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                                                                     Exhibit 4.1

                  CERTIFICATE AND AMENDMENT TO RIGHTS AGREEMENT

        CERTIFICATE AND AMENDMENT, dated September 23, 2001 (this "AMENDMENT"),
to the Rights Agreement, dated as of November 20, 1998 (the "RIGHTS AGREEMENT"),
between Illuminet Holdings, Inc., a Delaware corporation ("COMPANY"), and UMB
Bank, N.A. ("RIGHTS AGENT").

        WHEREAS, concurrently with the execution and delivery of this Amendment,
Company, VeriSign, Inc., a Delaware corporation ("VERISIGN"), and Illinois
Acquisition Corporation, a Delaware corporation and wholly-owned subsidiary of
VeriSign ("MERGER SUB"), have entered into an Agreement and Plan of Merger (the
"MERGER AGREEMENT"), which provides for the merger of Merger Sub with and into
Company (the "MERGER"), and a Stock Option Agreement (the "OPTION AGREEMENT"),
between Company and VeriSign, pursuant to which Company has granted to VeriSign
an option to purchase Company capital stock, and the Board of Directors of
Company has approved the Merger Agreement, the Merger, the Option Agreement and
this Amendment; and

        WHEREAS, Section 5.4 of the Rights Agreement provides that (i) prior to
the Distribution Date (as defined in the Rights Agreement), Company and Rights
Agent may amend the Rights Agreement as Company deems necessary or desirable,
and (ii) Rights Agent shall execute such an amendment upon receipt of a
certificate from an appropriate officer of Company that states that such
amendment is in compliance with such Section 5.4, unless Rights Agent determines
in good faith that such amendment would adversely affect Rights Agent's
interests under the Rights Agreement.

        THEREFORE, in consideration of the foregoing and the mutual agreements
set forth herein, the parties agree as follows:

        1. Company represents and warrants that, as of the date hereof, no
Distribution Date (as defined in the Rights Agreement) has occurred.

        2. Company certifies to Rights Agent that this Amendment is in
compliance with Section 5.4 of the Rights Agreement, and that Rights Agent is
entitled to rely upon such certification.

        3. Section 1.1 of the Rights Agreement is hereby amended by inserting
the following sentence at the end of the definition of "Acquiring Person":

            "Furthermore, notwithstanding anything in this Rights Agreement to
        the contrary, neither VeriSign, Inc., a Delaware corporation
        ("VeriSign"), nor any of its Subsidiaries, Affiliates or Associates,
        shall be or become an Acquiring Person as a result of the execution,
        delivery or performance, or public announcement thereof, of the
        Agreement and Plan of Merger among Company, VeriSign and Illinois
        Acquisition Corporation, or the Stock Option Agreement between Company
        and VeriSign, or the consummation of any of the transactions
        contemplated thereby."


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        4. This Amendment shall be governed by, and construed in accordance with
the laws of the State of Delaware applicable to contracts to be made and
performed entirely within such state. If any term of this Amendment is held by a
court of competent jurisdiction or other competent authority to be invalid or
unenforceable, the remainder of the terms hereof shall remain in full force and
effect, and this Amendment shall be construed in order to give the maximum
effect to the remaining terms and intent hereof. Verisign, its subsidiaries,
affiliates and associates (as defined in the Rights Agreement) are intended
third party beneficiaries of the terms hereof. This Amendment may be executed in
one or more counterparts, all of which shall together constitute one instrument.

        IN WITNESS WHEREOF, the parties have cause this Amendment to be executed
by their duly authorized officers as of the date first written above.

                                 ILLUMINET HOLDINGS, INC.


                                 By:     /s/ Roger H. Moore
                                         ---------------------------------------
                                 Name:   Roger H. Moore
                                 Title:  President and Chief Executive Officer

                                 UMB BANK, N.A., AS RIGHTS AGENT

                                 By:     /s/ Frank C. Bramwell
                                         ---------------------------------------
                                 Name:   Frank C. Bramwell               9/25/01
                                 Title:  Senior Vice President