<PAGE>   1
                                                                   EXHIBIT 10.21

                                AMENDMENT NO. 1
                                       TO
                                LEASE AGREEMENT


         THIS AMENDMENT NO. 1 TO LEASE AGREEMENT (the "Amendment") is made as
of the 31st day of January, 1998 by and between BOSTON PROPERTIES LIMITED
PARTNERSHIP (successor-in-interest to "Landlord") and NETWORK SOLUTIONS, INC.
("Tenant").

                                   WITNESSETH

         WHEREAS, by Lease Agreement dated as of May 30, 1997 (the "Lease"),
Landlord leased to Tenant for an initial term of five (5) years and two (2)
months 31,247 square feet of rentable area located in the office/flex building
(the "Building") known as Sugarland I and located at 365 Herndon Parkway,
Herndon, Virginia, as such space is more particularly described in the Lease;
and

         WHEREAS,  Tenant desires to lease additional space in the Building
contiguous to the Premises on the terms and conditions set forth herein;  and

         WHEREAS, Landlord and Tenant wish to amend the Lease (i) to include in
the description of the premises covered thereby an additional 9,509 square feet
of rentable area in the Building (the "Additional Space"), as depicted on
Exhibit 1 attached hereto and made a part hereof and (ii) to modify the terms
and provisions of the Lease as hereinafter set forth.

         NOW, THEREFORE, in consideration of the mutual covenants and premises
contained herein and other good and valuable consideration the receipt and
sufficiency of which hereby is acknowledged, Landlord and Tenant hereby agree
to amend the Lease as follows:

         1.  Defined Terms.  Except as otherwise provided herein, all of the
capitalized terms used herein shall have the same meanings as provided in the
Lease.

         2.  The Premises.  Commencing on the Lease Commencement Date-II, as
defined in Paragraph 3(a) below, the definition of the "Premises" in the Lease
hereby is amended to include therein the Additional Space.  In addition,
commencing on the Lease Commencement Date-II, Exhibit 1 attached hereto
depicting the floor plan for the Additional Space, hereby is added to Exhibit A
to the Lease.  As a result, commencing on the Lease Commencement Date-II, the
aggregate number of square feet of rentable area comprising the Premises hereby
is increased to a total of 40,756 square feet.  Commencing on the Lease
Commencement Date-II, the Additional Space shall be subject to all of the terms
and conditions of the Lease, except as otherwise provided below.

         (b)     Landlord and Tenant hereby agree that (i) the rentable area of
the Building (including the space used for the 900 KW generator and
uninterrupted power supply equipment) is 53,136 square feet and (ii) the
rentable area of the entire Premises including the Additional Space is 40,756
square feet.
<PAGE>   2
         (c)     The provisions of Section 1.4 of the Lease shall not apply to
the Additional Space.

         (d)     In the event the Lease Commencement Date-II is delayed, then,
promptly after the Lease Commencement Date-II is ascertained, Landlord and
Tenant shall execute a written declaration in substantially the form attached
as Exhibit E to the Lease, setting forth the Lease Commencement Date-II and
such other information as is provided therein.

         3.  Term.  (a) The term of the Lease with respect to the Additional
Space shall commence on February 1. 1998 (the "Lease Commencement Date-II") and
shall be coterminous with the Lease Term (i.e. through July 31, 2002);
provided, however, that in the event delivery of the Additional Space is
delayed beyond February 1, 1998, then the Lease Commencement Date-II shall be
the day after the day Landlord notifies Tenant that the Additional Space is
available for Tenant to commence the work described in Paragraph 6 below.

         (b) In the event that the delivery of possession of the Additional
Space to Tenant is delayed beyond February 1, 1998, regardless of the reasons
or causes of such delay, the Lease with respect to the Additional Space shall
not be rendered void or voidable as a result of such delay and the term of the
Lease with respect to the Additional Space shall commence on the Lease
Commencement Date-II, as determined in accordance with Paragraph 3(a) above.
In no event shall Landlord have any liability whatsoever to Tenant on account
of any such delay.

         4.  Base Rent. (a)  Commencing on February 1, 1998 (the ?Rent
Commencement Date-II), Tenant shall pay to Landlord as annual base rent for the
Additional Space an amount equal to the product of (i) the dollar amount per
square foot of annual base rent being paid by Tenant on the Rent Commencement
Date-II with respect to the original Premises multiplied by (ii) the number of
square feet of rentable area in the Additional Space. The annual base rent for
the Additional Space shall be payable as provided in Sections 3.1 and 3.3 of
the Lease and shall be adjusted each Lease Year at the same times and in the
same manner as provided in Section 3.2 of the Lease with respect to adjustments
in the annual base rent payable for the original Premises.  If the Rent
Commencement Date-II falls on a date other than on the first day of a month,
rent from such date until the first day of the following month shall be
prorated on a per diem basis and such prorated rent shall be payable in advance
on the Rent Commencement Date-II.

