Document:

<PAGE>   1
                                                                    EXHBIT 10.6

                       NET2000 COMMUNICATIONS GROUP, INC.
                               2180 Fox Mill Road
                             Herndon, Virginia 20171

                                              December 5, 2000

Mr. Clayton A. Thomas, Jr.
811 Water Place
Alexandria, Virginia  22314

         We are pleased to inform you that Net2000 Communications Group, Inc.
(the "Company") has agreed to your request to commit (the "Commitment") to
lend you (the "Borrower") loans in an aggregate amount of up to $1,500,000
(collectively, the "Loan") upon the terms and conditions set forth herein.

         All the details regarding the Loan are outlined in the following
paragraph of this letter.

         1.       Type of Loan: Subject to the terms and conditions hereof,
the Company agrees to make loans to you from time to time from the date hereof
to and including June 6, 2001; provided that in no event shall the aggregate
outstanding principal amount of the loans exceed $1,500,000. The Loan matures,
and shall be due and payable (together with all accrued interest thereon) on
the date (the "Maturity Date") which is six (6) months from the date the
initial advance of the Loan is made. The Borrower may borrow on any Business
Day, provided that the Borrower shall give the Company written notice (which
notice must be received by the Company prior to 1:00 p.m., Eastern Standard
Time, on the requested borrowing date). The proceeds of each advance of the
Loan shall be made available to the Borrower by check or wire transfer at an
account specified by the Borrower in writing to the Company. The Company has
established a bank account at a commercial bank of prominent reputation at
which the funds have been deposited for purposes of making the Loan.

         2.       Note. The obligation of the Borrower to repay the Loan shall
be evidenced by a promissory note (the "Note") in the form set forth as
Exhibit A hereto.

         3.       Interest Rate. Interest on the unpaid balance of the Loan
will be charged at a rate per annum as set forth in the Note and will be
computed on the basis set forth in the Note.

         4.       Prepayment.  Subject to the terms and conditions of this
letter (herein referred to as this "Agreement"), the Borrower may prepay this
Loan in whole or in part at any time without penalty or premium, together with
any accrued but unpaid interest thereon. Amounts so prepaid may not be
reborrowed.

<PAGE>   2

Mr. Clayton A. Thomas, Jr.
December 5, 2000
Page 2

         5.       Payment Terms.  (a) Accrued interest shall be due and
payable on the last day of each calendar month, commencing with the first such
day following the date the initial advance of the Loan is made.

                  (b)      The outstanding principal amount, together with all
accrued interest on the Loan, shall be due and payable on the Maturity Date.

         6.       Mandatory Repayment. In the event that the Borrower is no
longer employed by the Company for any reason whatsoever (whether because of
termination, death or otherwise) (the first date on which Borrower is no
longer employed, the "Termination Date"), the Loan, together with all accrued
interest thereon and all other obligations payable pursuant to this Agreement,
the Note and the Pledge Agreement, shall become due and payable no later than
thirty (30) days from the Termination Date.

         7.       Collateral.  The Loan shall be secured by a first priority
lien on, security interest in and pledge of all of the shares of stock of
Net2000 Communications, Inc., a Delaware corporation, owned by the Borrower,
pursuant to a pledge agreement (the "Pledge Agreement") in the form set forth
as Exhibit B hereto.

         8.       Event of Default.  (a) Each of the following events or
occurrences shall be an event of default (an "Event of Default") under this
Agreement and the Note:

                           (i)      The failure by the Borrower to pay the
         principal of the Loan or any interest on the Loan when the same shall
         have become due.

                           (ii)     The Borrower shall die or become legally
         incompetent.

                           (iii)    The Pledge Agreement shall cease, for any
         reason, to be in full force and effect or the Borrower shall so
         assert in writing; or the Pledge Agreement shall cease to be
         effective to grant a Lien on the collateral described therein with
         the priority purported to be created thereby; or

                           (iv)     (A) The Borrower shall commence any case,
         proceeding or other action (1) under any existing or future law of
         any jurisdiction, domestice or foreign, relating to bankruptcy,
         insolvency, reorganization or relief of debtors, seeking to have an
         order for relief entered with respect to it, or seeking to adjudicate
         it a bankrupt or insolvent, or seeking reorganization, arrangement,
         winding up, liquidation, dissolution, or other relief with respect to
         it or its debts, or (2) seeking appointment of a receiver, trustee,
         custodian or other similar official for it or for all or any
         substantial part of its assets, or the Borrower shall make a general
         assignment for the benefit of its creditors; or (B) there

