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                                                                  Exhibit 10.3.4

                                                                  EXECUTION COPY

                              EMPLOYMENT AGREEMENT

         This EMPLOYMENT AGREEMENT (this "Agreement") is dated as of June 16,
2000 and effective as of the consummation of the merger among Concentric Network
Corporation ("Concentric"), NEXTLINK Communications, Inc. ("Nextlink") and NM
Acquisition Corp. ("Employer") pursuant to the Agreement and Plan of Merger and
Share Exchange Agreement, dated as of January 9, 2000 and as amended and
restated as of May 10, 2000, by and among Concentric, Employer, Eagle River
Investments, L.L.C., Craig O. McCaw and NM Acquisition Corp. (the "Effective
Date"), between Henry R. Nothhaft ("Executive") and Employer.

         In consideration of the premises and the mutual covenants hereinafter
set forth and other good and valuable consideration, the receipt and sufficiency
of which is hereby acknowledged, the parties hereto hereby agree as follows:

1.       EMPLOYMENT OF EXECUTIVE

         Employer hereby agrees to employ Executive, and Executive hereby agrees
to be and remain in the employ of Employer, upon the terms and conditions
hereinafter set forth.

2.       EMPLOYMENT PERIOD

         Subject to earlier termination as provided in section 5, the term of
Executive's employment under this Agreement shall commence as of the Effective
Date and shall continue for a period of two (2) years (the "Initial Employment
Period"). Unless either party gives written notice of an intent not to extend
the Initial Employment Period, or any extension thereof, at least one year prior
to the expiration of the Initial Employment Period, or any extension thereof,
the Initial Employment Period, and any extension thereof, shall be automatically
extended for an additional one-year period such that until a notice of intent
not to extend is given by either party, the unexpired term of the Agreement is
always at least one full year (the Initial Employment Period and any extension
thereof is hereafter referred to as the "Employment Period").

3.       DUTIES AND RESPONSIBILITIES

         During the Employment Period, Executive shall devote all of his
business time and expend his best efforts, energies and skills to the Employer;
provided, however, that the Executive shall be allowed to serve on (i) the Board
of Directors of Asia Online, Ltd., Vertical Networks Incorporated and
WaveSplitter
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Technologies, Inc., (ii) the Telecommunications Advisory Board of Compaq
Computer Corp. and (iii) the Technical Advisory Board of SonicWALL, Inc. and on
such other Boards of Directors and Advisory Boards as are approved by the Chief
Executive Officer of the Employer (the "CEO"), or his designee, so long as such
service does not materially interfere with Executive's duties and
responsibilities hereunder. In addition, Executive shall be permitted to engage
in certain investments as a venture capitalist or angel investor as approved by
the CEO, or his designee, so long as such investment does not materially
interfere with Executive's duties and responsibilities hereunder. For purposes
of obtaining the approval of the CEO, or his designee, with respect to service
on various Boards of Directors and Advisory Boards and investment activities as
set forth above, permission will be deemed to be given by the CEO on the tenth
day following written notice by Executive to the CEO, unless prior to such date
the CEO provides written notice to the Executive of his disapproval. The notice
required in this Section 3 may be sent via electronic mail, in which case such
notice will be deemed given when actually received in the designated party's
electronic inbox. Executive shall perform such duties as he may be assigned from
time to time by the Board of Directors of the Employer (the "Board"), the Chief
Executive Officer of the Employer or any officer of the Employer senior in rank
to Executive.

4.       COMPENSATION AND RELATED MATTERS

         4.1 Base Salary. For each twelve-month period during the Employment
Period, commencing with the twelve-month period beginning on the Effective Date,
Employer shall pay to Executive an annual base salary equal to $412,000, subject
to increase at the discretion of the Board (the initial base salary, including
any Board approved increase thereof, the "Base Salary"). The Base Salary shall
be payable in accordance with the Employer's normal payroll practices.

         4.2 Annual Bonus. For each fiscal year during the Employment Period, at
the sole discretion of the Employer, Executive may receive a bonus (the "Bonus")
based upon attainment of annual performance objectives to be established in the
sole discretion of the Employer for the Bonus year.

