Document:

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                                                                   Exhibit 10.33

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                            STOCK EXCHANGE AGREEMENT

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                                 by and between

                                  Yahoo! Inc.

                                      and

                                NET2PHONE, INC.

                           Dated as of March 30, 2000
<PAGE>

                            STOCK EXCHANGE AGREEMENT

     THIS STOCK EXCHANGE AGREEMENT is made and entered into as of March 30,
2000, by and between Yahoo! Inc., a Delaware corporation (the "Yahoo!"), and
Net2Phone, Inc., a Delaware corporation ("Net2Phone").

     THE PARTIES AGREE AS FOLLOWS:

                                   ARTICLE I
                        AUTHORIZATION AND SALE OF STOCK
                        -------------------------------

     Section 1.1  Authorization of the Shares.  Yahoo! has authorized the
                  ---------------------------
issuance and exchange of 806,452 shares of Common Stock of Yahoo!, par value
$0.001 per share (the "Yahoo! Shares"), at a price per share equal to $186.00,
and Net2Phone has authorized the issuance and exchange of 2,777,778 shares of
Common Stock of Net2Phone, par value $0.01 per share (the "Net2Phone Shares"),
at a price per share equal to $54.00, all pursuant to the terms and conditions
of this Agreement.

     Section 1.2  Exchange of Shares.  Subject to the terms and conditions
                  ------------------
hereof, contemporaneously with the execution of this Agreement, Yahoo! will
issue to Net2Phone the Yahoo! Shares in exchange for the Net2Phone Shares, and
Net2Phone shall issue to Yahoo! the Net2Phone Shares in exchange for the Yahoo!
Shares.

                                   ARTICLE II
                             CLOSING DATE; DELIVERY
                             ----------------------

     Section 2.1  Closing Date.  The consummation of the exchange of the Yahoo!
                  ------------
Shares and the Net2Phone Shares hereunder (the "Closing") shall be held at the
offices of Venture Law Group, A Professional Corporation, 2800 Sand Hill Road,
Menlo Park, California, contemporaneously with the execution of this Agreement
or at such other time and place as Yahoo! and Net2Phone mutually agree upon in
writing (the "Closing Date").

     Section 2.2  Delivery.  At the Closing, Yahoo! shall deliver to Net2Phone
                  --------
certificate(s) representing the Yahoo! Shares and Net2Phone shall deliver to
Yahoo! certificate(s) representing the Net2Phone Shares.

                                  ARTICLE III
                    REPRESENTATIONS AND WARRANTIES OF YAHOO!
                    ----------------------------------------

     Yahoo! hereby represents and warrants to Net2Phone, at and as of the date
of this Agreement and at and as of the Closing Date, as follows:

     Section 3.1  Organization  Yahoo! is a corporation duly organized, validly
                  ------------
existing and in good standing under the laws of the State of Delaware and has
all requisite corporate power to own, lease and operate its property and to
carry on its business as now being conducted and is duly qualified or licensed
to do business and is in good standing in each jurisdiction in which the
<PAGE>

failure to be so qualified or licensed would be reasonably likely to have a
material adverse effect on the business, assets (including intangible assets),
liabilities, condition (financial or otherwise), prospects, value, property or
results or operations (a "Material Adverse Effect") of Yahoo!.

     Section 3.2  Valid Issuance of Common Stock.  The Yahoo! Shares, when
                  ------------------------------
issued and paid for in accordance with this Agreement will be duly authorized,
validly issued, fully paid, and non-assessable, and issued in compliance with
all applicable federal or state securities laws.

     Section 3.3  Authority; No Conflict; Required Filings and Consents.
                  -----------------------------------------------------

          (a) Yahoo! has all requisite corporate power and authority to enter
into this Agreement and to consummate the transactions contemplated by this
Agreement, including the acquisition of the Net2Phone Shares.  The execution and
delivery of this Agreement and the consummation of the transactions contemplated
by this Agreement have been duly authorized by all necessary corporate action on
the part of Yahoo!.  This Agreement has been duly executed and delivered by
Yahoo!, and constitutes the valid and binding obligation of Yahoo!, enforceable
in accordance with its terms, except to the extent that enforceability may be
limited by applicable bankruptcy, reorganization, insolvency, moratorium or
other laws affecting the enforcement of creditors' rights generally and by
general principles of equity.

          (b) The execution and delivery by Yahoo! of this Agreement does not,
and consummation of the transactions contemplated by this Agreement will not,
(i) conflict with, or result in any violation or breach of any provision of the
Certificate of Incorporation or Bylaws of Yahoo!, (ii) result in any violation
or breach of, or constitute (with or without notice or lapse of time, or both) a
default (or give rise to a right of termination, cancellation or acceleration of
any obligation or loss of any material benefit) under any of the terms,
conditions or provisions of any note, bond, mortgage, indenture, lease, contract
or other agreement, instrument or obligation to which Yahoo! is a party or by
which any of its properties or assets may be bound, or (iii) conflict or violate
any permit, concession, franchise, license, judgment, order, decree, statute,
law, ordinance, rule or regulation applicable to Yahoo! or any of its properties
or assets, except in the case of (ii) and (iii) for any such conflicts,
violations, defaults, terminations, cancellations or accelerations which would
not be reasonably likely to have a Material Adverse Effect on Yahoo! and its
subsidiaries, taken as a whole.

          (c) No consent, approval, order or authorization of, or registration,
declaration or filing with, any court, administrative agency or commission or
other governmental authority or instrumentality ("Governmental Entity") is
required by or with respect to Yahoo! in connection with the execution and
delivery of this Agreement or the consummation of the transactions contemplated
hereby, except for (i) such consents, approvals, orders, authorizations,
registrations, declarations and filings as may be required under applicable
federal and state securities laws and the laws of any foreign country, and (ii)
such other consents, authorizations, filings, approvals and registrations which,
if not obtained or made, would be reasonably expected to have a Material Adverse
Effect on Yahoo! and its subsidiaries, taken as a whole.

     Section 3.4  Commission Filings; Financial Statements.
                  ----------------------------------------

                                      ii
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          (a) Yahoo! has filed with the Securities and Exchange Commission (the
"Commission") and made available to Net2Phone or its representatives all forms,
reports and documents required to be filed by Yahoo! with the Commission since
June  30, 1999 (collectively, the "Yahoo! Commission Reports").  Yahoo!
Commission Reports (i) at the time filed, complied in all material respects with
the applicable requirements of the Securities Act of 1933, as amended, (the
"Securities Act"), and the Securities Exchange Act of 1934, as amended (the
"Exchange Act"), as the case may be, and (ii) did not at the time they were
filed (or if amended or superseded by a filing prior to the date of this
Agreement, then on the date of such filing) contain any untrue statement of a
material fact or omit to state a material fact required to be stated therein or
necessary in order to make the statements therein, in the light of the
circumstances under which they were made, not misleading.

