<PAGE>

                                                                   EXHIBIT 10.25

                              AGREEMENT OF SUBLEASE

     Agreement of Sublease ("Sublease") dated this 24th day of December, 1998
between PAREXEL International Corporation, having a principal place of business
at 195 West Street, Waltham, Massachusetts 02154 ("Sublessor") and Network- I
Security Solutions, Inc., having a principal place of business at 70 Walnut
Street, Wellesley, Massachusetts 02181 ("Sublessee").

                                   WITNESSETH:

     WHEREAS, Sublessor has leased premises at Reservoir Place II, 1601 Trapelo
Road, Waltham, MA ("Premises") pursuant to a Lease Agreement dated March 25,
1994, as amended April 5, 1996; September 24, 1996; April 27, 1997; and November
11, 1997 between Sublessor, as tenant, and Arnold B. Tofias and Donald Tofias,
Trustees of Reservoir Place Realty Trust, as landlord and assigned to Boston
Properties Limited Partnership on November 3, 1998 ("Landlord"), a copy of which
lease and amendments (collectively "Lease") has been reviewed by Sublessee and
is annexed hereto as Exhibit A.

     WHEREAS, Sublessee desires to sublease a portion of the Premises from
Sublessor on the first floor of the Premises consisting of 9,001 rentable square
feet and more particularly shown on Exhibit B annexed hereto ("Subleased
Premises");

     NOW, THEREFORE, in consideration of the Premises and other good and
valuable consideration, it is hereby mutually covenanted and agreed as follows:

          1. The defined terms herein shall have the same meaning as the defined
     terms in the Lease unless otherwise indicated. In the event of any
     ambiguities between the provisions of the Lease and the provisions of this
     Sublease, the provisions of this Sublease shall govern.

          2. Sublessor hereby subleases the Subleased Premises to Sublessee and
     Sublessee hereby subleases the Subleased Premises from Sublessor upon and
     subject to the terms, covenants, rentals and conditions herein set forth
     for a term ("Term") commencing on the earlier to occur of (i) occupancy or
     (ii) February 1, 1999 and expiring on August 31, 2001, provided however, if
     the Lease shall expire prior to said date for any reason then this Sublease
     shall expire on the date that the Lease shall expire.

<PAGE>


          3. The rents reserved under this Sublease for the term hereof shall be
     and consist of Annual Rent calculated as hereinafter set forth below, which
     shall be payable in equal, monthly installments in advance on the first day
     of each and every calendar month during the Term of this Sublease ("Monthly
     Rent"). The Monthly Rent and any additional rent shall be paid to Sublessor
     at its office or such other place as Sublessor shall designate. In the
     event the Commencement Date or Expiration Date occur other than on the
     first and last days, respectively, of the calendar month, the Monthly Rent
     and any additional rent shall be pro-rated as to the applicable month.

     RENT/P.S.F.                     MONTHLY RENT               ANNUAL RENT
     -----------                     ------------               -----------
       $27.00                        $20,252.25                 $243,027.00

          4. Sublessee shall use and occupy the Subleased Premises for the
     Permitted Use and for no other purpose.

          5. Sublessee shall pay as additional rent its pro rata share of any
     operating expense increase above a Calendar year 1998 base. Sublessee shall
     pay as additional rent its pro rata share of any real estate tax increases
     above a Fiscal Year 1998 base ($3.29/r.s.f.). Sublessee shall also pay as
     additional rent its pro rata share of electricity for lights, outlets,
     variable air volume boxes, ventilation and air conditioning.

          6. The Subleased Premises shall be accepted in its "AS IS" condition.
     Sublessor shall provide Sublessee with an alterations allowance of
     $8.00/rentable square foot, which shall be paid to Sublessee upon
     substantial completion of the improvements and payment of Sublessee's first
     rent payment. Sublessee shall be wholly responsible for any and all
     demising work to segregate the premises. Sublessee may have access to the
     Premises to commence any Sublessee improvement work and/or install
     telecommunications equipment and furniture in the Premises upon execution
     of this Sublease.

