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                                                                     EXHIBIT 5.1



                [BLACKWELL SANDERS PEPER MARTIN LLP LETTERHEAD]



                               October 7, 1999


Illuminet Holdings, Inc.
P.O. Box 8
4501 Intelco Loop, SE
Lacey, Washington  98503

        Re:  Registration Statement on Form S-1

Ladies and Gentlemen:

        This opinion is furnished to you in connection with a Registration
Statement on Form S-1 (File No. 333-85779) (the "Registration Statement") filed
with the Securities and Exchange Commission (the "Commission") under the
Securities Act of 1933, as amended (the "Securities Act"), for the registration
of 3,900,000 shares of Common Stock, $.01 par value per share with attached
rights to purchase Series B Participating Cumulative Preference Stock (the
"Shares") of Illuminet Holdings, Inc., a Delaware corporation (the "Company")
(including 585,000 Shares issuable upon exercise of an over-allotment option
granted by the Company).

        The Shares are to be sold by the Company pursuant to an underwriting
agreement (the "Underwriting Agreement") to be entered into by and among the
Company and Morgan Stanley & Co. Incorporated, BancBoston Robertson Stephens
Inc. and Donaldson, Lufkin & Jenrette Securities Corporation, as representatives
of the underwriters named therein (the "Underwriters"), the form of which has
been filed as Exhibit 1.1 to the Registration Statement.

        We are acting as counsel for the Company in connection with the issue
and sale by the Company of the Shares. We have examined signed copies of the
Registration Statement as filed with the Commission. We have also examined and
relied upon the Underwriting Agreement, minutes of meetings of the stockholders
and the Board of Directors of the Company as provided to us by the Company,
stock record books of the Company as provided to us by the Company, the
Certificate of Incorporation and Bylaws of the Company, each as amended to date,
the form of Amendment to the Certificate of Incorporation, which has been filed
as Exhibit 3.2 to the Registration Statement, and such other documents as we
have deemed necessary for purposes of rendering the opinions hereinafter set
forth.

        In our examination of the foregoing documents, we have assumed the
genuineness of all signatures, the authenticity of all documents submitted to us
as originals, the conformity to

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Illuminet Holdings, Inc.
October 7, 1999
Page 2


original documents of all documents submitted to us as copies, the authenticity
of the originals of such latter documents and the legal competence of all
signatories to such documents.

        We express no opinion herein as to the laws of any state or jurisdiction
other than the state laws of the State of Delaware and the federal laws of the
United States of America. To the extent that any other laws govern the matters
as to which we are opining herein, we have assumed that such laws are identical
to the state laws of the State of Delaware, and we are expressing no opinion
herein as to whether such assumption is reasonable or correct.

        Based upon the foregoing, we are of the opinion that the Shares to be
issued and sold by the Company will have been duly authorized for issuance and,
when such Shares are issued and paid for in accordance with the terms and
conditions of the Underwriting Agreement, such Shares will be validly issued,
fully paid and nonassessable.

        It is understood that this opinion is to be used only in connection with
the offer and sale of the Shares while the Registration Statement is in effect.

        Please note that we are opining only as to the matters expressly set
forth herein, and no opinion should be inferred as to any other matters. This
opinion is based upon currently existing statutes, rules, regulations and
judicial decisions, and we disclaim any obligation to advise you of any change
of these sources of law or subsequent legal or factual developments which might
affect any matters or opinions set forth herein.

        We hereby consent to the filing of this opinion with the Commission as
an exhibit to the Registration Statement in accordance with the requirements of
Item 601(b)(5) of Regulation S-K under the Securities Act and to the use of our
name therein and in the related Prospectus under the caption "Legal Matters." In
giving such consent, we do not hereby admit that we are in the category of
persons whose consent is required under Section 7 of the Securities Act or the
rules and regulations of the Commission.


                                       Very truly yours,

                                       /s/ Blackwell Sanders Peper Martin LLP

                                       BLACKWELL SANDERS PEPER MARTIN LLP