         5.  Additional Rent.  (a)  Commencing on the Rent Commencement
Date-II, the provisions of Article IV of the Lease respecting the payment by
Tenant of its proportionate share of Expenses incurred by Landlord in the
operation of the Building, shall be applicable to this lease of the Additional
Space.  Accordingly, commencing on the Rent Commencement Date-II, Tenant's
proportionate share of Expenses shall be determined with reference to the
entire Premises, including the Additional Space.

         (b)     Commencing on the Rent Commencement Date-II, the second (2nd)
sentence of Section 4.3 of the Lease hereby is deleted in its entirety and the
following substituted therefore:





                                       2





<PAGE>   3
                 For the purposes hereof, Tenant's proportionate share of
         Expenses shall be Seventy-Six and Seven tenths percent (76.7%), which
         is the proportion that the number of square feet of rentable area in
         the Premises (40,756) bears to the total number of square feet of
         rentable area in the Building (53,136).

         6.  Condition of the Additional Space.  (a) Except as expressly
provided otherwise in this Amendment, the Additional Space shall be delivered
to and accepted by Tenant in its present "as-is" condition.  It is understood
and agreed that Landlord will not make, and is under no obligation to make, any
structural or other alterations, decorations, additions or improvements in or
to the Additional Space.  Notwithstanding the foregoing, on the Lease
Commencement Date-II, Landlord shall deliver the Base Building and the
mechanical systems serving the Additional Space to Tenant in substantially the
same condition in which Landlord was required to deliver the Base Building and
the equivalent systems serving the Premises in accordance with the Lease, in
particular Section 9.8 thereof,  except that (i) there shall be no ceiling
tiles in the Additional Space and Landlord shall not be obligated to replace or
install or pay for the replacement or installation of such ceiling tiles, (ii)
there shall be no fire alarm system serving the Additional Space and Landlord
shall not be required to purchase or to install a fire alarm system for the
Additional Space and (iii) there are no ducts or diffusers downstream of the
VAV boxes serving the Additional Space.  In addition to the Additional Space
Improvement Allowance described in subparagraph (b) below, Landlord shall
provide to Tenant an allowance in an amount to be agreed upon by Landlord and
Tenant which shall be applied solely to the costs and expenses incurred by
Tenant in connection with the installation of the ducts and diffusers required
to serve the Additional Space.

            (b)  It is understood and agreed that Tenant intends to make
certain alterations, renovations and improvements to the Additional Space
(collectively, the "Additional Space Improvements").  The Additional Space
Improvements (i) shall be done in accordance with Tenant's plans therefor which
shall be subject to the prior written approval of Landlord,  which shall not be
unreasonably withheld, conditioned or delayed provided such plans comply with
all applicable Legal Requirements, (ii) shall be subject to the provisions of
Article IX and Article XIII of the Lease and (iii) shall be made at Tenant's
sole cost and expense; provided, however, that Landlord agrees to provide
Tenant with an improvement allowance (the "Additional Space Improvement
Allowance") in an amount equal to the product of (x) Fifteen Dollars ($15.00)
multiplied by (y) the number of square feet of rentable area in the Additional
Space, multiplied by (z) a fraction the numerator of which is the number of
full calendar months in the period commencing on the Rent Commencement Date-II
and continuing through the expiration of the Lease Term, excluding any Renewal
Term, as defined in Rider No. 1 to the Lease and the denominator of which is
sixty (60) (the number of complete calendar months commencing on the Rent
Commencement Date and continuing through the expiration of the initial Lease
Term).  The Additional Space Improvement Allowance shall be applied, as
hereinafter set forth, to all "hard" and "soft" costs incurred in connection
with the design, construction and installation of the Additional Space
Improvements, including, without limitation, any and all architectural,
engineering and consulting fees and telephone and data cabling costs.  The
funding of the Additional Space Improvement Allowance is subject to the
fulfillment by Tenant of all covenants and conditions set forth in the Lease
and this Amendment.  In the event the entire Additional Space Improvement
Allowance is not utilized for the Additional Space Improvements  on or before
December 31, 1998, such unused portion of the Additional Space Improvement





                                       3





<PAGE>   4
Allowance shall be applied against the installment(s) of base rent due
commencing January 1, 1999 with respect to the Additional Space pursuant to
Paragraph 3 hereof.

