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                                                                     Exhibit 4.6

                   EXCHANGE AND REGISTRATION RIGHTS AGREEMENT

         THIS AGREEMENT (the "Agreement") is made as of August 23, 1996 by and
among BBN CORPORATION, a Massachusetts corporation (the "Company"), BBN PLANET
CORPORATION, a Massachusetts corporation ("Planet"), and THE BOARD OF TRUSTEES
OF THE LELAND STANFORD JUNIOR UNIVERSITY, a body having corporate power under
the laws of the State of California ("Stanford" or the "Holder").

         WHEREAS, the Company owns 23,800,000 of the issued and outstanding
shares of Planet common stock, Stanford owns 200,000 of the issued and
outstanding shares of Planet common stock, Harvard University owns 10,000 of the
issued and outstanding shares of Planet common stock, Boston University owns
10,000 of the issued and outstanding shares of Planet common stock, and
Massachusetts Institute of Technology owns 10,000 of the issued and outstanding
shares of Planet common stock, together constituting all of the issued and
outstanding shares of Planet common stock;

         WHEREAS, AT&T Venture Company, L.P. owned 1,000,000 of the issued and
outstanding shares of Planet Series A Convertible Preferred Stock, $.01 par
value, constituting all of the issued and outstanding shares of Planet preferred
stock and together with the Planet common stock, constituting all of the capital
stock of Planet;

         WHEREAS, AT&T Venture Company L.P., the Company, and Planet have
recently entered into an exchange agreement, pursuant to which AT&T Venture
Company, L.P. transferred to the Company all of the shares of Planet Series A
Convertible Preferred Stock owned by it, in exchange for 400,000 shares of the
Company's Common Stock; and

         WHEREAS, the Company wishes to acquire, and Stanford wishes to
transfer, all of the issued and outstanding shares of Planet's common stock
owned by Stanford.

         NOW, THEREFORE, in consideration of the mutual covenants and agreements
herein contained, the parties to this Agreement agree as follows:

         1. Exchange of Holder's Planet Common Stock. Subject to the terms and
conditions hereof, the Company agrees to assign, transfer, and deliver to
Stanford 80,000 shares (the "Shares") of the Company's Common Stock, $1.00 par
value (the "Common Stock") in exchange for, and Stanford agrees to accept the
Shares in complete exchange for and to transfer to the Company 200,000 shares of
Planet's Common Stock, $1.00 par value (the "Planet Common Stock") and for the
cancellation of the rights of Stanford relating to such shares of Planet Common
Stock under the agreement canceled hereunder as provided in Section 6.2 of this
Agreement. This exchange of Planet Common Stock for the Shares shall
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occur at a closing (the "Closing") to be held at the offices of Ropes & Gray,
Boston, Massachusetts on August , 1996 (the "Closing Date") at 10:00 a.m., or on
such other date and at such other place as the parties may agree.

         At the Closing, Stanford will cause to be delivered to the Company
certificate(s) representing the Planet Common Stock, duly endorsed in blank or
accompanied by separate stock powers, in each case in proper form for transfer
and with signature guaranteed. At the Closing, the Company will deliver to
Stanford, in exchange, certificate(s) representing the Shares, duly registered
in the name of Stanford.

         2.  Registration Rights.

         (a) Demand Registration Rights. If following October 1, 1996 and prior
to the first date that all of the Shares may be sold immediately in accordance
with the provisions of Rule 144 promulgated under the Securities Act of 1933
(the "Demand Period"), Holder requests the Company to file a registration
statement under the Securities Act of 1933 (hereinafter, the "Securities Act")
for a public offering of Registrable Securities (as defined in Section 2(c)
hereof) equal to at least the lesser of (i) 50% of the Shares, or (ii) the
number of Registrable Securities the proposed aggregate offering price of which
is at least $3,000,000, the Company shall use its best efforts to register under
the Securities Act such Registrable Securities so requested. Notwithstanding the
foregoing, (i) the Company shall not be obligated to effect the filing of a
registration statement pursuant to this Section 2(a) during the 180 days
following the effective date of a registration statement pertaining to a public
offering of securities for the account of the Company, and (ii) if the Company
shall furnish to Holder requesting such registration 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 not be in the best interests of the
Company and its stockholders generally for such registration statement to be
filed, the Company shall have the right to defer such filing for a period of not
more than 120 days after receipt of the request of Holder; provided, however,
that the Company may not utilize the right set forth in this subsection (ii) for
more than one (1) such 120-day period with respect to each such request. The
Company is obligated to effect no more than two (2) registrations pursuant to
this Section 2(a), provided that no such registration which shall not have
become and remained effective for a period of at least 120 days (or such shorter
period during which Holder shall have sold all Registrable Securities which were
so registered) shall be deemed to be a registration for any purpose of this
Section 2(a).