         4.3 Executive Retention Bonus Plan. During Executive's employment with
the Employer, Executive shall be entitled to participate in the Employer's
Executive Retention Bonus Plan (the "Executive Retention Bonus Plan"). Subject
to the terms and conditions of the Executive Retention Bonus Plan, Executive
shall be entitled to earn a Cash Bonus Target (as defined in the Executive
Retention Bonus Plan) and a Restricted Stock Award (as defined in the Executive
Retention Bonus Plan) in the amounts


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specified in the Executive Retention Bonus Plan. In the event that Executive
remains on the Board following his termination of employment with the Employer,
Executive's status as a non-employee director shall not cause him to be deemed
an employee for purposes of the Executive Retention Bonus Plan and such status
shall not in and of itself cause the vesting of his Cash Bonus Target or
Restricted Stock to continue beyond his termination of employment with the
Employer.

         4.4 Other Benefits. During the Employment Period, subject to, and to
the extent Executive is eligible under their respective terms, Executive shall
be entitled to receive such fringe benefits as are, or are from time to time
hereafter generally provided by Employer to Employer's senior management
employees or other employees (other than those provided under or pursuant to
separately negotiated individual employment agreements or arrangements) under
any pension or retirement plan, disability plan or insurance, group life
insurance, medical and dental insurance, accidental death and dismemberment
insurance, travel accident insurance or other similar plan or program of
Employer.

         4.5 Expense Reimbursement. During the Employment Period, Employer shall
reimburse Executive for all business expenses reasonably incurred by him in the
performance of his duties under this Agreement upon his presentation of signed,
itemized accounts of such expenditures, all in accordance with Employer's
procedures and policies as adopted and in effect from time to time and
applicable to its senior management employees.

         4.6 Vacations. Executive shall be entitled to vacation days and time
off for each calendar year during the Employment Period commensurate with
Employer's policy for similarly situated employees, which vacation days and time
off shall be taken at such time or times as shall not unreasonably interfere
with Executive's performance of his duties under this Agreement.

         4.7 Option Award. In order to provide further incentive to Executive
and align the interests of Executive with those of the stockholders of Employer
and in consideration of Executive's covenants under Sections 7, 8 and 9 herein,
Employer shall grant to Executive options, pursuant to the Employer's Stock
Option Plan (the "Option Plan"), to purchase 800,000 shares of Class A common
stock of the Employer with an exercise price equal to the closing price of the
Common Stock on the NASDAQ Stock Market as of the Effective Date (the
"Options"). Subject to Executive's continued employment with the Employer or
service as a member of the Board, the Options shall vest and become exercisable
over a period of four years from the Effective Date at the rate of 25% on the
first anniversary of the Effective Date and thereafter at the rate of 1/48th of
the shares underlying the Options each month commencing


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on the 16th of July 2001 and on the 16th of each month thereafter until all of
the Options have vested and become exercisable. The Options shall have such
other terms and conditions as set forth in an option agreement to be entered
into between the Employer and Executive. (the "Option Agreement").

5.       TERMINATION OF EMPLOYMENT PERIOD

         5.1 Termination Without Cause; Voluntary Termination by Executive.
Employer may, by notice to Executive at any time during the Employment Period,
terminate the Employment Period without Cause (as defined below). The effective
date of such termination of the Executive from the Employer shall be the date on
which such notice is given. Executive may at any time during the Employment
Period voluntarily resign from the Employer and terminate the Employment Period.

         5.2 By Employer for Cause. Employer may, at any time during the
Employment Period, by notice to Executive, terminate the Employment Period for
"Cause." As used herein, "Cause" shall mean (i) any act of dishonesty on the
part of the Executive in connection with Executive's responsibilities as an
employee and intended to result in substantial personal enrichment of the
Executive, (ii) the Executive's commission of a felony or an act of fraud
against the Employer or its affiliates, (iii) a willful act by the Executive
that constitutes gross misconduct and that is injurious to the Employer or (iv)
continued violations by the Executive of the Executive's duties hereunder, which
are demonstrably willful and deliberate.