          (b) Each of the financial statements (including, in each case, any
related notes) contained in Yahoo! Commission Reports, including any such Report
filed after the date of this Agreement until the Closing, complied as to form in
all material respects with the applicable published rules and regulations of the
Commission with respect thereto, was prepared in accordance with generally
accepted accounting principles applied on a consistent basis throughout the
periods involved (except as may be indicated in the notes to such financial
statements or, in the case of unaudited statements, as permitted by Form 10-Q of
the Commission) and fairly presented the consolidated financial position of
Yahoo! and its subsidiaries as at the respective dates and the consolidated
results of its operations and cash flows for the periods indicated, except that
the unaudited interim financial statements were or are subject to normal and
recurring year-end adjustments which were not or are not expected to be material
in amount.

     Section 3.5  Purchase Entirely for Own Account.  The Net2Phone Shares to be
                  ---------------------------------
acquired by Yahoo! will be acquired for investment for Yahoo!'s own account, not
as a nominee or agent, and not with a view to the resale or distribution of any
part thereof, and that Yahoo! has no present intention of selling, granting any
participation in, or otherwise distributing the same.  Yahoo! further represents
that it does not presently have any contract, undertaking, agreement or
arrangement with any person to sell, transfer or grant participation to such
person or to any third person, with respect to any of the Net2Phone Shares.

     Section 3.6  Investment Experience.  Yahoo! is an "accredited investor" as
                  ---------------------
defined in Rule 501(a) under the Securities Act.  Yahoo! is aware of Net2Phone's
business affairs and financial condition and has had access to and has acquired
sufficient information about Net2Phone to reach an informed and knowledgeable
decision to acquire the Net2Phone Shares.  Yahoo! has such business and
financial experience as is required to give it the capacity to protect its own
interests in connection with the purchase of the Net2Phone Shares.

     Section 3.7  Restricted Securities. Yahoo! understands that the Net2Phone
                  ---------------------
Shares are characterized as "restricted securities" under applicable U.S.
federal and state securities laws inasmuch as they are being acquired from
Net2Phone in a transaction not involving a public offering and that, pursuant to
these laws and applicable regulations, Yahoo! must hold the Net2Phone Shares
indefinitely unless they are registered with the Commission and qualified by

                                      iii
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state authorities, or an exemption from such registration and qualification
requirements is available.  Yahoo! further acknowledges that if an exemption
from registration or qualification is available, it may be conditioned on
various requirements including, but not limited to, the time and manner of sale,
the holding period for the Net2Phone Shares, and on requirements relating to
Net2Phone which are outside of Yahoo!'s control, and which Net2Phone is under no
obligation and may not be able to satisfy.  In this connection, Yahoo!
represents that it is familiar with SEC Rule 144, as presently in effect, and
understands the resale limitations imposed thereby and by the Securities Act.

     Section 3.8  Legends.  Yahoo! understands that the Net2Phone Shares, and
                  -------
any securities issued in respect thereof or exchange therefor, may bear one or
all of the following legends:

          (a) "THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN
REGISTERED UNDER THE SECURITIES ACT OF 1933, AND HAVE BEEN ACQUIRED FOR
INVESTMENT AND NOT WITH A VIEW TO, OR IN CONNECTION WITH, THE SALE OR
DISTRIBUTION THEREOF.  THESE SECURITIES MAY NOT BE OFFERED, SOLD, TRANSFERRED OR
OTHERWISE DISPOSED OF UNLESS REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED, AND ANY APPLICABLE STATE SECURITIES LAWS OR AN EXEMPTION FROM SUCH
REGISTRATION IS AVAILABLE."

          (b) Any legend required by the Blue Sky laws of any state to the
extent such laws are applicable to the shares represented by the certificate so
legended.

     Section 3.9  Issuance and Registration of Yahoo! Shares.  The Yahoo! Shares
                  ------------------------------------------
to be issued to Net2Phone pursuant to this Agreement shall be issued pursuant to
the registration statement on Form S-4 filed by Yahoo! with the Commission on
January 12, 2000 (the "Shelf Registration Statement"), and as such shall be
deemed unrestricted securities for purposes of Rule 144 under the Securities Act
and shall be tradeable by Net2Phone without any contractual restriction (other
than as set forth in this Agreement) created under any contract, agreement or
similar instrument to which Yahoo! is a party or by which any of its properties
or assets may be bound or, to the knowledge of Yahoo!, created under any other
contract, agreement or similar instrument.

                                   ARTICLE IV
                  REPRESENTATIONS AND WARRANTIES OF NET2PHONE
                  -------------------------------------------

     Net2Phone hereby represents and warrants to Yahoo!, at and as of the date
of this Agreement and at and as of the Closing, as follows:

     Section 4.1  Organization  Net2Phone is a corporation duly organized,
                  ------------
validly existing and in good standing under the laws of the State of Delaware
and has all requisite corporate power to own, lease and operate its property and
to carry on its business as now being conducted and is duly qualified or
licensed to do business and is in good standing in each jurisdiction in

                                      iv
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which the failure to be so qualified or licensed would be reasonably likely to
have a Material Adverse Effect on Net2Phone.

     Section 4.2  Valid Issuance of Common Stock. The Net2Phone Shares, when
                  ------------------------------
issued and paid for in accordance with this Agreement will be duly authorized,
validly issued, fully paid, and non-assessable and issued in compliance with all
applicable federal or state securities laws.

     Section 4.3  Authority; No Conflict; Required Filings and Consents.
                  -----------------------------------------------------

          (a) Net2Phone has all requisite corporate power and authority to enter
into this Agreement and to consummate the transactions contemplated by this
Agreement, including the acquisition of the Yahoo! Shares.  The execution and
delivery of this Agreement and the consummation of the transactions contemplated
by this Agreement have been duly authorized by all necessary corporate action on
the part of Net2Phone.  This Agreement has been duly executed and delivered by
Net2Phone, and constitutes the valid and binding obligation of Net2Phone,
enforceable in accordance with its terms, except to the extent that
enforceability may be limited by applicable bankruptcy, reorganization,
insolvency, moratorium or other laws affecting the enforcement of creditors'
rights generally and by general principles of equity.

          (b) The execution and delivery by Net2Phone of this Agreement does
not, and consummation of the transactions contemplated by this Agreement will
not, (i) conflict with, or result in any violation or breach of any provision of
the Certificate of Incorporation or Bylaws of Net2Phone, (ii) result in any
violation or breach of, or constitute (with or without notice or lapse of time,
or both) a default (or give rise to a right of termination, cancellation or
acceleration of any obligation or loss of any material benefit) under any of the
terms, conditions or provisions of any note, bond, mortgage, indenture, lease,
contract or other agreement, instrument or obligation to which Net2Phone is a
party or by which any of its properties or assets may be bound, or (iii)
conflict or violate any permit, concession, franchise, license, judgment, order,
decree, statute, law, ordinance, rule or regulation applicable to Net2Phone or
any of its properties or assets, except in the case of (ii) and (iii) for any
such conflicts, violations, defaults, terminations, cancellations or
accelerations which would not be reasonably likely to have a Material Adverse
Effect on Net2Phone and its subsidiaries, taken as a whole.