          7. Upon the execution of this Sublease, Sublessee shall deliver cash
     in the amount of $81,009 to serve as security for the faithful performance
     and observance by Sublessee of the terms, provisions and conditions of this
     Sublease. It is agreed that in the event Sublessee defaults in respect of
     any of the terms, provisions and conditions of this Sublease including, but
     not limited to, the payment of the Annual and/or Monthly Rent and
     Additional Rent, Sublessor may, upon written notice to Sublessee and
     provided 

<PAGE>


     the monetary default is not cured within ten (10) days of receipt of the
     notice by Sublessee, (or any non-monetary default is not cured within
     thirty 30) days' ), may use, apply or retain the whole or any part of the
     cash security so deposited to the extent required for the payment of any
     such rent or any other sum as to which Sublessee is in default or for any
     sum which Sublessor may expend or may be required to expend by reason of
     Sublessee's default in respect of any of the terms, covenants and
     conditions of this Sublease including, but not limited to, any damages or
     deficiency in the re-letting of the Subleased Premises, whether such
     damages or deficiency accrued before or after summary dispossess
     proceedings or other re-entry by Sublessor. In the event that Sublessee
     shall fully and faithfully comply with all of the terms, provisions,
     covenants and conditions of this Sublease (subject to any applicable cure
     periods), the security shall be returned to Sublessee promptly following
     the date fixed as the expiration of the Sublease. Sublessee further
     covenants that it will not assign or encumber or attempt to assign or
     encumber the monies deposited herein as security and that neither Sublessor
     nor its successor or assigns shall be bound by any such assignment,
     encumbrance, attempted assignment or attempted encumbrance.

          8. Except as herein otherwise expressly provided, all of the terms of
     the Lease as they pertain to the Premises are hereby incorporated into and
     made a part of this Sublease as if stated at length herein, and Sublessee
     accepts this Sublease subject to, and hereby assumes all of the terms,
     covenants, conditions and agreements contained in the Lease with respect to
     the Premises, to be performed by Sublessor thereunder. The parties hereto
     agree that subject to the provisions of this Sublease, wherever the words
     "Premises" or words of similar import appear in the Lease, the same shall
     be deemed to mean the Subleased Premises and wherever the words "Landlord"
     and "Tenant" appear in the Lease, the words shall be deemed to refer to
     Sublessor and Sublessee respectively, so that, subject to the provisions of
     this Sublease and with respect only to the Subleased Premises, Sublessor
     shall have the rights and powers of Landlord under the Lease, and Sublessee
     shall have and does hereby agree to be bound by and accepts all of the
     rights, powers, duties and obligations of the Tenant under the Lease;
     provided, however, that notwithstanding the foregoing, but subject to
     Sublessor's obligation to provide information and cooperation to Sublessee
     to assist in enforcing Landlord's obligations under the Lease, Sublessor
     shall have no obligation to perform or furnish any of the work, services,
     repairs or maintenance undertaken to be made 

<PAGE>


     by Landlord under the Lease, or any other term, covenant or condition
     required to be performed by Landlord under the Lease. To the extent
     applicable to the Subleased Premises, Sublessee shall have the benefit of
     each and every covenant and agreement made by Landlord to Sublessor under
     the Lease. In the event that Landlord shall fail or refuse to comply with
     any of the respective provisions of the Lease, Sublessor shall have no
     liability on account of any such failure or refusal, provided that the
     Sublessee shall have the right to exercise in the name of the Sublessor all
     of the rights to enforce compliance on the part of Landlord as are
     available to the Sublessor with respect to the Premises. Sublessor hereby
     agrees to cooperate with and execute, all at Sublessee's expense, except to
     the extent that Landlord's non-compliance is due to the acts of Sublessor,
     all instruments and supply information reasonably required by Sublessee in
     order to enforce such compliance. Sublessee hereby agrees to indemnify,
     defend and hold Sublessor harmless from and against any and all damages,
     liabilities, obligations, costs, claims, losses, demands, expenses and
     injuries, including reasonable attorneys' fees and expenses incurred by
     Sublessor as a result of such cooperation and execution. In amplification
     and not in limitation of the foregoing and without any allowance to
     Sublessee or other reduction or adjustment of rent, Sublessor shall not be
     responsible for furnishing electrical, elevator, heating, air conditioning,
     cleaning, window washing, or other services, nor for any maintenance or
     repairs in or to the Subleased Premises or the building and/or property of
     which they form a part or any of the facilities or equipment therein.