<PAGE>   3

Mr. Clayton A. Thomas, Jr.
December 5, 2000
Page 3

         shall be commenced against the Borrower any case, proceeding or other
         action of a nature referred to in clause (A) above which (1) results
         in the entry of an order for relief or any such adjudication or
         appointment or (2) remains undismissed, undischarged or unbonded for
         a period of 60 days; or (C) there shall be commenced against the
         Borrower any case, proceeding or other action seeking issuance of a
         warrant of attachment, execution, distraint or similar process
         against all or any substantial part of its assets which results in
         the entry of an order for any such relief which shall not have been
         vacated, discharged, or stayed or bonded pending appeal within 60
         days from the entry thereof; or (D) the Borrower shall take any
         action in furtherance of, or indicating its consent to, approval of,
         or acquiescence in, any of the acts set forth in clause (A), (B), or
         (C) above; or (E) the Borrower shall generally not, or shall be
         unable to, or shall admit in writing its inability to, pay its debts
         as they become due.

                  (b)      If an Event of Default shall occur and be
continuing, the Company may, by notice to the Borrower terminate the
Commitment to make the Loan and declare the entire unpaid principal amount of
the Loan, all interest accrued and unpaid thereon and all other amounts
payable hereunder and under the other Documents (as hereinafter defined) to be
forthwith due and payable, whereupon the entire unpaid principal amount of the
Loan, all interest accrued and unpaid thereon and all other amounts payable
hereunder under the Note and under the other Documents shall be forthwith due
and payable, without presentment, demand, protest or further notice of any
kind, all of which are hereby expressly waived by the Borrower; provided,
however, that the occurrence of the events described in Paragraph 8(a)(iv) of
this Agreement shall result in an automatic termination of the Commitment and
immediate acceleration of the Loan and all amounts due by the Borrower under
the Note and this Agreement and the other Documents, whereupon all such
amounts shall be due and payable.

                  (c)      The remedies provided in this Paragraph 8 and in
the Note and Pledge Agreement are cumulative and not exclusive of any remedies
provided by law. The Company is entitled to exercise any remedies
simultaneously or in whatever order the Company deems appropriate.

                  (d)      Upon the occurrence and during the continuance of
any Event of Default, the Company may take any lawful action against the
Borrower to collect the payments then due and thereafter to become due under
the Note, this Agreement and the Pledge Agreement.

         9.       Notices. All notices, requests and demands to or upon the
respective parties hereto to be effective shall be in writing (including by
telecopy) and, unless otherwise expressly provided herein, shall be deemed to
have been duly given or made when delivered by hand, or, in the case of
telecopy notices, when sent (with confirmation received), addressed as follows
or to such other address as may be hereafter notified by the respective
parties hereto:

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Mr. Clayton A. Thomas, Jr.
December 5, 2000
Page 4

                  The Borrowers:            Mr. Clayton A. Thomas, Jr.
                                            811 Water Place
                                            Alexandria, Virginia 22314

                  The Company:              Net2000 Communications, Group, Inc.
                                            2180 Fox Mill Road
                                            Herndon, Virginia 20171
                                            Attn: Donald Clarke
                                            Telecopier No.: (703) 654-2025

provided that any notice, request or demand to or upon the Company shall not
be effective until received.

         10.      Expenses Etc. The Borrower shall pay or reimburse (on
demand) the Company for (a) all costs and expenses of the Company (including
fees and disbursements of counsel to the Company) in connection with the
negotiation, preparation, execution, delivery, administration and enforcement
of this Agreement, the Note, the Pledge Agreement, the Loan or any other
documents, agreements and instruments in connection therewith and all other
instruments or documents provided for herein or delivered or to be delivered
hereunder or in connection herewith (collectively, the "Documents") and of any
actual or proposed waiver or amendment to any of the foregoing (whether or not
such waiver or amendment shall become effective) and (b) all transfer, stamp,
documentary or other similar taxes, assessments or charges levied by any
governmental or revenue authority in respect of this Agreement, the Note, the
Pledge Agreement, the Loan or any other Document or any other document
referred to herein or therein. The Borrower hereby agrees to indemnify the
Company and its directors, officers, employees and agents (each an
"Indemnified Party") from, and hold each of them harmless against, any and all
losses, liabilities, claims, damages or expenses incurred by any of them
arising out of or by reason of any investigation or litigation or other
proceedings (including any threatened investigation or litigation or other
proceedings) relating to any actual or proposed use by the Borrower of the
proceeds of the Loan hereunder and under the Note, including, without
limitation, the reasonable fees and disbursements of counsel incurred in
connection with any such investigation or litigation or other proceedings (but
excluding any such losses, liabilities, claims, damages or expenses incurred
by reason of the gross negligence or willful misconduct of the Person to be
indemnified).

         11.      Amendments, Etc. Except as otherwise expressly provided in
this Agreement, any provision of this Agreement may be amended or modified
only by an instrument in writing signed by the Borrower and the Company.