          (c) No consent, approval, order or authorization of, or registration,
declaration or filing with, any Governmental Entity is required by or with
respect to Net2Phone in connection with the execution and delivery of this
Agreement or the consummation of the transactions contemplated hereby, except
for (i) such consents, approvals, orders, authorizations, registrations,
declarations and filings as may be required under applicable federal and state
securities laws and the laws of any foreign country, and (iii)  such other
consents, authorizations, filings, approvals and registrations which, if not
obtained or made, would be reasonably expected to have a Material Adverse Effect
on Net2Phone and its subsidiaries, taken as a whole.

     Section 4.4  Commission Filings; Financial Statements.
                  ----------------------------------------

                                       v
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          (a) Net2Phone has filed with the Commission and made available to
Net2Phone or its representatives all forms, reports and documents required to be
filed by Net2Phone with the Commission since July 30, 1999 (collectively, the
"Net2Phone Commission Reports").  Net2Phone Commission Reports (i) at the time
filed, complied in all material respects with the applicable requirements of the
Securities Act, and the Exchange Act, as the case may be, and (ii) did not at
the time they were filed (or if amended or superseded by a filing prior to the
date of this Agreement, then on the date of such filing) contain any untrue
statement of a material fact or omit to state a material fact required to be
stated therein or necessary in order to make the statements therein, in the
light of the circumstances under which they were made, not misleading.

          (b) Each of the financial statements (including, in each case, any
related notes) contained in Net2Phone Commission Reports, including any such
Report filed after the date of this Agreement until the Closing, complied as to
form in all material respects with the applicable published rules and
regulations of the Commission with respect thereto, was prepared in accordance
with generally accepted accounting principles applied on a consistent basis
throughout the periods involved (except as may be indicated in the notes to such
financial statements or, in the case of unaudited statements, as permitted by
Form 10-Q of the Commission) and fairly presented the consolidated financial
position of Net2Phone and its subsidiaries as at the respective dates and the
consolidated results of its operations and cash flows for the periods indicated,
except that the unaudited interim financial statements were or are subject to
normal and recurring year-end adjustments which were not or are not expected to
be material in amount.

     Section 4.5  Access to Information.  Net2Phone acknowledges that it has
                  ---------------------
received and reviewed a copy of the Shelf Registration Statement (including the
prospectus therein), together with Amendment No. 1 thereto (including the
prospectus therein) filed by Yahoo! with the Commission on February 11, 2000.

                                   ARTICLE V
                                   COVENANTS
                                   ---------

  Section 5.1  Post-Closing Lock-Up for Net2Phone.
               ----------------------------------

          (a)  Net2Phone agrees that:

               (i) Net2Phone will not sell in the open market in excess of
     100,000 of the Yahoo! Shares (subject to adjustment for future stock
     splits, dividends and the like) in any one trading day, without the prior
     written approval of Yahoo!;

               (ii) during 90-day period commencing on the Closing Date,
     Net2Phone will not sell in the open market more than one-third of the
     original number of Yahoo! Shares issued to Net2Phone pursuant to this
     Agreement;

                                      vi
<PAGE>

               (iii)  during 180-day period commencing on the Closing Date,
     Net2Phone will not sell in the open market more than two-thirds of the
     original number of Yahoo! Shares issued to Net2Phone pursuant to this
     Agreement; and

               (iv) following such 180-day period, Net2Phone shall be free to
     transfer any of the Yahoo! Shares without restriction under this Section
     5.1 (other than the restriction set forth in Section 5.1(a)(i) above).

          (b) Notwithstanding the foregoing, Net2Phone may sell or otherwise
transfer any of the Yahoo! Shares (i) to any wholly-owned subsidiary of
Net2Phone; provided, however, that in any such case, it shall be a condition to
           --------  -------
the transfer that the subsidiary execute an agreement stating that it is
receiving and holding such capital stock subject to the provisions of this
Agreement and there shall be no further transfer of such capital stock except in
accordance with this Agreement, and provided further that any such transfer
                                    -------- -------
shall not involve a disposition for value; or (ii) with the prior written
consent of Yahoo!.  Net2Phone also agrees and consents to the entry of stop
transfer instructions with Yahoo!'s transfer agent and registrar against the
transfer of the Yahoo! Shares except in compliance with the foregoing
restrictions.

          (c) Further, notwithstanding anything to the contrary in this Section
5.1, in the event that the Board of Directors of Net2Phone, following
consultation with its legal counsel, reasonably concludes in good faith that it
is required to transfer all or a portion of the Yahoo! Shares to avoid a
requirement to register under the Investment Company Act of 1940, as amended
(the "Investment Company Act"), Net2Phone shall so notify Yahoo! and may
      ----------------------
thereafter sell any shares it deems necessary to avoid any such registration or
other requirements under the Investment Company Act.

     Section 5.1  Post-Closing Lock-Up for Yahoo!.
                  -------------------------------

          (a)  Yahoo! agrees that:

               (ii) during 90-day period commencing on the Closing Date, Yahoo!
     will not sell in the open market more than one-third of the original number
     of Net2Phone Shares issued to Yahoo! pursuant to this Agreement;

               (iii)  during 180-day period commencing on the Closing Date,
     Yahoo! will not sell in the open market more than two-thirds of the
     original number of Net2Phone Shares issued to Yahoo! pursuant to this
     Agreement; and

               (iv) following such 180-day period, Yahoo! shall be free to
     transfer any of the Net2Phone Shares without restriction under this Section
     5.2.

          (b) Notwithstanding the foregoing, Yahoo! may sell or otherwise
transfer any of the Net2Phone Shares (i) to any wholly-owned subsidiary of
Yahoo!; provided, however, that in any such case, it shall be a condition to the
        --------  -------
transfer that the subsidiary execute an agreement stating that it is receiving
and holding such capital stock subject to the provisions of this Agreement and
there shall be no further transfer of such capital stock except in accordance
with this Agreement, and provided further that any such transfer shall not
                         -------- -------
involve a disposition for

                                      vii
<PAGE>

value; or (ii) with the prior written consent of Net2Phone. Yahoo! also agrees
and consents to the entry of stop transfer instructions with Net2Phone's
transfer agent and registrar against the transfer of the Net2Phone Shares except
in compliance with the foregoing restrictions.

     Section 5.3  Public Disclosure.  Yahoo! and Net2Phone shall consult with
                  -----------------
each other, and to the extent practicable, agree, before issuing any press
release or otherwise making any public statement with respect to the
transactions contemplated by this Agreement and will not issue any such press
release or make any such public statement prior to such consultation, except as
may be required by law or any listing agreement with the Nasdaq Stock Market or
a national securities exchange.

                                   ARTICLE VI
                                 MISCELLANEOUS
                                 -------------

     Section 6.1  Governing Law.  This Agreement shall be governed in all
                  -------------
respects by the laws of the State of Delaware (without reference to its
conflicts of laws principles).

     Section 6.2  Survival.  The representations, warranties, covenants and
                  --------
agreements made herein shall survive the closing of the transactions
contemplated hereby.

