1
                                                                     EXHIBIT 5.1

                                                                November 2, 2000

Netergy Networks, Inc.
2445 Mission College Blvd.
Santa Clara, CA 95054

        RE: Registration Statement on Form S-3

Ladies and Gentlemen:

        We have examined the registration statement on Form S-3 to be filed by
you with the Securities and Exchange Commission on or about November 2, 2000
(the "Registration Statement"), in connection with the registration under the
Securities Act of 1933, of 3,555,303 shares of Common Stock, to be sold by
certain stockholders listed in the Registration Statement. As your counsel, we
have examined the transactions taken and proposed to be taken in connection with
the sale of such shares by such stockholders in the manner set forth in the
Registration Statement.

        It is our opinion that such shares, if sold by such stockholders in the
manner set forth 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 thereof,
and any amendment thereto

                                          Very truly yours,

                                          WILSON SONSINI GOODRICH & ROSATI
                                          Professional Corporation

                                          /s/ WILSON SONSINI GOODRICH & ROSATI