         (b) Piggyback Registration Rights. Whenever the Company proposes to
register any Common Stock for its own or others' account under the Securities
Act for a public offering for cash, other than a registration relating to
employee benefit plans, the Company shall give Holder, if holding Registrable
Securities, prompt written notice of its intent to do so. Upon the written
request of Holder given within fifteen days of such notice, the Company will use
its best efforts to cause to be included in such registration all of the
Registrable Securities which Holder requests. If the Company is advised in
writing in good faith by any

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managing underwriter of the securities being offered pursuant to any
registration statement under this Section 2(b) that the number of shares to be
sold pursuant to such registration statement is greater than the number of such
shares which can be offered without adversely affecting the offering, the
Company shall first reduce pro rata the number of shares offered for the
accounts of holders of the Company's Common Stock that do not hold registration
rights similar to the rights granted to Holder pursuant to this Section 2(b),
and then may reduce pro rata the number of shares offered for the account of
Holder and for the accounts of holders of the Company's Common Stock that have
registration rights similar to the rights granted to Holder pursuant to this
Section 2(b) (based upon the number of shares of Common Stock proposed to be
sold pursuant to such registration statement by Holder and such other holders of
the Company's Common Stock) to a number deemed satisfactory by such managing
underwriter. In the event of such a limitation, shares of persons not having
registration rights will not be included in the registration unless all
Registrable Securities requested to be included in the registration have been
included. No piggyback registration right in this Section 2(b) shall be
construed to limit the demand registration right contained in Section 2(a)
hereof. The Company shall not after the date hereof grant piggyback registration
rights to any third party which provide such third party seniority over Holder
with respect to the inclusion of securities held by such third party over
Registrable Securities under this Section 2(b).

         (c) Definition of Registrable Securities. "Registrable Securities"
means (i) the Shares and (ii) shares of Common Stock or other securities
convertible into Common Stock received as a stock dividend or other distribution
in respect of the Shares; provided, however, that Registrable Securities shall
not include any such shares of Common Stock that as of the date of the
determination may be sold without limitation pursuant to Rule 144(k) under the
Securities Act.

         (d) Registration Procedures. All expenses incurred in connection with
the registrations under this Section 2 (including all registration, filing,
qualification, printer's, and accounting fees and the reasonable fees of one
counsel for the holders, but excluding underwriting commissions and discounts)
shall be borne by the Company. In connection with registrations under this
Section 2, the Company shall (i) use its best efforts to prepare and file with
the Commission as soon as reasonably practicable, a registration statement with
respect to the subject Registrable Securities and use its best efforts to cause
such registration to promptly become and remain effective for a period of at
least 120 days (or such shorter period during which Holder shall have sold all
Registrable Securities which were so registered); (ii) use its best efforts to
register and qualify the Registrable Securities covered by such registration
statement under applicable state securities laws as Holder shall reasonably
request for the distribution of the Registrable Securities; and (iii) take such
other actions as are reasonable and necessary to comply with the requirements of
the Securities Act and the regulations thereunder, or the reasonable request of
Holder, with respect to the registration and distribution of the Registrable
Securities. The Company is not obligated to effect registration or qualification
under this Section 2 in any jurisdiction requiring it to qualify to do business
(unless the Company is otherwise required to be so qualified) or to execute a
general consent

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to service of process.

         (e) Underwriting Arrangement. In connection with each registration
pursuant to Sections 2(a) and (b) above covering an underwritten public
offering, the Company and Holder agree to enter into a written agreement with
the managing underwriter in such form and containing such provisions as are
customary in the securities business for such an arrangement between such
underwriter and companies of the Company's size and investment stature;
provided, that Holder shall not be required to agree to any provision making
Holder jointly liable or jointly and severally liable with any other party
including the Company or make any representation about the Company.

         (f) Notification. The Company shall promptly notify Holder if
participating in a registration statement covering Registrable Securities of any
event which results in the prospectus included in such registration statement,
as then in effect, containing an untrue statement of a material fact or omitting
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. The Company shall use its best efforts to make appropriate changes to
such prospectus and provide copies of such revised prospectus to Holder.