     Section 6.3  Successors and Assigns.  Except as otherwise expressly
                  ----------------------
provided herein, the provisions hereof shall inure to the benefit of, and be
binding upon, the successors, assigns, heirs, executors and administrators of
the parties hereto.

     Section 6.4  Entire Agreement; Amendment.  This Agreement and the other
                  ---------------------------
documents delivered pursuant hereto constitute the full and entire understanding
and agreement among the parties with regard to the subjects hereof and thereof.
Any term of this Agreement may be amended and the observance of any term of this
Agreement may be waived (either generally or in a particular instance and either
retroactively or prospectively), only with the written consent of Yahoo! and
Net2Phone.

     Section 6.5  Notices and Other Communications.  Every notice or other
                  --------------------------------
communication required or contemplated by this Agreement by either party shall
be delivered either by (i) personal delivery, (ii) postage prepaid return
receipt requested registered or certified mail or the equivalent of registered
or certified mail under the laws of the country where mailed, (iii) nationally
recognized overnight courier, such as Federal Express or UPS, or (iv) facsimile
with a confirmation copy sent simultaneously by postage prepaid, return receipt
requested, registered or certified mail, in each case addressed to Yahoo! or
Net2Phone as the case may be at the following address:

                                     viii
<PAGE>

     To Yahoo!:     Yahoo! Inc.
                    3420 Central Expressway
                    Santa Clara, CA 95051
                    Attn:  Senior Vice President, Corporate Development
                    Facsimile:  (408) 328-7939

                    With a copy at the same address to the attention of the
                    General Counsel, and a copy to:

                    Venture Law Group
                    A Professional Corporation
                    2800 Sand Hill Road
                    Menlo Park, California 94025
                    Attn.:  Steven J. Tonsfeldt
                            Keith A. Miller
                    Facsimile:  (415) 233-8386

     To Net2Phone:  Net2Phone, Inc.
                    171 Main Street
                    Hackensack, New Jersey 07601
                    Attn:  Chief Executive Officer
                    Facsimile: (

                    With a copy to:

                    Kirkland & Ellis
                    Aon Center
                    200 East Randolph Drive
                    Chicago, Illinois 60601
                    Attn:  Richard W. Porter
                    Facsimile:  (312) 861-2200

or at such other address as the intended recipient previously shall have
designated by written notice to the other party (with copies to counsel as may
be indicated on the signature page).  Notice by registered or certified mail
shall be effective on the date it is officially recorded as delivered to the
intended recipient by return receipt or equivalent, and in the absence of such
record of delivery, the effective date shall be presumed to have been the fifth
(5th) business day after it was deposited in the mail.  All notices delivered in
person or sent by courier shall be deemed to have been delivered to and received
by the addressee and shall be effective on the date of personal delivery;
notices delivered by facsimile with simultaneous confirmation copy by registered
or certified mail shall be deemed delivered to and received by the addressee and
effective on the date sent.  Notice not given in writing shall be effective only
if acknowledged in writing by a duly authorized representative of the party to
whom it was given.

                                      ix
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     Section 6.6  Delays or Omissions.  No delay or omission to exercise any
                  -------------------
right, power or remedy accruing to any holder of any Shares, upon any breach or
default of Yahoo! under this Agreement, shall impair any such right, power or
remedy of such holder nor shall it be construed to be a waiver of any such
breach or default, or an acquiescence therein, or of or in any similar breach or
default thereafter occurring; nor shall any waiver of any single breach or
default be deemed a waiver of any other breach or default theretofore or
thereafter occurring.  Any waiver, permit, consent or approval of any kind or
character on the part of any holder of any breach or default under this
Agreement, or any waiver on the part of any holder of any provisions or
conditions of this Agreement, must be in writing and shall be effective only to
the extent specifically set forth in such writing.  All remedies either under
this Agreement, or by law or otherwise afforded to any holder, shall be
cumulative and not alternative.

     Section 6.7  Severability.  In case any provision of this Agreement shall
                  ------------
be invalid, illegal or unenforceable, the validity, legality and enforceability
of the remaining provisions shall not in any way be affected or impaired
thereby.

     Section 6.8  Finder's Fees.
                  -------------

          (a) Yahoo! (i) represents and warrants that it has retained no finder
or broker in connection with the transactions contemplated by this Agreement and
(ii) hereby agrees to indemnify and to hold Net2Phone harmless of and from any
liability for commission or compensation in the nature of a finder's fee to any
broker or other person or firm (and the costs and expenses of defending against
such liability or asserted liability) for which Yahoo!, or any of its employees
or representatives, is responsible.

          (b) Net2Phone (i) represents and warrants that it has retained no
finder or broker in connection with the transactions contemplated by this
Agreement and (ii) hereby agrees to indemnify and to hold Yahoo! harmless of and
from any liability for any commission or compensation in the nature of a
finder's fee to any broker or other person or firm (and the costs and expenses
of defending against such liability or asserted liability) for which Net2Phone,
or any of its employees or representatives, is responsible.

     Section 6.9  California Corporate Securities Law.  THE SALE OF THE
                  -----------------------------------
SECURITIES WHICH ARE THE SUBJECT OF THIS AGREEMENT HAS NOT BEEN QUALIFIED WITH
THE COMMISSIONER OF CORPORATIONS OF THE STATE OF CALIFORNIA, AND THE ISSUANCE OF
SUCH SECURITIES OR THE PAYMENT OR RECEIPT OF ANY PART OF THE CONSIDERATION
THEREFOR PRIOR TO SUCH QUALIFICATION IS UNLAWFUL, UNLESS THE SALE OF SECURITIES
IS EXEMPT FROM THE QUALIFICATION BY SECTION 25100, 25102, OR 25105 OF THE
CALIFORNIA CORPORATIONS CODE.  THE RIGHTS OF ALL PARTIES TO THIS AGREEMENT ARE
EXPRESSLY CONDITIONED UPON SUCH QUALIFICATION BEING OBTAINED, UNLESS THE SALE IS
SO EXEMPT.

     Section 6.10  Counterparts.  This Agreement may be executed in any number
                   ------------
of counterparts, each of which shall be an original, but all of which together
shall constitute one instrument.  Execution and delivery of this Agreement by
exchange of facsimile copies bearing

                                       x
<PAGE>

the facsimile signature of a party hereto shall constitute a valid and binding
execution and delivery of this Agreement by such party. Such facsimile copies
shall constitute enforceable original documents.

     Section 6.11  Attorneys' Fees.  If any action or proceeding shall be
                   ---------------
commenced to enforce this Agreement or any right arising in connection with this
Agreement, the prevailing party in such action or proceeding shall be entitled
to recover from the other party, the reasonable attorneys' fees, costs and
expenses incurred by such prevailing party in connection with such action or
proceeding or negotiation to avoid such action or proceeding.

                                      xi
<PAGE>

     IN WITNESS WHEREOF, the parties have executed this Agreement as of the day
and year set forth in the first paragraph hereof.

