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                                                                   EXHIBIT 4.2


                             DIGITAL ISLAND, INC.
                         REGISTRATION RIGHTS AGREEMENT

     THIS REGISTRATION RIGHTS AGREEMENT (the "Agreement") is entered as of July
1, 2000, by and among DIGITAL ISLAND, INC., a Delaware corporation ("Digital
Island"), and SRI International, a California nonprofit public benefit
corporation ("SRI").

     WHEREAS, Digital Island is obligated to issue shares of its Common Stock to
SRI pursuant to the Technology Purchase Agreement and Research Alliance, by and
between Digital Island and SRI, dated as of November 22, 1999 (the "Alliance
Agreement");

     WHEREAS, in connection with the transactions contemplated by the Alliance
Agreement, Digital Island wishes to grant to SRI certain registration rights
covering such shares, upon the terms and conditions set forth below;

     WHEREAS, the parties desire that this Agreement supercede and replace
Section 2.5.6 of the Alliance Agreement in its entirety;

     WHEREAS, the parties desire that all other provisions except Section
2.5.6 of the Alliance Agreement shall remain in full force and effect;

     NOW, THEREFORE, in consideration of these premises and for good and valid
consideration, the receipt and sufficiency of which are hereby acknowledged, the
parties hereto agree as follows:

     1.   Registration.  Upon request by SRI, at any time that Digital Island is
          ------------
eligible to use Form S-3 under the Securities Act of 1933, as amended (the
"Act"), Digital Island shall register for resale on Form S-3 under the Act all
of the shares of Digital Island common stock issued under the Alliance
Agreement, including any shares subsequent to the original effectiveness of such
registration statement, and shall keep such resale registration in effect for up
to one hundred eighty (180) days (or such earlier date on which the shares
registered for resale thereunder have been disposed of or may be sold or
disposed of in a 90-day period under Rule 144 under the Act). In addition,
Digital Island shall notify SRI in advance of any registration to be undertaken
by Digital Island while SRI holds of record any of the shares issued thereunder
and has not disposed of such shares and is not then permitted to resell such
shares in a 90-day period pursuant to Rule 144 under the Act, and will use its
reasonable best efforts to include shares of Digital Island common stock issued
thereunder which SRI requests in writing to be so included, provided that the
foregoing shares shall be subject to "cut back" by any managing underwriter of
the offering being registered if the inclusion thereof is believed in good faith
to be detrimental to the offering. Such registration rights shall be subject to
customary "black out" restrictions; provided that the effective period of such
registration statement shall be extended by the length of any such "black out"
so that SRI has the full one hundred eighty (180) days (or lesser period as
provided above) in which to sell securities thereunder.
<PAGE>

     2.   Business Combinations. The provisions of Section 1 shall be applicable
          ---------------------
to shares of capital stock into which Digital Island common stock will be
converted or exchanged, directly or indirectly, whether by merger,
consolidation, business combination or otherwise.

     3.   Governing Law: Attorney's Fees.  This Agreement shall be governed and
          ------------------------------
construed in accordance with the laws of the State of California, without regard
to the conflicts of law principles thereof. In any action to enforce this
Agreement the prevailing party will be entitled to costs and reasonable
attorney's fees.

     4.   Assignment. The obligations of this Agreement shall be fully binding
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upon, and the rights of this Agreement shall inure to the benefit of, the
permitted successors or assigns of either party.

     5.   Notices.  Any consent, notice or report required or permitted to be
          -------
given or made under this Agreement by one party to the other party shall be in
writing, delivered by any lawful means and addressed to the other party at its
address indicated below, or to such other address as the addressee shall have
last furnished in writing to the addressor. Except as otherwise provided in
this Agreement, such consent, notice or report shall be effective upon receipt
by the addressee.

To SRI:                                 To Digital Island:
SRI International                       Digital Island, Inc.
333 Ravenswood Avenue                   45 Fremont Street, 12th Floor
Menlo Park, CA 94025                    San Francisco, CA 94105
Attn: Contract Administration           Attn: Chris Albinson
Telephone: (650) 859-2004               Telephone: (415) 738-4196
Facsimile: (650) 859-5860               Facsimile: (415) 738-4397

                                        With a copy to:
                                        Brobeck, Phleger & Harrison LLP
                                        Two Embarcadero Place
                                        2200 Geng Road
                                        Palo Alto, CA 94303
                                        Attn: Curtis L. Mo, Esq.
                                        Telephone: (650) 424-0160
                                        Facsimile: (650) 496-2736

     6.   Severability. Any provisions of this Agreement which are determined to
          ------------
be invalid or unenforceable in any jurisdiction shall be ineffective to the
extent of such invalidity or unenforceability in such jurisdiction, without
rendering invalid or unenforceable the remaining provisions hereof and without
affecting the validity or enforceability of any of the terms of this Agreement
in any other jurisdiction.

     7.   Waiver and Amendments. No change, modification, extension or
          ---------------------
termination or waiver of this Agreement, or any of the provisions herein
contained, shall be valid unless made in writing and signed by a duly authorized
representative of the parties hereto.

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     8.   Alliance Agreement. The parties expressly agree that all provisions of
          ------------------
the Alliance Agreement other than Section 2.5.6 shall remain in full force and
effect.

     9.   Counterparts.  This Agreement may be executed in two or more
          ------------
counterparts, each of which shall be deemed an orginal, but all of which
together shall constitute one and the same instrument.

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<PAGE>

     IN WITNESS WHEREOF, the parties have caused this Registration Rights
Agreement to be executed on the date first written above.



DIGITAL ISLAND, INC.                         SRI INTERNATIONAL


By: /s/ Howard Lasky                         By: /s/ Thomas J. Furst
    _____________________________________        ____________________________

Name: Howard Lasky                           Name: Thomas J. Furst
      ___________________________________          __________________________

Title: Vice President and General Counsel    Title: Sr. Vice President, C.F.O.
       __________________________________           __________________________



               [SIGNATURE PAGE TO REGISTRATION RIGHTS AGREEMENT]

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