<PAGE>   5

Mr. Clayton A. Thomas, Jr.
December 5, 2000
Page 5

         12.      Representation and Warranty. The Borrower represents and
warrants to the Company that no part of the proceeds of the Loan will be used
to buy or carry any margin stock (within the meaning of Regulation T, U or X
of the Board of Governors of the Federal Reserve System).

         13.      Successors and Assigns. This Agreement shall be binding upon
and inure to the benefit of the Borrower and the Company, all future holders
of the Note and their respective successors and assigns, except that the
Borrower may not assign or transfer any of their rights or obligations under
this Agreement or the Note without the prior written consent of the Company.

         14.      Captions. Captions and section headings appearing herein are
included solely for convenience of reference and are not intended to affect
the interpretation of any provision of this Agreement.

         15.      Counterparts. This Agreement may be executed in any number
of counterparts, all of which taken together shall constitute one and the same
instrument and any of the parties hereto may execute this Agreement by signing
any such counterpart.

         16.      GOVERNING LAW. THIS AGREEMENT, THE NOTE AND THE OTHER
DOCUMENTS SHALL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAW OF THE
STATE OF NEW YORK. THE BORROWER AND THE COMPANY HEREBY IRREVOCABLY WAIVE, TO THE
FULLEST EXTENT PERMITTED BY LAW, ANY AND ALL RIGHT TO TRIAL BY JURY IN ANY LEGAL
PROCEEDING ARISING OUT OF OR RELATING TO THIS AGREEMENT OR THE TRANSACTIONS
CONTEMPLATED HEREBY.

         17.      Severability. In case any provision in or obligation under
this Agreement or the Note or the other Documents shall be invalid, illegal or
unenforceable in any jurisdiction, the validity, legality and enforceability of
the remaining provisions or obligations, or of such provision or obligation in
any other jurisdiction, shall not in any way be affected or impaired thereby.

         18.      Survival.  All indemnities set forth herein including,
without limitation, in Paragraph 10, shall survive the execution and delivery
of this Agreement, the Note and the other Documents and the making and
repayment of the Advances hereunder.

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Mr. Clayton A. Thomas, Jr.
December 5, 2000
Page 6

         If you are in agreement with the foregoing, kindly sign this
agreement in the space provided, whereupon this agreement shall become a
binding agreement on the parties hereto.

                                       Very truly yours,

                                       NET2000 COMMUNICATIONS GROUP, INC.

                                       By:      /s/Donald Clarke
                                                --------------------------
                                                 Donald Clarke
                                                 Chief Financial Officer

                           AGREEMENT AND ACCEPTANCE

With the intent to be legally bound hereby, the above terms and conditions are
hereby agreed to and accepted this 8th day of December, 2000.

                                                /s/Clayton A. Thomas, Jr.
                                                -------------------------------
                                                CLAYTON A. THOMAS, JR.

<PAGE>   7

COMMONWEALTH OF VIRGINIA   )
                           )        ss:
COUNTY OF FAIRFAX          )

                  On this 8th day of December, 2000, before me personally
appeared CLAYTON A. THOMAS, JR., to me known to be the person who executed the
foregoing instrument and who being by me duly sworn, did depose and say that
he resides at 811 Water Place, Alexandria, Virginia and that he signed his
name to the foregoing instrument.

                  SEAL
                                                /s/Jennifer S. Fabris
                                                -------------------------------
                                                NOTARY PUBLIC

My commission expires on:  5/31/2004<PAGE>   1

                                                                 EXHIBIT 10.6.1

                                 PROMISSORY NOTE

$1,500,000                                                    December 5, 2000

                FOR VALUE RECEIVED, the undersigned, CLAYTON A. THOMAS, JR. (the
"BORROWER") promises to pay to the order of NET2000 COMMUNICATIONS GROUP, INC.,
a Delaware corporation (the "LENDER"), the principal sum of ONE MILLION FIVE
HUNDRED THOUSAND DOLLARS ($1,500,000), or, if less, the aggregate outstanding
amount of the Loan defined in the Agreement referred to below, together with
interest thereon as set forth below, at its offices or such other place as the
Lender may designate in writing.

        1.      Agreement. This Promissory Note (the "NOTE") is executed and
delivered by the Borrower in connection with loans which may be made by the
Lender to the Borrower pursuant to the terms and conditions of that certain
letter agreement dated of even date herewith (as the same may be amended,
modified or restated from time to time, the "AGREEMENT") between the Borrower
and the Lender. This Note is subject to the terms and conditions of the
Agreement and is entitled to the benefits of the security described in and
provided for in the Pledge Agreement (as defined in the Agreement). Any
capitalized term used herein and not otherwise defined herein shall have the
meaning assigned to it in the Agreement.