                                    YAHOO! INC.

                                    By: /s/ Jeff Mallett
                                       ----------------------------------
                                       Name:  Jeff Mallett
                                       Title: President and Chief Operating
                                              Officer

                                    NET2PHONE, INC.

                                    By: /s/ Howard Balter
                                       ----------------------------------
                                       Name:  Howard Balter
                                       Title: Chief Executive Officer

                                      xii<PAGE>

                                                                   Exhibit 10.34
                                                                   -------------
                         REGISTRATION RIGHTS AGREEMENT
                         -----------------------------

     This Registration Rights Agreement (the "Agreement") is entered into as of
                                              ---------
March 30, 2000 by and between Net2Phone, Inc., a Delaware corporation (the

"Company"), and Yahoo! Inc, a Delaware corporation (the "Stockholder").
--------                                                 -----------

                                    RECITALS
                                    --------

     The Company and the Stockholder have entered into a Stock Exchange
Agreement of even date herewith (the "Exchange Agreement"), which provides for
                                      ------------------
the exchange of 2,777,778 shares of the Company's Common Stock (the "Shares")
                                                                     ------
for 806,452 shares of the Stockholder's Common Stock.  In connection with the
Stockholder's execution of the Exchange Agreement, the Stockholder desires to
obtain and the Company has agreed to grant certain registration rights to the
Stockholder with respect to the Shares.

                                   AGREEMENT
                                   ---------

     The parties hereby agree as follows:

     1.  Registration Rights.
         -------------------

          1.1  Definitions.  For purposes of this Section 1:
               -----------

               (a) The terms "register," "registered," and "registration" refer
                              --------    ----------        ------------
to a registration effected by preparing and filing a registration statement or
similar document in compliance with the Securities Act of 1933, as amended (the
"Securities Act"), and the subsequent declaration or ordering of the
 --------------
effectiveness of such registration statement or document.

               (b) The term "Registrable Securities" means:
                             ----------------------

                   (i)    the Shares; and

                   (ii)   any other shares of Common Stock of the Company issued
as (or issuable upon the conversion or exercise of any warrant, right or other
security which is issued as) a dividend or other distribution with respect to,
or in connection with a stock split, or in exchange for or in replacement of,
the Shares, excluding in all cases, however, any Registrable Securities sold by
a person in a transaction in which his or her rights under this Agreement are
not assigned; provided, however, that Common Stock or other securities shall
only be treated as Registrable Securities if and so long as they have not been
sold to or through a broker or dealer or underwriter in a public distribution or
a public securities transaction.
<PAGE>

                (c) The term "Holder" means the Stockholder, its successors and
                             ------
assigns.

                (d) The term "SEC" means the Securities and Exchange Commission.
                             ---

          1.2   Request for Registration.
                ------------------------

                (a) If one or more of the following events shall occur: (i) the
failure of the Company and the Stockholder to enter into on or before April 30,
2000 a definitive commercial agreement (the "Commercial Agreement") pursuant to
                                             --------------------
that certain letter of intent of even date herewith between the parties hereto,
in form and substance reasonably satisfactory to such parties, after negotiating
in good faith with the intention of completing such agreement, (ii) the
termination by Yahoo! of the Commercial Agreement following a breach of such
agreement by the Company, in accordance with the terms and conditions thereof,
or (iii) the effective date of a "Change in Control" of the Company (as defined
below), and following such occurrence, the Holder shall send to the Company a
written request that the Company file a registration statement under the
Securities Act covering the registration of any or all of the Registrable
Securities, then the Company shall use its best efforts to effect as
expeditiously as may be practicable the registration under the Securities Act of
all Registrable Securities which the Holder requests to be registered.
Notwithstanding the foregoing, the Company shall not be required to effect any
registration if the Registrable Securities that the Company shall have been
requested to register shall, in the aggregate, have a current market value of
less than (x) $5,000,000 if the Registrable Securities are to be registered on
Form S-1 (or any successor form thereto) or (y) $1,000,000 if the Registrable
Securities are to be registered on Form S-3 (or any successor form thereto).
The Holder shall have the right to only one (1) registration pursuant to this
Section 1.2(a).  For purposes of this Agreement, a "Change in Control" of the
                                                    -----------------
Company shall mean the sale, conveyance or other disposition by the Company of
all or substantially all of its property or business or the Company's merger
with or into or consolidation with any other corporation (other than a wholly-
owned subsidiary corporation) or the consummation of any other transaction or
series of related transactions in which more than 50% of the voting power of the
Company is disposed of, or the execution by the Company of any binding agreement
providing for any of the foregoing transactions: provided, however, that the
                                                 --------  --------
sale, disposition or conveyance of all or a portion of the Company's stock or
assets to, or the merger or consolidation of the Company with, AT&T Corp. or any
of its wholly owned subsidiaries, or the execution of a binding agreement
providing for the foregoing with AT&T Corp. or any of its wholly owned
subsidiaries, shall not constitute a Change in Control for purposes of this
Section 1.2.

                (b) If the Holder intends to distribute the Registrable
Securities covered by their request by means of an underwriting, it shall so
advise the Company as a part of its request made pursuant to this Section 1.2.
The underwriter will be selected by the Holder, subject to the Company's
approval, which shall not be unreasonably

                                      -2-
<PAGE>

withheld. Thereafter, the Holder and the Company shall enter into an
underwriting agreement in customary form with the underwriter or underwriters
selected for such underwriting.

                (c) Notwithstanding the foregoing, if the Company shall furnish
to the Holder a certificate signed by the President of the Company stating that
in the good faith judgment of the Board of Directors of the Company, it would be
materially detrimental to the Company and its stockholders for such registration
statement to be filed and it is therefore essential to defer the filing of such
registration statement, the Company shall have the right to defer such filing
for a reasonable period of time (but not more than 90 days) after receipt of the
request of the Holder (the "Demand Blackout Period").  The Company shall give
                            ----------------------
the Holder prompt written notice of any such determination to defer filing in
the form of a certificate signed by an executive officer of the Company.  The
Company shall promptly notify the Holder of the expiration or earlier
termination of the Demand Blackout Period.

          1.3   Company Registration.  If (but without any obligation to do so)
                --------------------
the Company proposes to register (including for this purpose a registration
effected by the Company for any Company stockholder other than the Holder) any
of its stock under the Securities Act in connection with the public offering of
such securities solely for cash (other than a registration relating solely to
the sale of securities to participants in a Company stock plan or a transaction
covered by Rule 145 under the Securities Act, a registration in which the only
stock being registered is Common Stock issuable upon conversion of debt
securities which are also being registered, or any registration on any form
which does not include substantially the same information as would be required
to be included in a registration statement covering the sale of the Registrable
Securities), the Company shall, at such time, promptly give the Holder written
notice of such registration.  Upon the written request of the Holder given
within twenty (20) days after mailing of such notice by the Company, the Company
shall, subject to the provisions of Section 1.5, cause to be registered under
the Securities Act all of the Registrable Securities that the Holder has
requested to be registered.  Nothing in this Section 1.3 shall create any
liability on the part of the Company or any other person to the Holder if the
Company, for any reason, decides not to file a registration statement proposed
to be filed pursuant to this Section 1.3 or to withdraw such registration
statement subsequent to its filing (except for the Company's obligation to pay
the expenses in connection therewith as provided in Section 1.11), regardless of
any action whatsoever that the Holder may have taken, whether as a result of the
issuance by the Company of any notice under this Section 1.3. or otherwise.