            (c)  Tenant shall be responsible for the design and construction of
the Additional Space Improvements and Tenant shall obtain all necessary permits
to construct such Additional Space  Improvements and legally occupy the
Additional Space. The timing of Tenant's completion of the Additional Space
Improvements and the receipt or issuance of any required building, special
exception, occupancy or other permits or approvals shall have no bearing on the
Lease Commencement Date-II, the Rent Commencement Date-II or Tenant's
obligation to pay base rent and additional rent as provided in Paragraph 4 and
Paragraph 5 above.  Any and all construction, installation and other related
activity by Tenant or its contractors in the Additional Space prior to the Rent
Commencement Date-II shall be coordinated with Landlord in accordance with the
terms of Section 9.1 of the Lease.  All terms and conditions of the Lease,
including,  without limitation, the insurance, release and waiver of liability
provisions of Article XII and XIV thereof, shall apply to and be effective
during such period of occupancy by Tenant, except for Tenant's obligation to
pay any base rent or additional rent attributable to Expenses.

            (d)  Landlord and Tenant shall cooperate with each other and shall
work together in good faith to effect the timely completion of the Additional
Space Improvements.  Where required all approval, consents or disapprovals or
denials of consent shall be delivered promptly after a request therefor.  All
disapprovals or denials of consent shall include a statement of the reason for
such disapproval or denial of consent.  Landlord and Tenant shall each use its
commercially reasonable efforts to comply with all construction schedules
created in connection with the performance of the work described in this Lease.

            (e) Landlord and Tenant agree that Landlord's Work as defined in
Section 9.4 of the Lease has been Substantially Completed, as defined in
Section 9.4 of the Lease.

            7.  Parking.  Landlord shall not be obligated to provide Tenant
reserved parking spaces  in connection with its lease of the Additional Space.
Effective on the Lease Commencement Date-II, Tenant's pro rata share of parking
spaces in the parking lot immediately adjacent to the Building shall increase
to account for the rentable area of the Additional Space.

            8.  Incorporation of Lease Terms.  All of the terms, conditions,
covenants and agreements set forth in the Lease (a) are incorporated herein by
reference, (b) shall remain in full force and effect, (c) shall apply to this
Amendment and the lease of the Additional Space and (d) shall be applicable to
and binding upon the parties hereto at all times during the term of the Lease
as if set forth herein, except to the extent expressly modified herein.

            9.  Broker.    Landlord and Tenant recognize Barnes, Morris, Pardoe
& Foster and CB Commercial Real Estate Group, Inc.  as the brokers procuring
this Amendment and Landlord shall pay said brokers a commission pursuant to a
separate agreement between said brokers and Landlord.  Landlord and Tenant each
represents and warrants to the other that, except as provided in the preceding
sentence, neither of them has employed or dealt with any broker, agent or
finder in carrying on the negotiations relating to this Amendment.  Each party
shall indemnify and hold the





                                       4





<PAGE>   5
other harmless from and against any claim or claims for brokerage or other
commissions asserted by any broker, agent or finder engaged by the indemnifying
party or with whom the indemnifying party has dealt in connection with this
Amendment, other than the brokers named in the first sentence of this Paragraph
9.

            IN WITNESS WHEREOF, Landlord and Tenant have executed this
Amendment No. 1 to Lease Agreement as of the date and year first above written.

<TABLE>
<S>                                                   <C>
                                                      BOSTON PROPERTIES LIMITED PARTNERSHIP,
                                                      a Delaware limited partnership
WITNESS:
                                                      By:   Boston Properties, Inc., its general partner


/s/ LORI E. SUTHERLAND                                 /s/ RAYMOND A. RITCHEY
--------------------------------------                 -------------------------------------------------
[SEAL]                                                Raymond A. Ritchey, Senior Vice President

                                                      TENANT:

                                                      NETWORK SOLUTIONS, INC., a Delaware corporation

                                                      By: /s/ ROBERT J. KORZENIEWSKI
                                                          --------------------------------------------
                                                      Name: Robert J. Korzeniewski
                                                            ------------------------------------------
                                                      Title: Chief Financial Officer
                                                             -----------------------------------------

ATTEST:

/s/ MARK MANDOLIA
-------------------------------------
[CORPORATE SEAL]
</TABLE>





                                       5





<PAGE>   6
                                     [MAP]