<PAGE>   1
                                                                 Exhibit 10.12.3


                     INCENTIVE STOCK OPTION GRANT AGREEMENT

                                    UNDER THE

             NET2000 COMMUNICATIONS, INC. 1999 STOCK INCENTIVE PLAN

        This Grant Agreement (the "Agreement") is entered into this 10th day of
February, 2000, by and between NET2000 COMMUNICATIONS, INC., a Delaware
corporation (the "Company"), and Donald E. Clarke (the "Employee"), effective as
of February 10, 2000 (the "Grant Date").

        In consideration of the premises, mutual covenants and agreements
herein, the Company and the Employee agree as follows:

        1. Grant of Option. The Company hereby grants to the Employee, pursuant
to the NET2000 COMMUNICATIONS, INC. 1999 Stock Incentive Plan (the "Plan"), an
incentive stock option to purchase from the Company, at a price of $12.00 per
share (the "Exercise Price"), 90,000 shares of Common Stock of the Company, $.01
par value per share ("Stock"), subject to the provisions of this Agreement and
the Plan (the "Option"). The Option shall expire at 5:00 p.m. Eastern Time on
the last business day preceding the tenth anniversary of the Grant Date (the
"Expiration Date"), unless fully exercised or terminated earlier.

        2. Terminology. Unless stated otherwise in this Agreement, capitalized
terms in this Agreement shall have the meaning set forth in the Plan. Except
where the context otherwise requires, the term "Company" shall include NET2000
COMMUNICATIONS, INC. and its Affiliates.

        3.     Exercise of Option.

               (a) Right to Exercise. Except as otherwise provided in this
Agreement, this Option may be exercised as to its vested portion at any time and
from time to time, in whole or in part, on or before the Expiration Date or
earlier termination of the Option. In the event of the Employee's death,
disability, or other termination of employment or service relationship, the
exercisability is governed by Section 4 below.

               (b) Vesting. The Option vests as follows: 25% of the shares shall
be vested on the first anniversary of the Grant Date, with the remainder of the
shares vesting 1/36 per month thereafter; provided, however, that the Employee
must be in the continuous employ of the Company from the Grant Date through the
applicable date upon which vesting is scheduled to occur. If, within 24 months
after a Change in Control, (i) the Employee's employment with the Company is
terminated by the Company without Cause; (ii) the Employee terminates his or her
employment with the Company due to the relocation of the Employee's principal
place of work at the time of the Change in Control to more than 30 miles away;
(iii) the Employee suffers a reduction of 5% or more of his or her cash
compensation; or (iv) the Employee suffers a material diminution of his or her
title, duties or responsibilities, then the Employee shall become 100% vested
upon the date of such termination, relocation, reduction or diminution.

        For the purposes of this Agreement, the term "Change in Control" shall
mean (i)the acquisition by any individual, entity or group within the meaning of
Section 13(d)(3) or 14(d)(2) of the Securities Exchange Act of 1934, as amended
(the "Exchange Act") (a "Person"), of beneficial ownership (within the meaning
of Rule 13d-3 promulgated under the Exchange Act) of 50% or more of either (A)
the then outstanding shares of common stock of Net2000 Communications, Inc. (the
"Outstanding Company Common Stock"); or (B) the combined voting power of the
then outstanding voting securities of the




                                       1
<PAGE>   2

Company entitled to vote generally in the election of directors (the
"outstanding Company Voting Securities); (ii) the closing of a sale or
conveyance of all or substantially all of the assets of the Company; or (iii)
the effective time of any merger, share exchange, consolidation or other
reorganization or business combination of the Company if immediately after such
transaction persons who hold a majority of the outstanding voting securities
entitled to vote generally in the election of directors of the surviving entity
are not persons who held voting capital stock of the Company immediately prior
to such transaction; provided, however, that for purposes of this Agreement, the
following acquisitions shall not constitute a Change in Control: (y) any
acquisition by the Company; or (z) any acquisition by any employee benefit plan
(or related trust) sponsored or maintained by the Company or any corporation
controlled by the Company.