         5.3 By Executive for Good Reason. Executive may, at any time during the
Employment Period by notice to Employer, terminate the Employment Period under
this Agreement for "Good Reason" (as defined below) effective immediately. For
the purposes hereof, "Good Reason" means any of the following without
Executive's consent: (i) the assignment to the Executive of any duties or the
significant reduction of the Employee's duties, either of which is inconsistent
with the Executives position with the Employer and responsibilities in effect
immediately prior to such assignment; provided, however, that a reduction in
duties, position or responsibilities solely by virtue of the Employer being
acquired and made part of a larger entity (as, for example, when the Chairman,
CEO & President of the Employer remains as such following a change of control
but is not made the Chairman, CEO & President of the acquiring corporation)
shall not constitute Good Reason; (ii) a substantial reduction, without good
business reasons, of the facilities and perquisites available to the Executive
immediately prior to such reduction; (iii) a reduction by the Employer in the
Base Salary and target bonus as in effect immediately prior to such reduction;
(iv) a


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material reduction by the Employer in the kind or level of employee benefits to
which the Executive is entitled immediately prior to such reduction, with the
result that the Executive's overall benefits package is significantly reduced;
(v) the relocation of the Executive to a facility or a location more than 35
miles from the Executive's then present location; (vi) any purported termination
of the Executive by the Employer that is not effected for Cause (other than on
account of Disability, as defined below), or any purported termination for which
the grounds relied upon are not valid; or (vii) the failure of the Employer to
obtain the assumption of this Agreement by any successors to the Employer.
Notwithstanding anything herein to the contrary, any change or diminishment in
Executive's duties or responsibilities which occurs solely as a result of the
Merger and the fact that Executive is employed by the combination of Concentric,
Nextlink and the Employer rather than solely by Concentric shall not constitute
Good Reason; provided, that Executive's duties and responsibilities are
substantially equivalent to those of a senior ranking executive of the Employer.

         5.4 Disability. During the Employment Period, if, as a result of
physical or mental incapacity or infirmity, Executive shall be unable to perform
his duties under this Agreement for (i) a continuous period of at least 90 days,
or (ii) periods aggregating at least 120 days during any period of 12
consecutive months (each a "Disability Period"), and at the end of the
Disability Period there is no reasonable probability that Executive can promptly
resume his duties hereunder, Executive shall be deemed disabled (the
"Disability") and Employer, by notice to Executive, shall have the right to
terminate the Employment Period for Disability at, as of or after the end of the
Disability Period. The existence of the Disability shall be determined by a
reputable, licensed physician selected by Employer in good faith, whose
determination shall be final and binding on the parties. Executive shall
cooperate in all reasonable respects to enable an examination to be made by such
physician. Notwithstanding the foregoing, Employer may conclusively determine
Executive to be disabled and terminate the Employment Period on account of
Disability at any time after Executive has commenced receiving benefits under
the Employer's long-term disability insurance policy.

         5.5 Death. The Employment Period shall end on the date of Executive's
death.

6.       TERMINATION COMPENSATION

         6.1 Termination Without Cause by Employer or for Good Reason by
Executive. If the Employment Period is terminated by


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Employer without Cause pursuant to the provisions of Section 5.1 hereof or by
Executive for Good Reason pursuant to the provisions of Section 5.3 hereof,
Employer will pay to Executive within five (5) days following the date of
termination Executive's Base Salary through the date of termination. In
addition, if the Employment Period is terminated by Employer without Cause
pursuant to the provisions of Section 5.1 hereof or by Executive for Good Reason
pursuant to the provisions of Section 5.3 hereof, (i) the Employer shall
continue to pay Executive's Base Salary for the remainder of the Employment
Period (the "Severance Period") in accordance with the Employer's normal payroll
practices, (ii) for purposes of the vesting of all unvested shares of Restricted
Stock previously granted to Executive pursuant to the Executive Retention Bonus
Plan and payment of any unpaid amounts of Executive's Cash Bonus Target pursuant
to the Executive Retention Bonus Plan, such shares shall continue to vest and
become transferable and no longer subject to forfeiture and the payment of any
unpaid portion of such Cash Bonus Target shall be paid at the times set forth in
the Executive Retention Bonus Plan, but only to the degree that the Performance
Goals in the Executive Retention Bonus Plan are met, as though Executive were
still employed with the Employer on the date or dates of such vesting and (iii)
all unvested portions of the Options granted to Executive in connection with
this Agreement and any unvested stock options held by executive with respect to
the common stock of Concentric that were assumed by the Employer on the
Effective Date shall immediately become vested and exercisable. In addition, if
the Employment Period is terminated by Employer without Cause pursuant to the
provisions of Section 5.1 hereof or by Executive for Good Reason pursuant to the
provisions of Section 5.3 hereof, Employer shall, for the two-year period
following such termination, make available to Executive the same health and
insurance benefits made available to Executive immediately prior to such
termination at the same cost to Executive as such benefits are made available to
other employees of Employer (who were similarly situated with Executive
immediately prior to his termination) during the course of the two-year period.
In the event that the same benefits cannot be made available to Executive
because he is no longer an employee of the Employer, the Employer shall provide
economically equivalent benefits for such period. Employer shall have no
obligation to continue any other benefits provided for in Section 4 past the
date of termination.

