207
C. Lexis King                                          138                   *                         138
                                                       ---                                             ---

   Total                                           172,372                   *                     172,372
                                                   =======                   =                     =======
</TABLE>


__________________________________
* Represents beneficial ownership of less than 1%.

(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 divided, stock split, recapitalization or
other similar transaction effected without the receipt of consideration which
results in an increase in the number of the Registrant's outstanding shares of
Common Stock.





                                       5.


<PAGE>   7
                                 LEGAL MATTERS

   The validity of the securities offered hereby will be passed upon for the
Company by Brobeck, Phleger & Harrison LLP, Palo Alto, California.


                                    EXPERTS

   The financial statements and the related financial statement schedule
incorporated in this prospectus by reference from the Company's Annual Report
on Form 10-K/A and Form 10-K, respectively, for the year ended April 30, 1996
have been audited by Deloitte and Touche LLP, independent auditors, as stated
in their reports, which are incorporated herein by reference, and have been so
incorporated in reliance upon the reports of such firm given upon their
authority as experts in accounting and auditing.





                                       6.


<PAGE>   8

                                    PART II
                     INFORMATION NOT REQUIRED IN PROSPECTUS


ITEM 14.  OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION

         The following table sets forth the costs and expenses, other than
underwriting discounts and commissions, payable by the Company in connection
with the sale of Common Stock being registered. All amounts are estimates
except the SEC registration fee.


<TABLE>
                         <S>                                   <C>
                         SEC Registration fee                  $  1,397
                         Legal fees and expenses                 15,000
                         Accounting fees and expenses             5,000
                         Miscellaneous                              500
                                                               --------
                              Total                            $ 21,897
                                                               ========
</TABLE>



ITEM 15.  INDEMNIFICATION OF DIRECTORS AND OFFICERS

         Section 317 of the California Corporations Code authorizes a court to
award, or a corporation's Board of Directors to grant, indemnity to directors
and officers in terms sufficiently broad to permit indemnification (including
reimbursement of expenses incurred) under certain circumstances for liabilities
arising under the Securities Act.  The Registrant's Restated Articles of
Incorporation, as amended and Amended Bylaws provide for indemnification of its
directors, officers, employees and other agents to the maximum extent permitted
by the California Corporations Code.  In addition, the Company has entered into
Indemnification Agreements with each of its directors and officers.


ITEM 16.  EXHIBITS


<TABLE>
<S>      <C>
2.1*     Agreement and Plan of Reorganization.
5.1      Opinion of Brobeck, Phleger & Harrison LLP.
23.1     Independent Auditors' Consent.
23.2     Consent of Brobeck, Phleger & Harrison LLP (included in the Opinion of Counsel filed as Exhibit 5.1 hereto).
24.1     Power of Attorney (included on page II-3 of this Registration Statement).
</TABLE>


- -----------------
* Incorporated by reference to the Registrant's Current Report on Form 8-K
  filed on March 28, 1997.

ITEM 17.  UNDERTAKINGS

         The undersigned Registrant hereby undertakes:

         (1)  To file, during any period in which offers or sales are being
made, a post-effective amendment to this Registration Statement: (i) to include
any prospectus required by section 10(a)(3) of the Securities Act; (ii) to
reflect in the prospectus any facts or events arising after the effective date
of the Registration Statement (or the most recent post-effective amendment
thereof) which, individually or in the aggregate, represent a fundamental
change in the information set forth in the Registration Statement; and (iii) to
include any material information with respect to the plan of distribution not
previously disclosed in the Registration Statement or any material change to
such information in the Registration Statement; provided, however, that (i) and
(ii) do not apply if the Registration Statement is on Form S-3 or Form S-8, and
the





                                      II-1


<PAGE>   9
information required to be included in a post-effective amendment by (i) and
(ii) is contained in periodic reports filed with or furnished to the Commission
by the Registrant pursuant to Section 13 or Section 15(d) of the Exchange Act
that are incorporated by reference in the Registration Statement.

         (2)  That, for the purpose of determining any liability under the
Securities Act, each such post-effective amendment shall be deemed to be a new
registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

         (3)  To remove from registration by means of a post-effective
amendment any of the securities being registered which remain unsold at the
termination of the offering.

         Insofar as indemnification for liabilities arising under the
Securities Act may be permitted to directors, officers and controlling persons
of the Registrant pursuant to the foregoing provisions, or otherwise, the
Registrant has been advised that in the opinion of the Securities and Exchange
Commission such indemnification is against public policy as expressed in the
Securities Act and is, therefore, unenforceable.  In the event that a claim for
indemnification against such liabilities (other than the payment by the
Registrant of expenses incurred or paid by a director, officer or controlling
person of the Registrant in the successful defense of any action, suit or
proceeding) is asserted by such director, officer or controlling person in
connection with the securities being registered, the Registrant will, unless in
the opinion of its counsel the matter has been settled by controlling
precedent, submit to a court of appropriate jurisdiction the question whether
such indemnification by it is against public policy as expressed in the
Securities Act and will be governed by the final adjudication of such issue.