         (g) Furnishing of Documents. At the request of Holder, if participating
in a registration statement, the Company will furnish to each underwriter, if
any, and Holder, a legal opinion of its counsel and a letter from its
independent certified public accountants, each in customary form and substance,
at such time or times as such documents are customarily provided in the type of
offering involved.

         (h) Prospectuses. From time to time the Company shall amend or
supplement any registration statement pursuant to which any of the Registrable
Securities are being offered pursuant to a registration under this Agreement and
the prospectus contained therein to the extent necessary to comply with the
Securities Act and any applicable state securities or Blue Sky laws. The Company
shall also provide Holder with as many copies of the prospectus contained in any
such registration statement in which Registrable Securities are included as it
may reasonably request.

         (i) Indemnification by the Company. The Company will indemnify and hold
harmless Holder and each underwriter of the Registrable Securities being sold by
Holder, and each controlling person of Holder and underwriter, against all
claims, losses, damages, and liabilities (or actions in respect thereof) arising
out of or based on any untrue statement (or alleged untrue statement) of a
material fact contained in any registration statement relating to such
Registrable Securities (or in any related registration statement, notification,
or the like) or any 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 any violation or alleged violation by the Company of any rule
or regulation promulgated under the Securities Act, the Securities Exchange Act
of 1934, or other federal or state law applicable to the Company and

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relating to action or inaction required of the Company in connection with any
such registration, qualification, or compliance, and will reimburse Holder and
each such underwriter and controlling person for any legal or any other expenses
reasonably incurred in connection with investigating or defending any such
claim, loss, damage, liability, or action and will enter into an indemnification
agreement with Holder and such underwriter containing customary provisions,
including provisions for contribution, as Holder or any underwriter shall
reasonably request; provided, however, that the Company will not be liable in
any such case to the extent that any such claim, loss, damage, or liability
arises out of or is based on any untrue statement or omission based upon written
information furnished to the Company by Holder or underwriter and stated to be
specifically for use therein.

         (j) Indemnification by Holder. Holder, if so participating in a
registration statement, will indemnify and hold harmless the Company, each of
its directors, each of its officers who has signed the registration statement
and each person, if any, who controls the Company within the meaning of Section 
15 of the Securities Act, against all claims, losses, damages and liabilities
(or actions in respect thereof) arising out of or based on any untrue statement
(or alleged untrue statement) of a material fact contained in any registration
statement relating to the Registrable Securities (or in any related registration
statement, notification or the like) or any omission (or alleged omission) to
state therein a material fact required to be stated therein or necessary to make
the statements therein not misleading, and will reimburse the Company and each
such director, officer or controlling person for any legal or any other expenses
reasonably incurred in connection with investigating or defending any such
claim, loss, damage, liability, or action and will enter into an indemnification
agreement with the Company containing customary provisions, including provisions
for contribution, as the Company or each such person shall reasonably request;
provided, however, that Holder will not be liable in any such case except to the
extent that any such claim, loss, damage, or liability arises out of or is based
on any untrue statement or omission based upon, and such indemnity shall be
expressly limited to cover only, written information furnished to the Company by
Holder and stated to be specifically for use therein. Notwithstanding any of the
foregoing to the contrary, in no event shall the liability of Holder for
indemnification under this Section 2(j) exceed the lesser of (i) that percentage
of the total amount of such losses, claims, damages or liabilities indemnified
against which equals the percentage obtained by dividing the total number of
Registrable Securities sold by Holder pursuant to the registration statement by
the total number of securities of the Company sold pursuant to the registration
statement, and (ii) the net proceeds received by Holder in the offering.

         (k)      Indemnification Procedures.

                  (i) Promptly after receipt by any person entitled to
indemnification under Sections 2(i) or 2(j) (and "Indemnified Party") of notice
of the commencement of any action in respect of which indemnity may be sought
against any person under Sections 2(i) or 2(j) (and "Indemnifying Party"), such
Indemnified Party shall notify all Indemnifying Parties in writing of the
commencement thereof (provided, however, that failure to so notify an
Indemnifying

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Party shall not relieve any Indemnifying Party from any liability it may have
hereunder except to the extent that the Indemnifying Party who did not receive
such notice shall have been materially prejudiced by such failure) and, subject
to the provisions hereinafter stated, the Indemnifying Party shall be entitled
to assume the defense of such action (including the employment of counsel, who
shall be counsel reasonably satisfactory to such Indemnified Party), and the
payment of expenses insofar as such action shall relate to any alleged liability
in respect of which indemnity may be sought against the Indemnifying Party.