         6.2 Certain Other Terminations. If the Employment Period is terminated
on account of a voluntary resignation by Executive pursuant to Section 5.1 or by
Employer pursuant to the provisions of Sections 5.2 or 5.4, or by death,
pursuant to the provisions of Section 5.5, Employer shall pay to Executive,
within five (5) days of the date of termination, Executive's Base Salary through
the date of termination. Employer shall have no obligation to


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continue any other benefits provided for in Section 4 past the date of
termination. Notwithstanding the foregoing, if Executive voluntarily resigns
after December 31, 2000, the Employer shall nominate Executive to serve as a
member of the Board for the two-year period following Executive's resignation.
In such event, the outstanding unvested Options granted pursuant to Section 4.7
herein and all outstanding unvested options held by Executive with respect to
the common stock of Concentric that were assumed by the Employer on the
Effective Date shall continue to vest according to the vesting schedules with
respect to such Options and options as in effect immediately prior to such
voluntary resignation during the period Executive serves as a member of the
Board. In the event that Executive voluntarily resigns after December 31, 2000
and following an Acceleration Change in Control (as defined in the Plan) the
Employer or any successor to the Employer fails to nominate Executive to serve
as a member of the Board or the board of directors of the successor for the
two-year period following Executive's resignation, the number of outstanding
unvested Options granted pursuant to Section 4.7 herein and outstanding unvested
options held by Executive with respect to the common stock of Concentric that
were assumed by the Employer on the Effective Date that would have become vested
during such two-year period of service on the Board shall become fully vested
and exercisable.

         6.3 No Other Termination Compensation. Executive shall not, except as
set forth in this Section 6, be entitled to any compensation following
termination of the Employment Period.

7.       CONFIDENTIALITY

         Unless otherwise required by law or judicial process, Executive shall
retain in confidence during the Employment Period and after termination of
Executive's employment with Employer pursuant to this Agreement all confidential
information known to the Executive concerning Employer and its businesses.
Confidential information does not include any information which has become
publicly known and made generally available through no wrongful acts of
Executive or of others who were under confidentiality obligations as to any
particular information. The obligations of Executive pursuant to this Section 7
shall survive the termination of Executive's employment and the expiration or
termination of this Agreement.

8.       NONCOMPETITION.

         For the duration of the Severance Period, if Executive's employment
with the Employer is terminated by the Employer without Cause pursuant to the
provisions of Section 5.1 hereof or by Executive for Good Reason pursuant to the
provisions of Section