               (c) Exercise Procedure. Subject to the conditions set forth in
this Agreement, this Option shall be exercised by delivery of written notice of
exercise on any business day to the Corporate Secretary of the Company, or any
individual designated by the Corporate Secretary, in such form as the Plan
Administrator may require from time to time. Such notice shall specify the
number of shares in respect of which the Option is being exercised and shall be
accompanied by full payment of the Exercise Price for such shares in accordance
with Section 3(d) of this Agreement. The exercise shall be effective upon
receipt by the Corporate Secretary of the Company, or any individual designated
by the Corporate Secretary, of such written notice accompanied by the required
payment or properly executed, irrevocable instructions to effectuate a
broker-assisted cashless exercise. The Option may be exercised only in multiples
of whole shares and may not be exercised at any one time as to fewer than one
hundred shares (or such lesser number of shares as to which the Option is then
exercisable). No fractional shares shall be issued pursuant to this Option.

               (d) Method of Payment. Payment of the Exercise Price shall be by
any of the following, or a combination thereof, as determined by the
Administrator in its discretion at the time of exercise:

       i.   by delivery of cash, certified or cashier's check, money order or
            other cash equivalent acceptable to the Administrator in its
            discretion;

       ii.  by tender (via actual delivery or attestation) to the Company of
            other shares of Stock of the Company through a registered broker
            which have a Fair Market Value on the date of tender equal to the
            Exercise Price, provided that such shares have been owned by the
            Employee for a period of at least six months or were purchased on
            the open market without assistance, direct or indirect, from the
            Company;

       iii. by a broker-assisted cashless exercise; or

       iv.  by any other method approved by the Plan Administrator.

               (e) Issuance of Shares upon Exercise. Upon due exercise of the
Option, in whole or in part, in accordance with the terms of this Agreement, the
Company shall issue to the Employee, the brokerage firm specified in the
Employee's delivery instructions pursuant to a broker-assisted cashless
exercise, or such other person exercising the Option, as the case may be, the
number of shares of Stock so paid for, in the form of fully paid and
nonassessable stock and shall deliver certificates therefor as soon as
practicable thereafter. The stock certificates for any shares of Stock issued
hereunder shall, unless such shares are registered or an exemption from
registration is available under applicable federal and state law, bear a legend
restricting transferability of such shares.



                                       2
<PAGE>   3

        4.     Termination of Employment or Service.

               (a) Exercise Period Following Cessation of Employment or Service
Relationship, In General. If the Employee ceases to be employed by, or in a
service relationship with, the Company for any reason other than death, total
and permanent disability (as defined in Section 4(b) below) or discharge for
"Cause" (as defined in Section 4(d) below), (i) this Option shall terminate
immediately upon such cessation to the extent it is unvested, and (ii) this
Option shall be exercisable during the 90-day period following such cessation to
the extent it is vested, but in no event after the Expiration Date. Unless
sooner terminated, this Option shall terminate in its entirety upon the
expiration of such 90-day period.

               (b) Disability of Employee. Notwithstanding the provisions of
Section 4(a) above, if the Employee ceases his or her employment or other
service relationship with the Company as a result of his or her total and
permanent disability, (i) this Option shall terminate immediately upon such
cessation to the extent it is unvested, and (ii) this Option shall be
exercisable during the 12-month period following such cessation to the extent it
is vested, but in no event after the Expiration Date. Unless sooner terminated,
this Option shall terminate in its entirety upon the expiration of such 12-month
period. For purposes of this Agreement, "total and permanent disability" shall
mean the inability to engage in any substantial gainful activity by reason of
any medically determinable physical or mental impairment which can be expected
to result in death or which has lasted or can be expected to last for a
continuous period of not less than twelve months. The Plan Administrator may
require such proof of total and permanent disability as the Plan Administrator
in its sole discretion deems appropriate and the Plan Administrator's good faith
determination as to whether the Employee is totally and permanently disabled
shall be final and binding on all parties concerned.

               (c) Death of Employee. If the Employee dies prior to the
Expiration Date or other termination of the Option, (i) this Option shall
terminate immediately upon the Employee's death to the extent it is unvested,
and (ii) this Option shall be exercisable during the 12-month period following
the date of death of the Employee to the extent it is vested, but in no event
after the Expiration Date, by the Employee's executor, personal representative,
or the person(s) to whom this Option is transferred by will or the laws of
descent and distribution. Unless sooner terminated, this Option shall terminate
in its entirety upon the expiration of such 12-month period.