                  (ii) The Indemnified Party shall have the right to employ
separate counsel and assume its own legal defense in any such action and to
participate in the defense thereof, but the fees and expenses of such counsel
subsequent to any assumption of the defense by the Indemnifying Party shall not
be at the expense of the Indemnifying Party unless the employment of such
counsel has been specifically authorized in writing by the Indemnifying Party.
The Indemnifying Party shall not be liable to indemnify any Indemnified Party
for any settlement of any such action effected without the Indemnifying Party's
written consent. The Company shall not, except with the approval of each party
being indemnified under this Agreement, consent to entry of any judgment or
enter into any settlement which does not include as an unconditional term
thereof the giving by the claimant or plaintiff to the parties being so
indemnified of a release from all liability in respect to such claim or
litigation.

         (l) Contribution. In order to provide for just and equitable
contribution to joint liability under the Securities Act in any case in which
any Indemnified Party exercising rights under this Agreement, or any controlling
person of any such Indemnified Party, makes a claim for indemnification pursuant
to Section 2(i) or 2(j) but it is judicially determined (by the entry of a final
judgment or decree by a court of competent jurisdiction and the expiration of
time to appeal or the denial of the last right of appeal) that such
indemnification may not be enforced in such case notwithstanding the fact that
this Agreement provides for indemnification in such case, then the Indemnifying
Party and such Indemnified Party will contribute to the aggregate losses,
claims, damages or liabilities to which they may be subject (after contribution
from others) 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 which resulted in such
losses, claims, damages or liabilities, as well as any other relevant equitable
considerations. The relative fault of the Indemnifying Party on the one hand and
of the Indemnified Party on the other shall be determined by reference to, among
other things, whether the untrue or alleged untrue statement of a material fact
or omission or alleged omission to state a material fact relates to information
supplied by the Indemnifying Party on the one hand or by the Indemnified Party
on the other, and each party's relative intent, knowledge, access to information
and opportunity to correct or prevent such statement or omission; provided,
however, that, in any such case, no person guilty of fraudulent
misrepresentation (within the meaning of Section 11(f) of the Securities Act)
will be entitled to contribution from any person who was not guilty of such
fraudulent misrepresentation.

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         (m) Exchange Act Registration. As long as the Company is subject to the
reporting requirements of either Section 13 or Section 15(d) of the Securities
Exchange Act of 1934 (the "Exchange Act"), the Company will use its reasonable
efforts to timely file with the Commission such information as the Commission
may require under either Section 13 or Section 15(d) of the Exchange Act; and in
such event, the Company shall use its reasonable efforts to take all action as
may be required as a condition of the availability of Rule 144 under the
Securities Act (or any successor exemptive rule hereafter in effect) with
respect to the Registrable Securities. The Company shall furnish to Holder
forthwith upon request (i) a written statement by the Company as to the
compliance with the reporting requirements of Rule 144, (ii) a copy of the most
recent annual or quarterly report of the Company as filed with the Commission,
and (iii) such other reports and documents as Holder may reasonably request in
availing itself of any rule or regulation of the Commission allowing Holder to
sell any such Registrable Securities pursuant to Rule 144 without registration.
The Company shall use its reasonable efforts to facilitate and expedite
transfers of the Registrable Securities pursuant to Rule 144 under the
Securities Act, which efforts shall include prompt notice to its transfer agent
to expedite such transfers of Registrable Securities.

         3. Purchase for Investment. Stanford acknowledges that the Shares will
be sold and exchanged without registration under the Securities Act on the basis
that the transactions contemplated by this Agreement do not involve any public
offering of securities. Stanford hereby confirms to the Company that it is
acquiring such securities for its own account, for investment only and without a
view toward the distribution thereof. Stanford hereby confirms that it is able
to bear the economic risk of an investment in the Shares acquired by it pursuant
to this Agreement and can afford to sustain a total loss on such investment and
has such knowledge and experience in financial and business matters that it is
capable of evaluating the merits and risks of the proposed investment and
therefore has the capacity to protect its own interests in connection with the
purchase of the Shares. Stanford acknowledges that it has been fully informed of
the limitations that are implicit in the fact that the Shares are not registered
under the Securities Act for sale to it, and may not be transferred absent
registration or an exemption therefrom, that the Company has no obligation to
register any of the Shares sold to Stanford other than as specifically provided
in this Agreement, and that the exemptions from the registration requirements of
the Securities Act are limited, and agrees that such securities shall be
legended to note such restrictions on their transfer in substantially the
following form:

                           "THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN
                  REGISTERED UNDER THE SECURITIES ACT OF 1933 AND MAY NOT BE
                  OFFERED OR SOLD ABSENT AN OPINION OF COUNSEL REASONABLY
                  SATISFACTORY TO THE ISSUER THEREOF THAT SUCH REGISTRATION HAS
                  BEEN EFFECTED OR AN EXEMPTION THEREFROM IS AVAILABLE. TRANSFER
                  MAY BE REFUSED IN THE ABSENCE OF SUCH OPINION."

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         Stanford acknowledges receipt of financial and other information
concerning the Company (including copies of its annual report to shareholders
for fiscal year 1995, its 10-K filing for the year ended June 30, 1995, its 10-Q
filings for the periods ended September 30, 1995, December 31, 1995, and March
31, 1996 and its proxy statement for the 1995 annual meeting), and that it has
had the opportunity to obtain additional information concerning the Company.
Stanford represents and agrees that it has not taken prior to the date of this
Agreement, and will not take after the date of this Agreement, any action that
would make unavailable the exemption provided in Section 4(2) of the Securities
Act with respect to the receipt by it of the Shares.

         Neither Stanford nor the Company has relied upon any agreement,
representation, or warranty of the other which is not contained in this
Agreement or in the schedules hereto (including the publicly filed documents of
the Company listed in the second paragraph of this Section 3), in connection
with the consummation of the transactions contemplated hereby.

         4. Representation and Warranties of Stanford. As of the Closing,
Stanford represents and warrants to the Company and Planet that:

         (a) Ownership. Stanford owns the Planet Common Stock, free and clear of
all liens, claims, charges, encumbrances, and restrictions (other than
restriction imposed by applicable securities laws), and the transfer of the
shares of Planet Common Stock to the Company will not constitute a breach or
violation of or default under any agreement or other instrument by which
Stanford may be bound.

         (b) Authority. Stanford has all requisite power and authority to enter
into this Agreement and perform its obligations hereunder, and this Agreement
constitutes a valid and binding obligation of Stanford enforceable against
Stanford in accordance with its terms subject to bankruptcy, insolvency,
reorganization, and similar laws of general application affecting the rights and
relief of creditors and secured parties and to the further extent that the
availability of the remedies of specific enforcement, of injunctive relief, or
of other equitable remedies is subject to the discretion of the court before
which any proceeding therefor may be brought.

         (c) Investor. Stanford is an organization described in Section 
501(c)(3) of the Internal Revenue Code with total assets (including endowment,
annuity, and life income funds) of at least $5 million according to its most
recent audited financial, and is domiciled in the state of California.

         5. Representations and Warranties of the Company. As of the Closing,
the Company represents and warrants to Stanford that:

         (a) Corporate Status. Each of the Company and Planet is a corporation
duly organized, validly existing, and in good standing under the laws of the
Commonwealth of

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Massachusetts and has full corporate power and authority to carry on its
business as now conducted and is registered or qualified to do business as a
foreign corporation in each jurisdiction in which the nature of the Company's
operations and the Company's ownership of property requires the Company to be so
qualified and where the failure so to qualify would have a material adverse
effect upon the business of the Company as currently conducted. It is intended
that following the Closing, Planet will be merged with and into the Company.

         (b) Capitalization of the Company. The authorized capital stock of the
Company currently consists of 100,000,000 shares of Common Stock, par value
$1.00 per share. At the close of business on July 31, 1996, there were
20,402,365 shares of Common Stock of the Company validly issued and outstanding,
and approximately 6,380,000 shares of Common Stock were subject to authorized
options, convertible securities, and other outstanding rights to acquire Common
Stock of the Company. Except for the Common Stock Purchase Rights of the Company
and except as set forth in the preceding sentence, at the close of business on
July 31, 1996 there were no commitments, agreements, understandings, or
arrangements of the Company of any kind relating to the issuance of capital
stock of the Company, any securities convertible or exchangeable into capital
stock of the Company, or any such rights, warrants, or options. The shares of
Common Stock of the Company being acquired by Stanford pursuant to this
Agreement will, when delivered to Stanford in accordance with this Agreement, be
duly authorized, validly issued, fully paid, and nonassessable, free and clear
of all liens, claims, charges, encumbrances, and restrictions (other than
restrictions imposed by applicable securities laws).