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5.3 hereof, or for the longer of (i) two-years following Executive's termination
of employment hereunder for any other reason or (ii) Executive's tenure as a
member of the Board (in either event, such period referred to herein as the
"Non-Compete Period"), the Executive shall not directly or indirectly, engage in
any business, (including becoming an employee, director, officer, consultant,
independent contractor, lecturer or advisor of or to, or otherwise providing
services to, any business, individual, partnership, firm, association or
corporation (each an "Entity")) which is competitive with the business of the
Employer at the time. Nothing herein, however, shall prohibit Executive (i) from
acquiring or holding any issue of stock or securities of any Entity which has
any securities listed on a national securities exchange or quoted in the daily
listing of over-the-counter market securities, provided that at any one time
Executive and members of Executive's immediate family do not own more than five
percent of the voting securities of any such Entity and (ii) from remaining as a
member of any board of directors on which he may be serving on the date of any
such termination. Furthermore, nothing herein shall prohibit Executive from
engaging in certain investments as a venture capitalist or angel investor
provided such investments are first approved by the CEO, or his designee. For
purposes of obtaining the approval of the CEO, or his designee, with respect to
certain investment activities as a venture capitalist or angel investor, as set
forth above, permission will be deemed to be given by the CEO on the tenth day
following written notice by Executive to the CEO, unless prior to such date the
CEO provides written notice to the Executive of his disapproval. The notice
required in this Section 8 may be sent via electronic mail, in which case such
notice will be deemed given when actually received in the designated party's
electronic inbox. The obligations of Executive pursuant to this Section 8 shall
survive the termination of Executive's employment and the expiration or
termination of this Agreement.

9.       NONSOLICITATION.

         During the Non-Compete Period, Executive shall not directly or
indirectly solicit to enter into the employ of any other Entity, or hire, any of
the employees of the Employer (or individuals who were employees of the Employer
within one (1) year of such solicitation date). During the Non-Compete Period,
Executive shall not, directly or indirectly, solicit, hire or take away or
attempt to solicit, hire or take away (i) any customer or client of the Employer
or (ii) any former customer or client (that is, any customer or client who
ceased to do business with the Employer during the one (1) year period
immediately preceding such date) of the Employer or encourage any customer or
client of the Employer to terminate its relationship with the


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Employer without the Employer's prior written consent. The obligations of
Executive pursuant to this Section 9 shall survive the termination of
Executive's employment and the expiration or termination of this Agreement.

10.      SUCCESSORS; BINDING AGREEMENT

         This Agreement and all rights of the Executive hereunder shall inure to
the benefit of and be enforceable by Executive and Executive's personal or legal
representatives, executors, administrators, successors, heirs, distributees,
devisees and legatees. If Executive should die while any amounts would still be
payable to him hereunder if he had continued to live, all such amounts, unless
otherwise provided herein, shall be paid in accordance with the terms of this
Agreement to Executive's devisee, legatee, or other beneficiary or, if there be
no such beneficiary, to Executive's estate.

11.      SURVIVORSHIP

         The respective rights and obligations of the parties hereunder shall
survive any termination of this Agreement to the extent necessary to the
intended preservation of such rights and obligations.

12.      TAX REIMBURSEMENT PAYMENT

         12.1 To the extent that any payment or distribution by the Employer to
or for the benefit of Executive pursuant to the terms of this Agreement or
otherwise (the "Payment") is or becomes subject to the tax (the "Excise Tax")
imposed under Section 4999 of the Internal Revenue Code of 1986, as amended (the
"Code"), or any similar tax that may hereafter be imposed, the Employer shall
pay to the appropriate taxing authorities on behalf of the Executive at the time
specified in Section 12.7 below an additional amount (the "Tax Reimbursement
Payment") such that the net amount retained by the Executive with respect to
such Payment, after deduction of any Excise Tax on the Payment and any Federal,
state and local income tax and Excise Tax on the Tax Reimbursement Payment
provided for by this Section 12 (and any interest and penalties thereon), but
before deduction for any Federal, state or local income or employment tax
withholding on such Payment, shall be equal to the amount of the Payment.

         12.2 For purposes of determining whether any of the Payment will be
subject to the Excise Tax and the amount of such Excise Tax,

                  (i) such Payment will be treated as "parachute payments"
within the meaning of Section 280G of the Code, and all


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"parachute payments" in excess of the "base amount" (as defined under Section
280G(b)(3) of the Code) shall be treated as subject to the Excise Tax, unless
and except to the extent that, in the good faith judgment of the Employer, or
the firm then acting as independent auditors to the Employer (the "Auditors"),
the Employer (based upon the advice of its counsel) has a reasonable basis to
conclude that such Payment (in whole or in part) either does not constitute
"parachute payments" or represents reasonable compensation for personal services
actually rendered (within the meaning of Section 280G(b)(4)(B) of the Code) in
excess of the "base amount," or such "parachute payments" are otherwise not
subject to such Excise Tax, and

                  (ii) the value of any non-cash benefits or any deferred
payment or benefit shall be determined by the Auditors in accordance with the
principles of Section 280G of the Code taking into account the application of
Section 68(a) of the Code.