          9. Sublessor shall promptly transmit to Landlord any notice or demands
     received from Sublessee and shall promptly transmit to Sublessee any notice
     or demands received from Landlord. Sublessee shall promptly transmit to
     Sublessor any notice or demands received from Landlord or any other party
     having an interest to which this Sublease is subordinate.

          10. Sublessor shall notify Sublessee in writing prior to a sublease
     proposal being sent to any third party for the adjacent 10,001 rentable
     square feet as set forth on Exhibit B attached hereto.

          11. Sublessee agrees that in any case where the provisions of this
     Sublease require the consent or approval of Sublessor prior to the taking
     of any action, it shall be a condition precedent to the taking of such
     action that the prior consent or approval of Landlord shall have been
     obtained if Landlord's consent must be obtained under the 

<PAGE>


     Lease in such cases. In the event Landlord's consent is so obtained,
     Sublessor agrees that its consent shall not be unreasonably withheld,
     delayed or conditioned. Sublessee agrees that Sublessor shall not have any
     duty or responsibility with respect to obtaining the consent or approval of
     Landlord when the same is required under the terms of the Lease, other than
     the transmission by Sublessor to Landlord of Sublessee's request for such
     consent or approval.

          12. Neither Sublessee nor Sublessor shall take any action or fail to
     take any action in connection with the Subleased Premises which would
     result in a violation or default under the Lease. To the maximum extent
     this Agreement may be made effective according to law, Sublessee agrees to
     indemnify and save harmless Sublessor from and against all claims of
     whatever nature arising from any act, omission or negligence of Sublessee,
     or Sublessee's contractors, licensee, invitees, agents, servants or
     employees, or arising from any accident, injury or damage whatsoever caused
     to any person, or to the property of any person, excluding acts, omissions,
     negligence, accident, injury or damage caused solely by Sublessor occurring
     at the Subleased Premises after the commencement of the Lease Terrn and
     until the end of the Lease Term and thereafter, so long as Sublessee is in
     occupancy of any part of the Subleased Premises. This indemnity and hold
     harmless agreement shall include all indemnity against all costs, expenses
     and liabilities incurred in or in connection with any such claim or
     proceeding brought thereof This indemnity and hold harmless agreement shall
     survive the expiration or earlier termination of the Lease Term. If
     Sublessee shall default in the performance of any of its obligations under
     this Sublease or under the Lease, Sublessor, without being under any
     obligation to do so and without thereby waiving such default, may remedy
     such default for the account and at the expense of Sublessee upon prior
     written notice. If Sublessor makes any expenditures or incurs any
     obligation for the payment of money in connection therewith, such sums paid
     or obligations incurred shall be deemed to be Additional Rent hereunder and
     shall be paid to it by Sublessee on demand, the default in which Sublessor
     shall have the same remedies as for a default in payment of fixed rent.
     Notwithstanding the foregoing, any consideration paid to Sublessee by its
     subtenant or assignee in excess of the amounts due Sublessor in this
     Sublease, shall be paid to Sublessor by Sublessee promptly upon receipt of
     same by Sublessee.

<PAGE>


          13. The parties hereto represent and warrant to each other and to
     Landlord that they have not dealt with any broker in connection with this
     Sublease other than Meredith & Grew Incorporated and Avalon Partners
     ("Brokers"). Sublessee agrees to indemnify, defend and hold Sublessor and
     Landlord harmless from and against any claim or liability for a commission
     by any broker other than Broker (including 'reasonable attorneys' fees
     incurred by the other party or Landlord in connection with any such claim).
     Sublessor shall be responsible for payment of Broker's commission and
     hereby indemnifies, defends and holds Landlord harmless for the commission
     due Broker in any event.