               (d) Cause. Notwithstanding anything to the contrary herein, this
Option shall terminate in its entirety, regardless of whether the Option is
vested in whole or in part, immediately upon the Employee's discharge of
employment or other service relationship for Cause or upon the Employee's
commission of any act that would constitute Cause during any period following
the cessation of employment or other service relationship during which the
Option otherwise would be exercisable. For purposes of this Agreement, "Cause"
means (i) Employee's failure or refusal to perform any assigned duty; (ii)
misconduct or dishonesty by Employee in connection with the performance of his
or her duties, including but not limited to, the Employee's failure to observe
the policies of the Company; (iii) disloyalty, misappropriation of funds by
Employee, Employee's commission of a felony or commission of a misdemeanor
involving moral turpitude, or fraudulent or unethical conduct by Employee
related to or affecting Employee's employment; or (iv) any breach of any
provision of any employment, confidentiality, non-disclosure, non-competition,
non-solicitation or other similar agreement (as amended, from time to time),
executed by the Employee for the benefit of the Company, all as determined by
the Plan Administrator, which determination shall be conclusive and binding for
all purposes hereunder.

        5.     Non-Guarantee of Employment or Service Relationship. Nothing in
the Plan or this Agreement shall alter the employment status or service
relationship of the Employee, nor be construed as




                                       3
<PAGE>   4

a contract of employment or other service relationship between the Company and
the Employee, or as a contractual right of Employee to continue in the employ
of, or in a service relationship with, the Company, or as a limitation of any
rights of the Company regarding the Employee's employment.

        6. No Rights as a Stockholder. The Employee shall not have any of the
rights of a stockholder with respect to the shares of Stock that may be issued
upon the exercise of the Option until such shares of Stock have been issued to
him or her upon the due exercise of the Option. No adjustment shall be made for
dividends or distributions or other rights for which the record date is prior to
the date such certificate or certificates are issued.

        7. Adjustments for Changes in Capitalization Business Combinations, Etc.
The provisions of this Section 7 shall supersede the provisions of Section 7(d)
of the Plan with respect to this Option.

               (a) In the event of a stock dividend of, or stock split or
reverse stock split affecting, Common Stock, the number of shares covered by and
the exercise price and other terms of this Agreement, shall, without further
action of the Board, be adjusted equitably to reflect such event. The Plan
Administrator may make adjustments, in its discretion, to address the treatment
of fractional shares and fractional cents that arise as a result of the stock
dividend, stock split or reverse stock split.

               (b) In the event of any other changes affecting the Common Stock,
the Company or its capitalization, by reason of any spin-off, split-up, dividend
or recapitalization, or any merger, consolidation or share exchange, that does
not result in a Change in Control, the Plan Administrator, in its discretion and
without the consent of the Employee, may make any adjustments in this Agreement
it deems appropriate, including but not limited to changes to the number and
type of securities subject to this Agreement. Nothing in this Section 7(b) shall
permit any change in this Agreement in connection with any merger, consolidation
or share exchange, that results in a Change in Control.

               (c) Notwithstanding anything in the Plan to the contrary, the
Plan Administrator, in its sole discretion and without the consent of the
Employee, may make any modifications to this Option, including but not limited
to cancellation, forfeiture, surrender or other termination of the Option in
whole or in part regardless of the vested status of the Option, but only to the
extent necessary to facilitate any business combination that is authorized by
the Board to comply with requirements for treatment as a pooling of interests
transaction for accounting purposes under generally accepted accounting
principles.

               (d) The Administrator is authorized to make, in its discretion
and without the consent of the Employee, adjustments in the terms and conditions
of, and the criteria included in, the Option in recognition of unusual or
nonrecurring events affecting the Company, or the financial statements of the
Company or any Affiliate, or of changes in applicable laws, regulations, or
accounting principles, whenever the Administrator determines that such
adjustments are appropriate in order to prevent dilution or enlargement of the
benefits or potential benefits intended to be made available under the Plan.

        8.     Qualified Nature of the Option. This Option is intended to
qualify as an incentive stock option within the meaning of Code section 422
("Incentive Stock Option"), to the fullest extent permitted by the law, and
this Agreement shall be so construed.