         12.3 For purposes of determining the amount of the Tax Reimbursement
Payment, the Executive shall be deemed to pay:

                  (i) Federal income taxes at the highest applicable marginal
rate of Federal income taxation for the calendar year in which the Tax
Reimbursement Payment is to be made, and

                  (ii) any applicable state and local income taxes at the
highest applicable marginal rate of taxation for the calendar year in which the
Tax Reimbursement Payment is to be made, net of the maximum reduction in Federal
incomes taxes which could be obtained from the deduction of such state or local
taxes if paid in such year.

         12.4 The Executive shall have no obligation to file any refund claim
with any taxing authority or to contest any determination by the Internal
Revenue Service; provided, however, that the Executive shall notify the Employer
in writing as soon as reasonably practicable of any written claim by the
Internal Revenue Service that, if successful, would require the payment by the
Employer of a Tax Reimbursement Payment. If the Employer notifies the Executive
in writing within 10 business days after the Executive's notice that it desires
to contest such claim, the Executive shall take such action in connection with
contesting such claim as the Employer shall reasonably request in writing from
time to time, including, without limitation, accepting legal representation with
respect to such claim by an attorney reasonably selected by the Employer,
cooperate with the Employer in good faith in order to effectively contest such
claim, and permit the Employer to participate in any proceedings relating to
such claim, in each case limited to issues with respect to which a Tax
Reimbursement Payment would be payable hereunder.

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         12.5 In the event that the Excise Tax is subsequently determined by the
Auditors or pursuant to any proceeding or negotiations with the Internal Revenue
Service to be less than the amount taken into account hereunder in calculating
the Tax Reimbursement Payment made, the Executive shall repay to the Employer,
at the time that the amount of such reduction in the Excise Tax is finally
determined, the portion of such prior Tax Reimbursement Payment that would not
have been paid if such Excise Tax had been applied in initially calculating such
Tax Reimbursement Payment, plus interest on the amount or such repayment at the
rate provided in Section 1274(b)(2)(B) of the Code. Notwithstanding the
foregoing, in the event any portion of the Tax Reimbursement Payment to be
refunded to the Employer has been paid to any Federal, state or local tax
authority, repayment thereof shall not be required until actual refund or credit
of such portion has been made to the Executive, and interest payable to the
Employer shall not exceed interest received or credited to the Executive by such
tax authority for the period it held such portion. The Executive and the
Employer shall mutually agree upon the course of action to be pursued if the
Executive's good faith claim for refund or credit is denied.

         12.6 In the event that the Excise Tax is later determined by the
Auditors or pursuant to any proceeding or negotiations with the Internal Revenue
Service to exceed the amount taken into account hereunder at the time the Tax
Reimbursement Payment is made (including, but not limited to, by reason of any
payment the existence or amount of which cannot be determined at the time of the
Tax Reimbursement Payment), the Employer shall make an additional Tax
Reimbursement Payment in respect of such excess (plus any interest or penalty
payable with respect to such excess) at the time that the amount of such excess
is finally determined.

         12.7 The Tax Reimbursement Payment (or portion thereof) provided for in
Section 12.1 above shall be paid to the appropriate taxing authorities on behalf
of the Executive not later than the required deposit date for taxes withheld in
respect of the Payment; provided, however, that if the amount of such Tax
Reimbursement Payment (or portion thereof) cannot be finally determined on or
before the date on which payment is due, the Employer shall pay to the
appropriate taxing authorities on behalf of the Executive by such date an amount
estimated in good faith by the Auditors to be the minimum amount of such Tax
Reimbursement Payment and shall pay the remainder of such Tax Reimbursement
Payment (together with interest at the rate provided in Section 1274(b)(2)(b) of
the Code) as soon as the amount thereof can be determined. In the event that the
amount of the estimated Tax Reimbursement Payment exceeds the amount



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subsequently determined to have been due, such excess shall constitute a loan by
the Employer to the Executive, payable on the fifth business day after written
demand by the Employer for payment (together with interest at the rate provided
in Section 1274(b)(2)(B) of the Code).