          1.4   Obligations of the Company.  Whenever required under this
                --------------------------
Section 1 to effect the registration of any Registrable Securities, the Company
shall, as expeditiously as reasonably possible:

                (a) Prepare and file with the SEC a registration statement with
respect to such Registrable Securities and use its best efforts to cause such
registration

                                      -3-
<PAGE>

statement to become effective, and, upon the request the Holder, keep such
registration statement effective for up to 90 days (20 days in the case of a
registration statement filed on Form S-1 under the Securities Act) or such
shorter period during which the Holders complete the distribution described in
the registration statement relating thereto, whichever first occurs.

                (b) Prepare and file with the SEC such amendments and
supplements to such registration statement and the prospectus used in connection
with such registration statement as may be necessary to comply with the
provisions of the Act with respect to the disposition of all securities covered
by such registration statement for up to 90 days;

                (c) Cause all such Registrable Securities registered pursuant
hereunder to be listed on each securities exchange on which similar securities
issued by the Company are then listed;

                (d) Furnish to the Holder participating in the registration such
reasonable number of copies of the registration statement, preliminary
prospectus, final prospectus and such other documents as the Holder may
reasonably request;

                (e) Use its best efforts to register and qualify the securities
covered by such registration statement under such securities or Blue Sky laws of
such jurisdictions as shall be reasonably requested by the Holder, provided that
the Company shall not be required in connection therewith or as a condition
thereto to qualify to do business or to file a general consent to service of
process in any such states or jurisdictions;

                (f) Notify the Holder at any time when a prospectus relating
thereto is required to be delivered under the Securities Act of the happening of
any event as a result of which the prospectus included in such registration
statement, as then in effect, includes an untrue statement of a material fact or
omits to state a material fact required to be stated therein or necessary to
make the statements therein not misleading in the light of the circumstances
then existing, and at the request of the Holder, prepare and furnish to the
Holder a reasonable number of copies of a supplement to or an amendment of such
prospectuses as may be necessary so that, as thereafter delivered to the
purchasers of such shares, such prospectuses shall not include an untrue
statement of a material fact or omit to state a material fact required to be
stated therein or necessary to make the statements therein no misleading in
light of the circumstances then existing;

                (g) Provide a transfer agent and registrar for all Registrable
Securities registered pursuant hereunder not later than the effective date of
such registration.

                (h) Use best efforts to cause the transfer agent to remove
restrictive legends on certificates representing the Registrable Securities
covered by a

                                      -4-
<PAGE>

registration statement hereunder, as the Company determines to be appropriate,
upon advice of counsel;

                (i) In the event of any underwritten public offering hereunder,
enter into and perform its obligations under an underwriting agreement, in usual
and customary form, with the managing underwriter of such offering. The Holder
shall also enter into and perform its obligations under such an agreement; and

                (j) Use its best efforts to furnish, at the request of the
Holder, on the date that the Registrable Securities are delivered to the
underwriters for sale in connection with a registration pursuant to this Section
1, if such securities are being sold through underwriters, or, if such
securities are not being sold through underwriters, on the date that the
registration statement with respect to such securities becomes effective, (i) an
opinion, dated such date, of the counsel representing the Company for the
purposes of such registration, in form and substance as is customarily given to
underwriters in an underwritten public offering, addressed to the underwriters,
if any, and to the Holder and (ii) a letter dated such date, from the
independent certified public accountants of the Company, in form and substance
as is customarily given by independent certified public accountants to
underwriters in an underwritten public offering, addressed to the underwriters,
if any, and to the Holder.

          1.5   Underwriting Requirements.  In connection with any offering
                -------------------------
involving an underwriting of shares of the Company's capital stock, the Company
shall not be required under Section 1.3 to include any of the Holder's
securities in such underwriting unless they accept the terms of the underwriting
as agreed upon between the Company and the underwriters selected by it (or by
other persons entitled to select the underwriters), and then only in such
quantity as the underwriters determine in their sole discretion will not
jeopardize the success of the offering by the Company.  If the total amount of
securities, including Registrable Securities, requested by stockholders to be
included in such offering exceeds the amount of securities sold other than by
the Company that the underwriters determine in their sole discretion is
compatible with the success of the offering, then the Company shall be required
to include in the offering only that number of such securities, including
Registrable Securities, which the underwriters determine in their sole
discretion will not jeopardize the success of the offering (the securities so
included to be apportioned pro rata among the selling stockholders according to
the total amount of securities entitled to be included therein owned by each
selling stockholder or in such other proportions as shall mutually be agreed to
by such selling stockholders).  The Holder shall not be entitled to participate
in an underwritten offering unless the Holder enters into, and performs its
obligations under, one or more underwriting agreements and any related
agreements and documents (including an escrow agreement and/or a power of
attorney with respect to the disposition of the Registrable Securities), in the
form that the Holder shall agree to with the lead managing underwriter of the
transaction.  If the Holder disapproves of the terms of any underwriting, it may
elect, prior to the execution of any underwriting agreement, to withdraw
therefrom by written notice to the Company and the lead managing underwriter.

                                      -5-
<PAGE>

          1.6  Delay of Registration.  The Holder shall not have any right to
               ---------------------
obtain or seek an injunction restraining or otherwise delaying any registration
as the result of any controversy that might arise with respect to the
interpretation or implementation of this Section 1.

          1.7  Furnish Information.  It shall be a condition precedent to the
               -------------------
obligations of the Company to take any action pursuant to this Section 1 with
respect to the Registrable Securities of the Holder that the Holder shall
furnish to the Company such information regarding itself, the Registrable
Securities held by it, and the intended method of disposition of such securities
as shall be required to effect the registration of the Holder's Registrable
Securities.