        2.      Interest Rate Provisions. Except as provided in Section 2.2
hereof, from the date hereof and thereafter until the outstanding amount hereof
is paid in full, interest shall accrue on the principal balance of this Note
outstanding from time to time at the Prime Rate (as defined below) plus a margin
of 1% per annum (the "INTEREST RATE"), and shall be paid currently in cash on a
monthly basis on each Payment Date (as defined below).

                2.2     Default Interest Rate. If the Borrower shall default in
the payment of the principal of or interest on the Note or any other amount
becoming due hereunder after any applicable cure period, by acceleration or
otherwise, or under any other Document, whether or not such default is declared,
the Interest Rate shall increase to the Prime Rate plus a margin of 3% per annum
and interest shall be paid currently in cash on a monthly basis on each Payment
Date (as defined below) until the payment in full of all overdue payments or
other cure of such Event of Default.

                2.3     Calculation of Interest. Interest shall be computed for
each payment period on the principal balance for the actual number of days
outstanding over a year of 365 or 366 days, as applicable.

                As used herein, "Prime Rate" means, at any time, the greater of
the rate of interest per annum then most recently announced or established by
The Toronto Dominion Bank at its principal office in New York City as its
highest commercial prime or base rate then in effect. Each change in any
interest rate provided for herein based upon the Prime Rate resulting from a
change in the Prime Rate shall take effect without notice to the Borrower at the
time of such change in the Prime Rate.

<PAGE>   2

        3.      Payment Provisions.

                3.1     Interest Payments.

                        (a)     Pay Rate Interest. Commencing on the last day of
 the calendar month following the date the initial advance of the Loan is made
and continuing on the last day of each succeeding calendar month thereafter
(each such date, a "PAYMENT DATE"), the Borrower shall pay in cash to the Lender
monthly installments of interest only (in arrears) at the applicable Interest
Rate on the then outstanding principal balance under this Note.

                3.2     Principal Payments and Payment at Maturity. On the
Maturity Date, the entire then outstanding principal balance of this Note,
together with all accrued but unpaid interest, and all other sums owed hereunder
shall be due and payable in full in cash without further notice or demand.

                3.3     Prepayments; Application of Payments. The Borrower may
prepay the then outstanding principal amount of this Note in whole or in part at
any time without premium or penalty.

                3.4.    Mandatory Prepayment of the Note. In the event that the
Borrower is no longer employed by the Lender for any reason whatsoever (whether
because of termination, death or otherwise) (the first date on which Borrower is
no longer employed, the "TERMINATION DATE") the Loan, together with all accrued
interest therein and all other obligations payable pursuant to this Agreement,
the Note and the Pledge Agreement, shall become due and payable no later than
thirty (30) days from the Termination Date.

                3.5     Payments Generally. All payments of principal and
interest in respect of this Note shall be made in lawful money of the United
States of America in same day funds to the Lender, at its office located at 2180
Fox Mill Road, Herndon, Virginia 20171 or at such other place as shall be
designated in writing to the Borrower for such purpose.

        4.      Assignment. This Note and the obligations hereunder may not be
assigned by the Borrower without the prior written consent of the Lender.

        5.      Default and Remedies. The occurrence of an Event of Default
under the Agreement shall constitute a default hereunder and, so long as such
Event of Default shall continue, shall entitle the Lender to exercise the rights
and remedies specified in the Agreement, as well as those available at law or in
equity.

        6.      Waivers. The Borrower hereby waive presentment, demand, protest,
or further notice of any kind.

        7.      Controlling Law. This Note and all matters related hereto shall
be governed, construed and interpreted strictly in accordance with the laws of
the State of New York.
                           [Signatures on next page]

                                      - 2 -

<PAGE>   3

               IN WITNESS WHEREOF, the undersigned has caused this Note to be
executed and its seal affixed on the day and year first above written.

                                     /s/Clayton A. Thomas, Jr.
                                     -------------------------------------------
                                     CLAYTON A. THOMAS, JR.

                                     - 3 -

<PAGE>   4

COMMONWEALTH OF VIRGINIA)
                        )       ss:
COUNTY OF FAIRFAX       )

               On this 8th day of December, 2000, before me personally appeared
CLAYTON A. THOMAS, JR., to me known to be the person who executed the foregoing
instrument and who being by me duly sworn, did depose and say that he resides at
811 Water Place, Alexandria, Virginia and that he signed his name to the
foregoing instrument.

               SEAL
                                     /s/Jennifer S. Fabris
                                     -------------------------------------------
                                     NOTARY PUBLIC

My commission expires on:  5/31/2004

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