          14. Sublessee shall not assign, mortgage or encumber this Sublease,
     nor underlet, nor suffer or permit the Subleased Premises or any part
     thereof to be used by others except with the prior written consent of
     Sublessor, which consent shall not be unreasonably withheld, and subject to
     provisions of the Lease. In no event shall any assignment or subletting
     relieve Sublessee of its liability and responsibility to Sublessor under
     this Sublease in the absence of any express written agreement by Sublessor
     or Landlord, as the case may be, to that effect.

          15. In the event of any taking by eminent domain or damage by fire or
     other casualty to the Subleased Premises thereby rendering the Subleased
     Premises wholly or in part untenantable, Sublessee shall acquiesce in and
     be bound by any action taken by or agreement entered into between Landlord
     and Sublessor with respect thereto.

          16. All notices or demands under this Sublease shall be in writing and
     shall be sent by registered or certified mail, return receipt requested, or
     hand delivered, to Sublessor at its address set forth above, Attention:
     Jeanette Indorato, with copies to William T. Sobo and to Sublessee at the
     Subleased Premises, Attention: Murray Fish, Chief Financial Officer,
     Network 1 Security Solutions, with a copy to: Solovay Edlin & Eiseman P.C.,
     845 Third Avenue New York, New York 10022, Attention: Sam Schwartz, Esquire
     or such other address or person as either of the parties may designate by
     written notice.

<PAGE>


          17. Sublessee acknowledges that it has inspected the Subleased
     Premises and the improvements therein, or has caused an inspection thereof
     to be made, and is fully familiar and satisfied therewith and no
     representations have been made or are made or responsibility assumed IV by
     Sublessor as to the condition, value or suitability of the Subleased
     Premises or any improvements thereon. Sublessor shall not be required to do
     any work to make the Subleased Premises ready for occupancy by Sublessee.

          18. Sublessee covenants at all times during the term of this Sublease
     not to make any alterations or additions to the Subleased Premises of any
     nature whatsoever without the written consent of Sublessor, which consent
     shall not be unreasonably withheld or delayed. Any proposed alteration or
     addition remains subject to the provisions of the Lease and Landlord's
     consent. Sublessor approves of the alterations to the Subleased Premises as
     shown on Exhibit C which is attached hereto, incorporated into and made a
     part of this Agreement. The execution of this Sublease Agreement is also
     subject to receiving Landlord's consent to the proposed alterations and
     additions set forth in Exhibit C, in a consent agreement executed between
     Landlord, Sublessor and Sublessee. Sublessee shall be wholly responsible
     for the construction of and any and all improvements to the Subleased
     Premises.

          19. Upon the expiration or other termination of the Term of this
     Sublease, Sublessee covenants to quit and surrender to Sublessor the
     Subleased Premises, broom clean, in good order and condition, ordinary wear
     and tear and damage by fire or other casualty excepted, and at Sublessee's
     expense, to remove all property of Sublessee. Any property not so removed
     shall be deemed to have been abandoned by Sublessee and may be retained or
     disposed of at Sublessee's expense by Sublessor, as Sublessor shall desire.
     Sublessee's obligation to observe or perforrii this covenant shall survive
     the expiration or termination of the Term of this Sublease. Immediately
     upon the failure of Sublessee to perform any covenant of this Paragraph,
     Sublessor may, without notice, do so and shall be entitled to receive from
     Sublessee as damages the then cost of performance of such covenant, such
     damages to be paid in addition to and separate and independently from
     damage accruing by reason of any other covenant of this Sublease.

<PAGE>


          20. A separate sublease consent agreement between Landlord, Sublessor
     and Sublessee shall be executed in conjunction with the execution of this
     Sublease.

          21. Sublessor shall be entitled to 32 unreserved parking spaces, free
     of charge, of which 18 unreserved parking spaces will be located in the
     garage below the building and 14 located on outdoor surface lots.