          1.8  Indemnification.  In the event any Registrable Securities are
               ---------------
included in a registration statement under this Section 1:

               (a) To the extent permitted by law, the Company will indemnify
and hold harmless the Holder and each person, if any, who controls the Holder
within the meaning of the Securities Act or the Securities Exchange Act of 1934,
as amended (the "Exchange Act"), against any losses, claims, damages, or
                 ------------
liabilities (joint or several) to which they may become subject, under the
Securities Act, the Exchange Act or other federal or state law, insofar as such
losses, claims, damages, or liabilities (or actions in respect thereof) arise
out of or are based upon any of the following statements, omissions or
violations (collectively a "Violation"): (i) any untrue statement or alleged
                            ---------
untrue statement of a material fact contained in such registration statement,
including any final prospectus contained therein or any amendments or
supplements thereto, (ii) the omission or alleged omission to state therein a
material fact required to be stated therein, or necessary to make the statements
therein not misleading, or (iii) any violation or alleged violation by the
Company of the Securities Act, the Exchange Act, any state securities law or any
rule or regulation promulgated under the Securities Act, the Exchange Act or any
state securities law; and the Company will pay, as incurred, any legal or other
expenses reasonably incurred by them in connection with investigating or
defending any such loss, claim, damage, liability, or action; provided, however,
that the indemnity agreement contained in this subsection 1.8(a) shall not apply
to amounts paid in settlement of any such loss, claim, damage, liability, or
action if such settlement is effected without the consent of the Company, which
consent shall not be unreasonably withheld, nor shall the Company be liable in
any such case for any such loss, claim, damage, liability, or action to the
extent that it arises out of or is based upon a Violation which occurs in
reliance upon and in conformity with written information furnished expressly for
use in connection with such registration by such Holder or controlling person.

               (b) To the extent permitted by law, the Holder will indemnify and
hold harmless the Company, each of its directors, each of its officers who has
signed the registration statement, each person, if any, who controls the Company
within the meaning of the Securities Act or the Exchange Act against any losses,
claims, damages,

                                      -6-
<PAGE>

or liabilities (joint or several) to which any of the foregoing persons may
become subject, under the Securities Act, the Exchange Act or other federal or
state law, insofar as such losses, claims, damages, or liabilities (or actions
in respect thereof) arise out of or are based upon any Violation, in each case
to the extent (and only to the extent) that such Violation occurs in reliance
upon and in conformity with written information furnished by such Holder
expressly for use in connection with such registration; and each such Holder
will pay, as incurred, any legal or other expenses reasonably incurred by any
person intended to be indemnified pursuant to this subsection 1.8(b), in
connection with investigating or defending any such loss, claim, damage,
liability, or action; provided, however, that the indemnity agreement contained
in this subsection 1.8(b) shall not apply to amounts paid in settlement of any
such loss, claim, damage, liability or action if such settlement is effected
without the consent of the Holder, which consent shall not be unreasonably
withheld; provided that in no event shall any indemnity under this subsection
1.8(b) exceed the net proceeds from the offering received by the Holder, unless
such liability arises out of or is based on the willful conduct of the Holder.

               (c) Promptly after receipt by an indemnified party under this
Section 1.8 of notice of the commencement of any action (including any
governmental action), such indemnified party will, if a claim in respect thereof
is to be made against any indemnifying party under this Section 1.8, deliver to
the indemnifying party a written notice of the commencement thereof and the
indemnifying party shall have the right to participate in, and, to the extent
the indemnifying party so desires, jointly with any other indemnifying party
similarly noticed, to assume the defense thereof with counsel mutually
satisfactory to the parties; provided, however, that an indemnified party shall
have the right to retain its own counsel, with the fees and expenses to be paid
by the indemnifying party, if representation of such indemnified party by the
counsel retained by the indemnifying party would be inappropriate due to actual
or potential differing interests between such indemnified party and any other
party represented by such counsel in such proceeding. The failure to deliver
written notice to the indemnifying party within a reasonable time of the
commencement of any such action, if prejudicial to its ability to defend such
action, shall relieve such indemnifying party of any liability to the
indemnified party under this Section 1.8, but the omission so to deliver written
notice to the indemnifying party will not relieve it of any liability that it
may have to any indemnified party otherwise than under this Section 1.8.

               (d) If the indemnification provided in this Section 1.8 is held
by a court of competent jurisdiction to be unavailable to an indemnified party
with respect to any loss, liability, claim, damage or expense referred to
therein, then the indemnifying party, in lieu of indemnifying such indemnified
party hereunder, shall contribute to the amount paid or payable by such
indemnified party as a result of such loss, liability, claim, damage or expense
in such proportion as is appropriate to reflect the relative fault of the
indemnifying party on the one hand and of the indemnified party on the other in
connection with the statements or omissions that resulted in such loss,
liability, claim, damage or expense as well as any other relevant equitable
considerations. The relative fault of the indemnifying party and of the
indemnified party shall be

                                      -7-
<PAGE>

determined by reference to, among other things, whether the untrue or alleged
untrue statement of a material fact or the omission to state a material fact
relates to information supplied by the indemnifying party or by the indemnified
party and the parties' relative intent, knowledge, access to information, and
opportunity to correct or prevent such statement or omission.

               (e) The obligations of the Company and the Holder under this
Section 1.8 shall survive the completion of any offering of Registrable
Securities in a registration statement under this Section 1, and otherwise.

          1.9  Reports Under Securities Exchange Act of 1934.  With a view to
               ---------------------------------------------
making available to the Holder the benefits of Rule 144 promulgated under the
Securities Act and any other rule or regulation of the SEC that may at any time
permit the Holder to sell securities of the Company to the public without
registration, the Company agrees to use its best efforts to:

               (a) make and keep public information available, as those terms
are understood and defined in SEC Rule 144, at all times;

               (b) file with the SEC in a timely manner all reports and other
documents required of the Company under the Securities Act and the Exchange Act;
and

               (c) furnish to the Holder, so long as the Holder owns any
Registrable Securities, forthwith upon request (i) a written statement by the
Company that it has complied with the reporting requirements of SEC Rule 144,
the Securities Act and the Exchange Act, (ii) a copy of the most recent annual
or quarterly report of the Company and such other reports and documents so filed
by the Company, and (iii) such other information as may be reasonably requested
in availing the Holder of any rule or regulation of the SEC which permits the
selling of any such securities without registration.

          1.10  Assignment of Registration Rights.  The rights to cause the
                ---------------------------------
Company to register Registrable Securities pursuant to this Section 1 may be
assigned by a Holder to a transferee or assignee of all of the Holder's
Registrable Securities, provided the Company is, within a reasonable time after
such transfer, furnished with written notice of the name and address of such
transferee or assignee and the securities with respect to which such
registration rights are being assigned; and provided, further, that such
assignment shall be effective only if immediately following such transfer the
further disposition of such securities by the transferee or assignee is
restricted under the Securities Act.

          1.11  Expenses of Registration.
                ------------------------

                (a) Demand Registration.  All expenses other than underwriting
                    -------------------
discounts and commissions incurred in connection with registrations, filings or
qualifications pursuant to Section 1.2, including (without limitation) all
registration,

                                      -8-
<PAGE>

filing and qualification fees, printers' and accounting fees, fees and
disbursements of counsel for the Company, and the reasonable fees and
disbursements of one counsel for the Holder shall be borne by the Company;
provided, however, that the Company shall not be required to pay for any
--------  -------
expenses of any registration proceeding begun pursuant to Section 1.2 if the
registration request is subsequently withdrawn at the request of the Holder,
unless, at the time of such withdrawal, the Holder has learned of a material
adverse change in the condition, business, or prospects of the Company from that
known to the Holder at the time of its request and has withdrawn the request
with reasonable promptness upon obtaining knowledge of such material adverse
change.