         The undersigned Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act, each filing of the
Registrant's annual report pursuant to Section 13(a) or Section 15(d) of the
Exchange Act that is incorporated by reference in the Registration Statement
shall be deemed to be a new registration statement relating to the securities
offered therein, and the offering of such securities at that time shall be
deemed to be the initial bona fide offering thereof.





                                      II-2


<PAGE>   10

                                   SIGNATURES

         Pursuant to the requirements of the Securities Act of 1933, as
amended, the Registrant has duly caused this Registration Statement to be
signed on its behalf by the undersigned, thereunto duly authorized in the City
of Santa Clara, State of California, on this 30th day of April, 1997.

                            NETWORK APPLIANCE, INC.


<TABLE>
<S>                            <C>     <C>
Dated: April 30, 1997          By      /s/ DANIEL J. WARMENHOVEN              
                                       -----------------------------------------
                                       Daniel J. Warmenhoven
                                       President and Chief Executive Officer
</TABLE>


KNOW ALL MEN BY THESE PRESENTS, that each person whose signature appears below
constitutes and appoints Daniel J. Warmenhoven and Jeffry R.  Allen, and each
of them, as his true and lawful attorneys-in-fact and agents, with full power
of substitution and resubstitution, for him and in his name, place and stead,
in any and all capacities, to sign any and all amendments (including
post-effective amendments) to this Registration Statement, and to file same,
with all exhibits thereto, and other documents in connection therewith, with
the Securities and Exchange Commission, granting unto said attorneys-in-fact
and agents, and each of them, full power and authority to do and perform each
and every act and thing requisite and necessary to be done in connection
therewith, as fully to all intents and purposes as he might or could do in
person, hereby ratifying and confirming all that said attorneys-in-fact and
agents, or any of them, or their or his substitutes, may lawfully do or cause
to be done by virtue thereof.

        Pursuant to the requirements of the Securities Act of 1933, as amended,
this report has been signed below by the following persons on behalf of the
Company and in the capacities and on the dates indicated:



<TABLE>
<CAPTION>
SIGNATURES                              TITLE                                      DATE
- ----------                              -----                                      ----
<S>                                     <C>                                        <C>
/s/ DANIEL J. WARMENHOVEN               President and Chief Executive              April 30, 1997
- ---------------------------------       Officer (Principal Executive Officer)
Daniel J. Warmenhoven                   

</TABLE>



                                      II-3


<PAGE>   11

<TABLE>
<CAPTION>
Signatures                              Title                                      Date
- ----------                              -----                                      ----
<S>                                     <C>                                        <C>


/s/ JEFFRY R. ALLEN                     Vice President, Finance and                April 30, 1997
- ---------------------------------       Operations, Chief Financial Officer
Jeffry R. Allen                         (Principal Financial and Accounting
                                        Officer)




/s/ DONALD T. VALENTINE                 Director                                   April 30, 1997
- ---------------------------------                                                                
Donald T. Valentine




/s/ CAROL A. BARTZ                      Director                                   April 30, 1997
- ---------------------------------                                                                
Carol A. Bartz



/s/ MICHAEL R. HALLMAN                  Director                                   April 30, 1997
- ---------------------------------                                                                
Michael R. Hallman



/s/ KURT R. JAGGERS                     Director                                   April 30, 1997
- ---------------------------------                                                                
Kurt R. Jaggers



/s/ ROBERT T. WALL                      Director                                   April 30, 1997
- ---------------------------------                                                                
Robert T. Wall
</TABLE>






                                      II-4


<PAGE>   12

                               Index to Exhibits
                               -----------------


<TABLE>
<CAPTION>
Exhibit
Number                                         Exhibit Title
- ------                                         -------------
<S>       <C>
2.1*      Agreement and Plan of Reorganization
5.1       Opinion of Brobeck, Phleger & Harrison LLP
23.1      Independent Auditors' Consent
23.2      Consent of Brobeck, Phleger & Harrison LLP (included in the Opinion of Counsel
          filed as Exhibit 5.1)
24.1      Power of Attorney (included on page II-3 of this Registration Statement)
</TABLE>


- -------------
* Incorporated by reference to the Registrant's Current Report on Form 8-K
  filed on March 28, 1997.