        9.     Notice of Disqualifying Disposition. If the Employee makes a
disposition (as that term is defined in Code section 424(c)) of any shares of
Common Stock acquired pursuant to this Option within two years of the Grant
Date or within one year after the shares of Common Stock are transferred to the




                                       4
<PAGE>   5

Employee, the Employee shall notify the Administrator of such disposition in
writing within 30 days of the disposition.

        10.    Withholding of Taxes. At the time the Option is exercised, in
whole or in part, or at any time thereafter as requested by the Company, the
Employee hereby authorizes withholding from payroll or any other payment of any
kind due the Employee and otherwise agrees to make adequate provision for
federal, state, local or other taxes required by law to be withheld, if any,
which arise in connection with the Option (including upon a disqualifying
disposition within the meaning of Code section 421(b)). The Company may require
the Employee to make a cash payment to cover any withholding tax obligation as
a condition of exercise of the Option. If the Employee does not make such
payment when requested, the Company may refuse to issue any stock certificate
under the Plan until arrangements satisfactory to the Administrator for such
payment have been made.

        11.    Nontransferability of Option. This Option is nontransferable
otherwise than by will or the laws of descent and distribution and during the
lifetime of the Employee, the Option may be exercised only by the Employee or,
during the period the Employee is under a legal disability, by the Employee's
guardian or legal representative. Except as provided above, the Option may not
be assigned, transferred, pledged, hypothecated or disposed of in any way
(whether by operation of law or otherwise) and shall not be subject to
execution, attachment or similar process.

        12.    Notices. All notices and other communications made or given
pursuant to this Agreement shall be in writing and shall be sufficiently made
or given if hand delivered or mailed by certified mail or overnight commercial
courier, addressed to the Employee at the address contained in the records of
the Company, or addressed to the Administrator, care of the Company for the
attention of its Corporate Secretary at its principal office or, if the
receiving party consents in advance, transmitted and received via telecopy or
via such other electronic transmission mechanism as may be available to the
parties.

        13.    Entire Agreement. This Agreement contains the entire agreement
between the parties with respect to the stock option granted hereunder. Any
oral or written agreements, representations, warranties, written inducements,
or other communications made prior to the execution of this Agreement with
respect to the stock option granted hereunder shall be void and ineffective for
all purposes.

        14.    Amendment. This Agreement may be amended from time to time by
the Administrator in its discretion; provided, however, that this Agreement may
not be modified in a manner that would have a materially adverse effect on the
Option except as provided in Section 7 of this Agreement or in a written
document signed by each of the parties hereto.

        15.    Conformity with Plan. This Agreement is intended to conform in
all respects with, and is subject to all applicable provisions of, the Plan,
which is incorporated herein by reference. Inconsistencies between this
Agreement and the Plan shall be resolved in accordance with the terms of the
Plan, except as specifically provided in Section 7. In the event of any
ambiguity in this Agreement or any matters as to which this Agreement is
silent, the Plan shall govern. A copy of the Plan is provided to you with this
Agreement.

        16.    Governing Law. This Agreement shall be governed by and construed
in accordance with the laws of the Commonwealth of Virginia, other than the
conflict of laws principles thereof.



                                       5
<PAGE>   6

        17.    Headings. The headings in this Agreement are for reference
purposes only and shall not affect the meaning or interpretation of this
Agreement.

        18.    Execution. This Agreement is voidable by the Company if the
Employee does not execute and return the Agreement to the Company within 30
days of its execution by the Company.

        IN WITNESS WHEREOF, the Company has caused this Agreement to be executed
by its duly authorized officer as of the date first above written.

                                            NET2000 COMMUNICATIONS, INC.

                                            Signature:    /s/ Clayton Thomas
                                                      --------------------------

                                            Print Name:   Clayton Thomas
                                                       -------------------------

                                            Date:         2/10/00
                                                 -------------------------------

The undersigned hereby acknowledges that he/she has carefully read this
Agreement and the Plan and agrees to be bound by all of the provisions set forth
in such documents.

                                            EMPLOYEE

                                            Signature:   /s/ Donald E. Clarke
                                                      --------------------------
                                            Print Name:   Don Clarke
                                                       -------------------------
                                            Date:         2/10/00
                                                 -------------------------------
Enclosure:  Net2000 Communications, Inc. 1999 Stock Incentive Plan


                                       6