                (b) Company Registration.  All expenses other than underwriting
                    --------------------
discounts and commissions incurred in connection with registrations, filings or
qualifications of Registrable Securities pursuant to Section 1.3 for the Holder,
including (without limitation) all registration, filing, and qualification fees,
printers' and accounting fees, fees and disbursements of counsel for the Company
and the reasonable fees and disbursements of one counsel for the Holder selected
by it with the approval of the Company, which approval shall not be unreasonably
withheld, shall be borne by the Company.

          1.12  Termination of Registration Rights.  The rights granted under
                ----------------------------------
this Section 1 shall terminate upon the earlier of (a) two years following the
date of this Agreement, or (b)  at such time as the Holder may sell all of its
Registrable Securities in any single three month period pursuant to Rule 144 (or
such successor rule as may be adopted).

     2.  Miscellaneous.
         -------------

          2.1  Amendments and Waivers.  Any term of this Agreement may be
               ----------------------
amended or waived with the written consent of the Company and the Holders of a
majority of the Shares then held by all Holders.  Any amendment or waiver
effected in accordance with this Section 2.1 shall be binding upon the parties
and their respective successors and assigns.

          2.2  Successors and Assigns.  Subject to the provisions of Section
               ----------------------
1.10, the terms and conditions of this Agreement shall inure to the benefit of
and be binding upon the respective successors and assigns of the parties.
Nothing in this Agreement, express or implied, is intended to confer upon any
party other than the parties hereto or their respective successors and assigns
any rights, remedies, obligations, or liabilities under or by reason of this
Agreement, except as expressly provided in this Agreement.

          2.3  Governing Law.  This Agreement and all acts and transactions
               -------------
pursuant hereto and the rights and obligations of the parties hereto shall be
governed, construed and interpreted in accordance with the laws of the State of
New York, without giving effect to principles of conflicts of law.

                                      -9-
<PAGE>

          2.4  Counterparts.  This Agreement may be executed in two or more
               ------------
counterparts, each of which shall be deemed an original and all of which
together shall constitute one instrument.

          2.5  Titles and Subtitles.  The titles and subtitles used in this
               --------------------
Agreement are used for convenience only and are not to be considered in
construing or interpreting this Agreement.

          2.6  Notices. Any notice required or permitted by this Agreement shall
               -------
be in writing and shall be deemed sufficient upon receipt, when delivered
personally or by courier, overnight delivery service or confirmed facsimile, or
48 hours after being deposited in the regular mail as certified or registered
mail (airmail if sent internationally) with postage prepaid, if such notice is
addressed to the party to be notified at such party's address or facsimile
number as set forth below (or as subsequently modified by written notice):

     To the Holder:  Yahoo! Inc.
                     3420 Central Expressway
                     Santa Clara, CA 95051
                     Attn:  Senior Vice President, Corporate Development
                     Facsimile:  (408) 328-7939

                     With a copy at the same address to the attention of the
                     General Counsel, and a copy to:

                     Venture Law Group
                     A Professional Corporation
                     2800 Sand Hill Road
                     Menlo Park, California 94025
                     Attn.:  Steven J. Tonsfeldt
                             Keith A. Miller
                     Facsimile:  (415) 233-8386

    To the Company:  Net2Phone, Inc.
                     171 Main Street
                     Hackensack, New Jersey 07601
                     Attn:  Chief Executive Officer
                     Facsimile: (___) ____________

                     With a copy to:

                     Kirkland & Ellis
                     Aon Center
                     200 East Randolph Drive
                     Chicago, Illinois 60601

                                      -10-
<PAGE>

                     Attn:  Richard W. Porter
                     Facsimile:  (312) 861-2200

          2.7  Severability.  If one or more provisions of this Agreement are
               ------------
held to be unenforceable under applicable law, the parties agree to renegotiate
such provision in good faith, in order to maintain the economic position enjoyed
by each party as close as possible to that under the provision rendered
unenforceable.  In the event that the parties cannot reach a mutually agreeable
and enforceable replacement for such provision, then (i) such provision shall be
excluded from this Agreement, (ii) the balance of the Agreement shall be
interpreted as if such provision were so excluded and (iii) the balance of the
Agreement shall be enforceable in accordance with its terms.

          2.8  Entire Agreement.  This Agreement is the product of all of the
               ----------------
parties hereto, and constitutes the entire agreement between such parties
pertaining to the subject matter hereof, and merges all prior negotiations and
drafts of the parties with regard to the transactions contemplated herein.  Any
and all other written or oral agreements existing between the parties hereto
regarding such transactions are expressly canceled.

          2.9  Advice of Legal Counsel.  Each party acknowledges and represents
               -----------------------
that, in executing this Agreement, it has had the opportunity to seek advice as
to its legal rights from legal counsel and that the person signing on its behalf
has read and understood all of the terms and provisions of this Agreement.  This
Agreement shall not be construed against any party by reason of the drafting or
preparation thereof.

          2.10  Delays or Omissions.  No delay or omission to exercise any
                -------------------
right, power or remedy accruing to any party to this Agreement, upon any breach
or default of the other party, shall impair any such right, power or remedy of
such non-breaching party nor shall it be construed to be a waiver of any such
breach or default, or an acquiescence therein, or of or in any similar breach or
default thereafter occurring; nor shall any waiver of any single breach or
default be deemed a waiver of any other breach or default theretofore or
thereafter occurring.  Any waiver, permit, consent or approval of any kind or
character on the part of any party of any breach or default under this
Agreement, or any waiver on the part of any party of any provisions or
conditions of this Agreement, must be made in writing and shall be effective
only to the extent specifically set forth in such writing.  All remedies, either
under this Agreement, or by law or otherwise afforded to any Holder, shall be
cumulative and not alternative.

          2.11  Third Parties.  Nothing in this Agreement, express or implied,
                -------------
is intended to confer upon any party, other than the parties hereto, and their
respective successors and assigns, any rights, remedies, obligations or
liabilities under or by reason of this Agreement, except as expressly provided
herein.

          2.12  Parity of Rights.  The registration rights granted to the Holder
                ----------------
under this Agreement are not intended by the parties to be senior to any of the
registration

                                      -11-
<PAGE>

rights granted to holders of the Company's capital stock under that certain
Series A Preferred Shareholder Registration Rights Agreement dated as of May 13,
1999, but rather to rank on a pari passu basis with such rights. The parties
agree to interpret the terms of this Agreement in a manner consistent with the
foregoing intention.

                            [Signature Page Follows]

                                      -12-
<PAGE>

     The parties have executed this Agreement as of the date first above
written.

                                    NET2PHONE, INC.

                                    By: Howard Balter
                                       ----------------------------------
                                    Title: Chief Executive Officer
                                          -------------------------------
                                    Address:  171 Main Street
                                              Hackensack, NJ 07601

                                    YAHOO! INC.

                                    By: Jeff Mallett
                                       -----------------------------------
                                    Title: President and Chief Executive Officer
                                          --------------------------------
                                    Address:  3420 Central Expressway
                                              Santa Clara, CA 95051

                                      -13-

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