<PAGE>   1


                                                                     EXHIBIT 5.1


                                 March 17, 1997



Internet Middleware Corporation
50 Airport Parkway
San Jose, CA  95110

Ladies and Gentlemen:

                 We have acted as counsel for Network Appliance, Inc., a
California corporation ("Acquiror"), in connection with the merger (the
"Merger") of Internet Middleware Corporation, a California corporation
("Target"), with and into Acquiror pursuant to the Agreement and Plan of
Reorganization by and between Acquiror and Target, dated as of March 17, 1997
(the "Reorganization Agreement"), and the Agreement of Merger between Acquiror
and Target, dated as of March 17, 1997 (the "Merger Agreement").  This opinion
is rendered to you pursuant to Section 6.2(c) of the Reorganization Agreement.
Capitalized terms used herein and not otherwise defined shall have the same
meaning given to such terms in the Reorganization Agreement.

                 In connection with this opinion, we have examined originals,
or copies certified or otherwise identified to our satisfaction, of such
documents, corporate records, certificates, including certificates of public
officials, and other instruments as we have deemed necessary or advisable for
purposes of this opinion, including the Restated Articles of Incorporation and
Bylaws of Acquiror
 and the records of its Board of Directors relating to the
Merger.  In addition, in connection with the Merger, we have reviewed the
Reorganization Agreement, the Merger Agreement and the Shareholder's Agreements
between certain Target shareholders and Acquiror.

                 In such examination and review we have assumed the genuineness
of all signatures, the legal capacity of natural persons, the authenticity of
all documents submitted to us as originals, the conformity to original
documents of all documents submitted to us as certified or photostatic copies,
and the authenticity of the originals of such copies; and that there are no
extrinsic agreements or understandings among the parties to the Reorganization
Agreement and the Merger Agreement that would modify or interpret the terms of
the Reorganization Agreement and the Merger Agreement or the respective rights
or obligations of the parties thereunder.  As to any facts material to the
opinions hereinafter expressed which we did not independently establish or
verify, we have relied without investigation, upon certificates, statements and
representations of representatives of Acquiror.  During the course of our
discussion with such officers and representatives and our review of the
documents described above in connection with the preparation of these opinions,
no facts were disclosed to us that caused us to conclude that any

<PAGE>   2
Internet Middleware Corporation
March 17, 1997
                                                                          Page 2

such certificate, statement or representation is untrue.  In making our
examination of the documents executed by entities other than Acquiror, we have
assumed that each such other entity had the power to enter into and perform all
its obligations thereunder and the due authorization, execution and delivery of
such documents by each such entity.

                 The opinions hereinafter expressed are qualified to the extent
that (a) the validity or enforceability of any of the agreements, documents or
obligations referred to herein may be subject to or affected by applicable
bankruptcy, insolvency, reorganization, moratorium or other laws relating to or
affecting the rights and remedies of creditors generally, and (b) the
enforceability of such agreements, documents or obligations may be limited by
general principles of equity, including, without limitation, concepts of
materiality, reasonableness, good faith and fair dealing, and public policy,
whether applied by a court of law or equity.  We do not express any opinion
herein as to the availability of any equitable or other specific remedy upon
breach of any of the agreements, documents or obligations referred to herein.
We render or imply no opinion with respect to compliance with applicable
anti-fraud statutes, rules or regulations of applicable state or Federal law.

                 Based upon and subject to the foregoing, and subject to the
further assumptions, limitations, qualifications, and exceptions set forth
herein, we are of the opinion that:

                 1.       Acquiror is a corporation duly organized, validly
existing and in good standing under the laws of the State of California, has
the corporate power and authority to own, operate and lease its properties and
carry on its business as now conducted;

                 2.       The authorized capital stock of Acquiror consists of
55,000,000 shares of Common Stock, no par value, and 5,000,000 shares of
Preferred Stock, no par value.  At the close of business on March 14, 1997, no
shares of Preferred Stock were issued and outstanding.  At the close of
business on March 14, 1997, 15,753,147 shares of Common Stock were issued and
outstanding, all of which have been duly authorized and are validly issued,
fully paid and nonassessable.  On the close of business on March 14, 1997,
2,148,351 shares of Common Stock were subject to issuance upon exercise of
outstanding options;

                 3.       Acquiror has full corporate power and authority to
execute, deliver, and perform its obligations under the Reorganization
Agreement and the Merger Agreement; Acquiror has taken all requisite corporate
action to approve and adopt the Reorganization Agreement and the Merger
Agreement and to approve and to authorize the carrying out of the transactions
contemplated thereunder; and the Reorganization Agreement and the Merger
Agreement have been duly executed and delivered by Acquiror and constitute
legal, valid and binding obligations of Acquiror; provided that enforceability
of the indemnity obligations contained in such agreements may be limited by
public policy;