         (c) SEC Documents. The Company has furnished to Stanford a true and
complete copy of the Company's Form 10-K for the year ended June 30, 1995 and
Form 10-Q's for the periods ended September 30, 1995, December 31, 1995, and
March 31, 1996, and proxy statement for the 1995 annual meeting (the "Company's
SEC Documents"). As of their respective filing dates, the Company's SEC
Documents complied in all material respects with the requirements of the federal
securities laws, and none of the Company's SEC Documents 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 made therein, in
light of the circumstances under which they were made, not misleading.

         (d) Authority for Agreement. The Company has all necessary corporate
power and authority to execute and deliver this Agreement and to carry out its
obligations hereunder. The execution and delivery of this Agreement and the
consummation of the transactions contemplated hereby have been duly authorized
by the Board of Directors of the Company and no other corporate action
(including but not limited to stockholder approval) is legally required to be
taken or obtained by the Company to duly authorize the execution, delivery, and
performance of this Agreement and the transactions contemplated hereby. This
Agreement constitutes the valid and legally binding obligation of the Company
and is enforceable against the Company in accordance with its terms subject to
bankruptcy, insolvency, reorganization, and similar laws of general application
affecting the rights and relief of creditors and secured

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parties and to the further extent that the availability of the remedies of
specific enforcement, of injunctive relief, or of other equitable remedies is
subject to the discretion of the court before which any proceeding therefor may
be brought. The execution and delivery of this Agreement and the consummation of
the transactions contemplated hereby will not conflict with or result in any
material violation of or material defaults under any provision of the Articles
of Organization or By-laws of the Company or in any violation of or default with
respect to any mortgage, indenture, lease, agreement, or other instrument,
permit, concession, grant, franchise, license, judgment, order, decree, statute,
law, ordinance, rule, or regulation applicable to the Company. Such execution,
delivery, and consummation will not accelerate the maturity of or otherwise
modify the material terms of any indebtedness of the Company, or, result in the
creation of any lien, charge, encumbrance, or security interest upon any of the
property or assets of the Company.

         No consent, approval, order, or authorization of, or registration,
declaration, or filing with, any governmental authority and no consent or other
agreement of any person other than the Company is required in connection with
the execution and delivery of this Agreement or the consummation of the
transactions contemplated hereby by the Company. No consent or other agreement
of any person other than the Company is required for the sale of the Shares by
the Company or for the acceptance by the Company of the Planet Common Stock to
be transferred or assigned under this Agreement.

         (e) Brokers, Finders, etc. All negotiations relating to this Agreement
and the transactions contemplated hereby and thereby have been carried on
without the intervention of any person acting on behalf of the Company or in
such manner as to give rise to any valid claim against Stanford for any
brokerage or finder's commission, fee, or similar compensation on behalf of the
Company.

         (f) Litigation. To the Company's knowledge, there are no judicial or
administrative actions, suits, proceedings, or investigations pending, or
threatened, which question the validity of this Agreement or of any action taken
or to be taken pursuant to or in connection with the provisions of this
Agreement, or which could reasonably be expected to result in any material
adverse change in the business or financial condition of the Company and which
have not been publicly disclosed by the Company.

         (g) Financial Statements. The Company has delivered to Stanford the
consolidated balance sheets of the Company as of June 30, 1995 and 1994 and
related consolidated statements of operations and shareholders' equity and cash
flow for the three years ended June 30, 1995 as stated in a copy of its annual
report to shareholders for the fiscal year ended June 30, 1995 and reported upon
therein by Coopers & Lybrand, independent public accountants. The foregoing
financial statements, including the accompanying notes, have been prepared in
accordance in all material respects with the requirements of the SEC with
respect to preparation of such annual report and fairly present the financial
position of the Company as of the dates of the respective balance sheets and the
results of its operations for the respective

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periods then ended.

         6.  Miscellaneous.

         6.1. Notices. All notices, requests, consents, and demands shall be in
writing and shall be personally delivered, mailed, postage prepaid; or
telecopied, to the Company at:

                           BBN Corporation
                           150 CambridgePark Drive
                           Cambridge, Massachusetts 02140
                           Attn:  President

to Holder at:

                           Stanford Management Company
                           2770 Sand Hill Road
                           Menlo Park, California  94025
                           Attn:  Daniel Kingston

to Planet at:

                           150 CambridgePark Drive
                           Cambridge, MA 02140
                           Attn:  President

or such other address as may be furnished in writing to the other parties
hereto.

