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                                                                     EXHIBIT 5.1

Netcom Systems, Inc.
20550 Nordhoff Street
Chatsworth, CA 91311

        Re:     Registration Statement on Form S-1

Ladies and Gentlemen:

        We have examined the Registration Statement on Form S-1 filed with the
Securities and Exchange Commission on May 14, 1999, (as such may be amended or
supplemented, the "Registration Statement"), in connection with the registration
under the Securities Act of 1933, as amended, of shares of Common Stock of
Netcom Systems, Inc. with a maximum aggregate offering price of $86,250,000 (the
"Shares"). The Shares to be offered by the Company are to be sold to the
underwriters as described in such Registration Statement for the sale to the
public or issued to the Representatives of the underwriters. As your counsel in
connection with the transaction, we have examined the proceedings proposed to be
taken in connection with said sale and issuance of the Shares.

        It is our opinion that, upon approval by the pricing committee duly
authorized by the Company's Board of Directors, the Shares when issued and sold
in the manner referred to in the Registration Statement will be legally and
validly issued, fully paid and nonassessable.

        We consent to the use of this opinion as an exhibit to the Registration
Statement, and further consent to the use of our name wherever appearing in the
Registration Statement, including the prospectus constituting a part hereof, and
any amendment thereto.


                                       Very truly yours,

                                       WILSON SONSINI GOODRIGH & ROSATI
                                       Professional Corporation

                                       /s/ WILSON SONSINI GOODRICH & ROSATI