         12.8 The Employer recognizes that if the Executive is required to repay
to the Employer all or any part of the Tax Reimbursement Payment theretofore
made or is required to pay interest to the Employer or to pay over any interest
received or credited to the Executive in respect of any Tax Reimbursement
Payment, in each case the amount re-payable or payable by the Executive may not
be deductible for Federal, state or local income tax or employment tax purposes
to the same extent that the receipt of such amount by the Executive was taxable
for Federal, state or local income tax or employment tax purposes. Therefore,
notwithstanding anything to the contrary in this Section 12, any amount
re-payable or payable by the Executive to the Employer shall be appropriately
adjusted to take into account the difference between the Federal, state and
local income and employment taxes payable by the Executive on the initial amount
received (whether from the Employer or as interest from any taxing authority)
and the amount of the Federal, state and local income and employment taxes
actually saved by the Executive by reason of any re-payment or payment over to
the Employer of any amount pursuant to this Section 12.

         12.9 All fees and expenses for, or related to, the determination of Tax
Reimbursement Amounts shall be borne solely by the Employer, and the Employer
shall bear and pay directly all costs and expenses (including all professional
fees and disbursement and any and all taxes, whether or not requiring a Tax
Reimbursement Payment, incurred by, or imposed upon, the Executive) relating to
any refund, or to any proceedings or negotiations with the Internal Revenue
Service arising out of any claim by the Internal Revenue Service that, if
successful, would require the payment by the Employer of a Tax Reimbursement
Payment.

13.      MISCELLANEOUS

         13.1 Notices. Unless otherwise specifically set forth herein, any
notice required or permitted hereunder shall be in writing and shall be
sufficiently given if personally delivered or if sent by telegram or telex or by
registered or certified mail, postage prepaid, with return receipt requested,
addressed: (a) in the case of the Employer to NEXTLINK Communications, Inc. 1505
Farm Credit Drive, Sixth Floor, McLean, VA 22102, Attention: General Counsel, or
to such other address and/or to the attention of such other person as the
Employer shall designate by written


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notice to Executive; and (b) in the case of Executive, to Executive's then
current home address as shown on the Employer's books, or to such other address
as Executive shall designate by written notice to the Employer. Any notice given
hereunder shall be deemed to have been given at the time of receipt thereof by
the person to whom such notice is given.

         13.2 Taxes. Employer is authorized to withhold (from any compensation
or benefits payable hereunder to Executive) such amounts for income tax, social
security, unemployment compensation and other taxes as shall be necessary or
appropriate in the reasonable judgment of Employer to comply with applicable
laws and regulations.

         13.3 Governing Law. This Agreement shall be governed by and construed
and enforced in accordance with the laws of the State of Delaware, without
reference to the principles of conflicts of laws therein.

         13.4 Arbitration. Any dispute or controversy arising under or in
connection with this Agreement shall be settled exclusively by arbitration in
Fairfax County Virginia or such other county in which the Employer's main
corporate headquarters is then located in accordance with the rules of the
American Arbitration Association then in effect. Judgment may be entered on the
arbitration award in any court having jurisdiction.

         13.5 Headings. All descriptive headings in this Agreement are inserted
for convenience only and shall be disregarded in construing or applying any
provision of this Agreement.

         13.6 Counterparts. This Agreement may be executed in counterparts, each
of which shall be deemed to be an original, but all of which together shall
constitute one and the same instrument.