     IN WITNESS WHEREOF, THE parties hereto have duly executed this instrument
the day and year first above written.


PAREXEL International Corporation             Network-1 Security Solutions, Inc.



By:/s/ William T. Sobo                        By:/s/ Murray P. Fish
   ------------------------------                -------------------------------
Name: William T. Sobo                         Name: Murray P. Fish
Title: CFO                                    Title: CFO

<PAGE>

                                                                   EXHIBIT 10.25

                               CONSENT TO SUBLEASE



     CONSENT TO SUBLEASE dated as of the 14th day of January, 1999 by and
between Boston Properties Limited Partnership, successor-in-interest to Arnold
B. Tofias and Donald Tofias, Trustees of Reservoir Place Realty Trust, and not
individually ("Landlord"), Parexel International Corporation, a Massachusetts
corporation ("Tenant"), and Network - 1 Security Solutions, Inc., a Delaware
corporation ("Subtenant").



                                    RECITALS


     By Lease dated March 25, 1994, as amended (the "Lease") Landlord did lease
to Tenant and Tenant did lease from Landlord certain premises in the building
(the "Building II") known as and numbered Reservoir Place II, 1601 Trapelo Road,
Waltham, Massachusetts, consisting of a portion of the first (1st) and second
(2nd) floors of the Building containing 62,591 square feet of rentable floor
area and certain premises in the building ("Building I") known as and numbered
Reservoir Place I, 1601 Trapelo Road, Waltham, Massachusetts, consisting of a
portion of the first (1st) floor of Building I containing 9,100 square feet of
rentable floor area, which premises are more particularly described in the Lease
(the "Premises").

     Tenant desires to sublease to Subtenant 9,001 square feet of rentable floor
area of the Premises (the "Subleased Premises") located in Building II upon the
terms and conditions contained in a sublease between Tenant and Subtenant dated
December 24, 1998 (the "Sublease"), a copy of which is attached hereto as
Exhibit A.

     Article V of the Lease provides that the Premises may not be sublet without
the written consent of Landlord.

     Landlord desires to consent to the Sublease upon the terms and conditions
contained in this Consent to Sublease (the "Consent").

<PAGE>


     NOW, THEREFORE, in consideration of One Dollar ($1.00) and other good and
valuable consideration, paid by each of the parties hereto to the other, the
receipt and sufficiency of which is hereby acknowledged, and in further
consideration of the provisions herein, Landlord, Tenant and Subtenant hereby
agree as follows:

1.   Landlord hereby consents to the Sublease subject to the terms and
     conditions of this Consent.

2.   Subtenant shall be directly bound and fully liable to Landlord for those
     terms, covenants, conditions and agreements contained in the Lease with
     respect to the Subleased Premises which are assumed by Subtenant pursuant
     to the terms of the Sublease, including, without limitation, the
     obligations to pay Annual Rent, additional rent and all other charges in
     the full amount, in the manner and at the time provided for under the
     Sublease.

3.   Tenant shall remain fully, primarily and directly liable to Landlord for
     all of the obligations of Tenant under the Lease relating to both the
     Premises and the Subleased Premises, including, without limitation, the
     obligations to pay Annual Fixed Rent, additional rent and all other charges
     in the full amount, in the manner and at the time provided for under the
     Lease notwithstanding (i) Landlord's consent to the Sublease, (ii) the
     terms and provisions of this Consent, (iii) the execution and delivery of
     the Sublease or (iv) Landlord's collecting or accepting from Subtenant any
     payments whether under the Lease or pursuant to or required by the Sublease
     and whether before or after any default of Tenant under the Lease. Further,
     the provisions of this Consent and the execution and delivery of the
     Sublease shall not constitute a recognition of the Sublease or the
     Subtenant thereunder, it being agreed that in the event of termination or
     expiration of the Lease, at Landlord's option, the Sublease shall be
     terminated and Subtenant shall have no further rights with respect to the
     Subleased Premises.

4.   Tenant and Subtenant shall be jointly and severally liable in respect of
     Subtenant's obligations to Landlord under the Lease as provided in Section
     2 of this Consent.