         All such notices, requests, demands, and other communication shall,
when mailed (registered or certified mail, return receipt requested, postage
prepaid) be effective four days after deposit in the mails, or when personally
delivered or when telecopied, shall be effective upon actual receipt.

         6.2. Entire Agreement. This Agreement constitutes the entire agreement
of the parties with respect to the matters contemplated herein. It supersedes
any and all prior understandings or agreements as to the subject matter of this
Agreement. Upon the effectiveness of this Agreement, the BBN Internet Services
Inc. Registration Rights Agreement dated as of August 19, 1994 between Planet
and Stanford is canceled and terminated in whole, without liability or claims to
any party thereto.

         6.3. Amendments, Waivers, and Consents. Any provision in this Agreement
to the contrary notwithstanding, changes in or additions to this Agreement may
be made, and compliance with any covenant or provision herein set forth may be
omitted or waived, upon the consent thereto in writing of the Company and
Holder.

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         6.4. Binding Effect; Assignment. This Agreement shall be binding upon
and inure to the benefit of the personal representatives, successors, and
permitted assigns, if any, of the respective parties hereto. Holder shall not
have the right to assign its rights or obligations hereunder or any interest
therein. The Company shall not have the right to assign its obligations
hereunder or any interest therein without obtaining the prior written consent of
Holder.

         6.5. General. The headings contained in this Agreement are for
reference purposes only and shall not in any way affect the meaning or
interpretation of this Agreement. In this Agreement the singular includes the
plural, the plural, the singular, the masculine gender includes the neuter,
masculine and feminine genders.

         6.6. Severability. If any provisions of this Agreement shall be found
by any court of competent jurisdiction to be invalid or unenforceable, the
parties hereby waive such provision to the extent that it is found to be invalid
or unenforceable. Such provision shall, to the maximum extent allowable by law,
be modified by such court so that it becomes enforceable, and, as modified,
shall be enforced as any other provision hereof, all the other provisions hereof
continuing in full force and effect.

         6.7. Counterparts. This Agreement may be executed in counterparts, all
of which together shall constitute one and the same instrument.

         6.8. Specific Performance. The Company recognizes that the rights of
Holder under this Agreement are unique, and, accordingly, Holder shall, in
addition to such other remedies as may be available to it at law or in equity,
have the right to enforce its rights hereunder by actions for injunctive relief
and specific performance to the extent permitted by law. This Agreement is not
intended to limit or abridge any rights of Holder which may exist apart from
this Agreement.

         The Company will cooperate with all reasonable requests by Holder for
such publicly- available information relating to the Company as may be called
for by an information- reporting form currently or hereafter required by the SEC
as a condition to the availability of an exemption from the Securities Act for
the sale of any Restricted Securities, including any publicly-available
information relating to the Company that may be required to be delivered by
Holder in connection with any sales to accredited investors (as defined in
Regulation D promulgated under the Securities Act or any successor regulation)
in a resale pursuant to the so-called "4(1 1/2) exemption" or to any qualified
institutional buyer in a resale pursuant to Rule 144A promulgated under the
Securities Act or any successor regulation. In this connection, the Company will
furnish to Holder, promptly upon request by Holder, copies of all financial
statements, reports, notices and proxy statements, sent or made available
generally by the Company to its stockholders, and copies of all regular and
periodic reports and all registration statements and prospectuses filed by the
Company with any securities exchange or with the SEC.

                                      -12-
   13
         IN WITNESS WHEREOF, the parties have caused this Exchange and
Registration Rights Agreement to be duly executed as of the date first above
written.

                                   BBN CORPORATION

                                   By  /s/ Paul R. Gudonis
                                       -------------------------------
                                       Title: Vice President

                                   BBN PLANET CORPORATION

                                   By  /s/ Paul R. Gudonis
                                      --------------------------------
                                       Title: President

                                   THE BOARD OF TRUSTEES OF THE LELAND
                                   STANFORD JUNIOR UNIVERSITY

                                   By  /s/ L. Hoagland, Jr.
                                      ---------------------------------
                                       Title: CEO, Stanford Management Co.


                                      -13-