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                 4.       The approval of the shareholders of Acquiror is not
required for the consummation of the transactions contemplated by the
Reorganization Agreement and the Merger Agreement; the shares of Acquiror
Common Stock to be delivered in exchange for shares of Target Common Stock
will, when issued as contemplated by the Reorganization Agreement and the
Merger Agreement, be validly issued, fully paid and non-assessable;

                 5.       The execution, delivery and performance of the
Reorganization Agreement by Acquiror, the execution and delivery of the Merger
Agreement by Acquiror and the carrying out of the transactions contemplated by
the Reorganization Agreement and by the Merger Agreement to be carried out by
Acquiror did not and will not conflict with or constitute a violation under the
charter documents of Acquiror;

                 6.       To our knowledge, no suit, action or legal,
administrative, arbitration or other proceeding or governmental investigation
is pending or threatened to which Acquiror or any of its assets or properties
is a party which seeks to prohibit, restrain or enjoin the transactions
contemplated by the Reorganization Agreement or the Merger Agreement;

                 7.       There is no consent, approval, authorization, order,
registration, qualification or filing of or with any court or any regulatory
authority or other governmental body (either foreign or domestic) required by
Acquiror or with respect to its assets or properties or otherwise for the
consummation of the transactions contemplated by the Reorganization Agreement
and the Merger Agreement that has not been obtained, except for (i) such
consents, approvals, authorizations, registration or qualifications as may be
required under state securities or Blue Sky laws in connection with the offer
and sale of Acquiror Common Stock pursuant to the Merger, and (ii) acceptance
for filing of the Merger Agreement together with any appropriate tax clearance
certificate by the California Secretary of State.  Our opinion herein, however,
is subject to the timely and proper completion of all filings and other actions
contemplated above in this paragraph 8 where such filings and actions are to be
undertaken on or after the date hereof; and

                 8.       Based in part upon the representations of Target
shareholders in shareholder questionnaires, the shares of Acquiror Common Stock
to be issued and delivered in exchange for shares of Target Common Stock
pursuant to the Reorganization Agreement and the Merger Agreement will, when
issued, be exempt from registration under Section 5 of the Securities Act of
1933, as amended (the "Act") pursuant to Section 4(2) of the Act.

                 Whenever our opinion herein with respect to the existence or
absence of facts is indicated to be based upon our knowledge, such expression
means that in the course of our representation of Acquiror in connection with
the Merger nothing has come to our attention that would give us actual
knowledge of the existence or absence of such facts.  We have undertaken

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no independent factual investigation to determine the existence or absence of
such facts.

                 With respect to the opinions set forth in the third and fourth
sentences of paragraph (2) as to the number of issued and outstanding shares of
Common Stock and to the effect that such shares have been validly issued and
are fully paid and nonassessable, and as to the number of shares of Common
Stock subject to outstanding options, we have:  (i) relied, without independent
verification, on a certificate of the transfer agent and registrar of Acquiror
("Transfer Agent") dated as of March 17, 1997 as to the matters set forth
therein and (ii) relied, without independent verification, on a certificate of
an officer of Acquiror, dated as of March 17, 1997 with respect to certain of
the factual matters set forth therein.  We note, however, that we have not (x)
reviewed Acquiror's stock records, (y) verified procedures used by or obtained
records from Acquiror's Transfer Agent, or (z) independently verified that each
issued and outstanding share of Common Stock has been fully paid.

                 This opinion relates solely to the laws of the State of
California and applicable Federal laws of the United States, and we express no
opinion with respect to the effect or applicability of the laws of other
jurisdictions.

                 The opinions expressed herein are solely for your benefit in
connection with the above transactions and may not be relied upon in any manner
or for any purpose by any other person.

                                        Sincerely,



                                        BROBECK, PHLEGER & HARRISON LLP



<PAGE>   1
                                                                EXHIBIT 23.1


                         INDEPENDENT AUDITORS' CONSENT


        We consent to the incorporation by reference in this Registration
Statement of Network Appliance, Inc. on Form S-3 of (1) our report dated July
23, 1996 on the financial statement schedule included in the Annual Report on
Form 10-K and (2) our report dated May 10, 1996 (August 16, 1996 as to Note
10), incorporated by reference in the Annual Report on Form 10-K/A of Network
Appliance, Inc. for the year ended April 30, 1996. We also consent to the
reference to us under the heading "Experts" in the Prospectus, which is part of
this Registration Statement.


Deloitte & Touche LLP

San Jose, California
April 28, 1997