         13.7 Validity and Enforceability. The Executive acknowledges and agrees
that the covenants set forth in Sections 8 and 9 hereof are reasonable and valid
in geographical and temporal scope and in all other respects. The invalidity or
unenforceability of any provision or provisions of this Agreement shall not
affect the validity or enforceability of any other provision or provisions of
this Agreement, which shall remain in full force and effect. If any provision of
this Agreement is held to be invalid, void or unenforceable in any jurisdiction,
any court or arbitrator so holding shall substitute a valid, enforceable
provision that preserves, to the maximum lawful extent, the terms and intent of
such provisions of this Agreement. If any of the provisions of, or covenants
contained in, this Agreement are hereafter construed to be invalid or



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<PAGE>   14
Nothaft Employment Agreement - Execution copy.doc

unenforceable in any jurisdiction, the same shall not affect the remainder of
the provisions or the enforceability thereof in any other jurisdiction, which
shall be given full effect, without regard to the invalidity or unenforceability
in such other jurisdiction. Any such holding shall affect such provision of this
Agreement, solely as to that jurisdiction, without rendering that or any other
provisions of this Agreement invalid, illegal, or unenforceable in any other
jurisdiction. If any covenant should be deemed invalid, illegal or unenforceable
because its scope, either geographical or temporal, is considered excessive,
such covenant will be modified so that the scope of the covenant is reduced only
to the minimum extent necessary to render the modified covenant valid, legal and
enforceable.

         13.8 Entire Agreement and Representation. This Agreement, the Option
Agreement and the Executive Retention Bonus Plan (the "Employment Agreements")
contain the entire agreement and understanding between Employer and Executive
with respect to the subject matter hereof. No representations or warranties of
any kind or nature relating to Employer or its several businesses, or relating
to Employer's assets, liabilities, operations, future plans or prospects have
been made by or on behalf of Employer to Executive. The Employment Agreements
supersede any prior agreement or agreements between the parties relating to the
subject matter hereof. Specifically, the Employment Agreements supersede and
replace any employment agreement, Executive Continuity Agreement or similar
agreement or understanding between Executive and Concentric, or any of its
affiliates, relating to the subject matter hereof (the "Concentric Agreements"),
which shall be of no further force or effect as of and following the Effective
Date.

         13.9 Violation of Confidentiality, Noncompetition or Nonsolicitation
Provisions. Notwithstanding anything to the contrary herein, the payments and
benefits made available pursuant to Sections 6 and 4.7 herein are subject to
Executive's full compliance with the provisions of Sections 7, 8, 9 and 13.10(b)
herein and the Employer retains the right to recover and/or revoke or retract
any payments or other benefits made available to Executive pursuant to Sections
6 or 4.7 herein in the event that Executive violates any of the provisions of
Sections 7, 8, 9 or 13.10(b) herein.

         13.10    Conflict of Interest.

                  (a) Notwithstanding anything to the contrary herein, in the
event that during Executive's employment with the Employer the Board reasonably
determines that (i) Executive's service on any board of directors or advisory
board listed in Section 3 herein, (ii) Executive's service on any board of
directors or advisory

                                      -14-
<PAGE>   15
board with respect to which the Employer has given Executive permission to serve
on as set forth in Section 3 or (iii) Executive's investment as venture
capitalist or an angel investor in any investment with respect to which the
Employer has given Executive permission to invest in pursuant to Section 3
herein is or creates a conflict of interest with Executives employment with the
Employer or represents service to or a material investment in an entity that
engages in business that is in competition with the Employer, then Executive
must resign from any such board or advisory board and/or take all reasonable
steps to diminish or liquidate (at the discretion of the Board) any such
investment.

                  (b) Notwithstanding anything to the contrary herein, in the
event that following Executive's termination of employment with the Employer and
during the Non-Compete Period the Board reasonably determines that (i)
Executive's service on any board of directors or advisory board with respect to
which he was a member of at the time of his termination of employment with the
Employer or (ii) Executive's investment as venture capitalist or an angel
investor in any investment with respect to which the Employer has given
Executive permission to invest in pursuant to Section 8 herein is or creates a
conflict of interest with his service on the Board or represents service to or a
material investment in an entity that engages in business that is in competition
with the Employer, then Executive must resign from any such board or advisory
board and/or take all reasonable steps to diminish or liquidate (at the
discretion of the Board) any such investment.

         IN WITNESS WHEREOF, the parties hereto have executed this Agreement as
of the date first above written.

                                                 NEXTLINK COMMUNICATIONS, INC.


                                                 By:/s/ Daniel F. Akerson
                                                    ----------------------


                                                 /s/ Henry R. Nothhaft
                                                 -------------------------
                                                           Executive