5.   All of the terms and conditions of Article V of the Lease shall apply to
     the Sublease and Tenant and Subtenant agree to be bound by such terms and
     conditions.

<PAGE>


6.   Landlord's consent to the Sublease shall not constitute a consent to any
     future subletting of the Premises or assignment of the Lease which shall be
     subject to the terms of Article V of the Lease.

7.   This Consent to Sublease consenting to the Sublease does not constitute
     approval by Landlord of any of the provisions of the Sublease document or
     agreement thereto or therewith; nor shall the same be construed to amend
     the Lease in any respect, any purported modifications being solely for the
     purpose of setting forth the rights and obligations as between Tenant and
     Subtenant, but not binding Landlord.

8.   Tenant represents and warrants to Landlord that Tenant has dealt with no
     broker in connection with the Sublease OTHER THAN Avalon Partners and
     Meredith & Grew Incorporated (collectively the '"Brokers"). In the event
     any claim is made against Landlord relative to dealings by Tenant with any
     broker in connection with the Sublease, Tenant shall defend the claim
     against Landlord with counsel of Tenant's selection and save harmless and
     indemnify Landlord on account of loss, cost or damage which may arise by
     reason of such claim. TENANT AGREES THAT IT SHALL BE SOLELY RESPONSIBLE FOR
     THE PAYMENT OF BROKERAGE COMMISSIONS TO THE BROKERS.

9.   Subtenant represents and warrants that Subtenant has dealt with no broker
     in connection with the Sublease OTHER THAN the Brokers. In the event any
     claim is made against Landlord relative to dealings by Subtenant with any
     broker in connection with the Sublease, Subtenant shall defend the claim
     against Landlord with counsel of Subtenant's selection first approved by
     Landlord and save harmless and indemnify Landlord on account of loss, cost
     or damage which may arise by reason of such claim.

10.  Reference is made to alterations to be made by Subtenant in the Subleased
     Premises pursuant to Paragraph 18 of the Sublease ("Subtenant's Work").
     Landlord hereby consents to the scope of the Subtenant's Work as shown on
     Exhibit C to the Sublease, provided however that neither Subtenant nor
     Tenant shall have the right to commence the Subtenant's Work unless and
     until Landlord has received and approved stamped architectural and
     engineering drawings and specifications from Subtenant for Subtenant's
     Work.

<PAGE>


11.  Tenant and Subtenant shall not amend in any respect the Sublease without
     the prior written approval of Landlord. In no event shall any such
     amendment affect or modify or be deemed to affect or modify the Lease in
     any respect. The terms of this Consent shall control in the event of a
     conflict between the terms of this Consent and the terms of the Sublease.

12.  All capitalized terms and words used in this instrument shall have the same
     meaning as set forth in the Lease unless a contrary meaning is expressly
     set forth herein.

13.  Except as expressly provided herein, the Lease and its terms and provisions
     shall remain unchanged and in full force and effect.



<PAGE>


EXECUTED under seal as of the date and year first above written.

WITNESS:                                   LANDLORD:
                                           BOSTON PROPERTIES LIMITED 
PARTNERSHIP

                                           By:      Boston Properties, Inc., 
its General Partner

      By:
         -----------------------------------
      Name:
           ---------------------------------
      Title:
            --------------------------------

ATTEST:                                    TENANT:

By:                                        PAREXEL INTERNATIONAL CORPORATION
    -----------------------------------
Name:                                      By:
     ----------------------------------       ----------------------------------
                                                        (Name)  
                                              (Title)
Title:  SECRETARY (OR ASSISTANT SECRETARY)     Hereunto Duly Authorized



ATTEST:                                    SUBTENANT

                                           NETWORK-1 SECURITY SOLUTIONS, INC.

By:                                        By:
     ----------------------------------       ----------------------------------
                                                        (Name)
                                               (Title)
Name:                                           Hereunto Duly Authorized
     ----------------------------------  
Title:  SECRETARY (OR ASSISTANT SECRETARY)