Document:

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                                                                   EXHIBIT 10.47

                              GUARANTY AGREEMENT
                            (FIBERNET HOLDCO, INC.)

     This GUARANTY (as amended, supplemented, amended and restated or otherwise
modified from time to time, this "Guaranty"), entered into as of July 14, 2000
by FIBERNET HOLDCO, INC., a Delaware corporation (the "Guarantor"), in favor of
DEUTSCHE BANK AG NEW YORK BRANCH, as administrative agent (together with any
successor(s) thereto in such capacity, the "Administrative Agent") for each of
the Secured Parties.

                                    RECITALS
                                    --------

     A.  Pursuant to the Credit Agreement, dated as of April 11, 2000 and
amended as of July 14, 2000 (as amended, supplemented, amended and restated or
otherwise modified from time to time, the "Credit Agreement"), among Fibernet
Telecom Group, Inc., a Delaware corporation (the "Borrower"), the financial
institutions from time to time parties thereto (collectively, the "Lenders"),
Deutsche Bank Securities Inc., as co-syndications agent, Toronto Dominion
(Texas), Inc., as co-syndications agent, and the Administrative Agent, the
Lenders have extended Commitments to make Loans and issue Letters of Credit to,
and for the benefit of, the Borrower;

     B.  The Guarantor has duly authorized the execution, delivery and
performance of this Guaranty;

     C.  It is in the best interests of the Guarantor to execute this Guaranty
as the Guarantor will derive substantial direct and indirect benefits from the
Loans made to the Borrower, and the Letters of Credit issued for the benefit of
the Borrower, from time to time pursuant to the Credit Agreement, including
receiving proceeds thereof which will enable the Guarantor to acquire
Telecommunication Assets comprising a portion of the System; and

     D.  This Guaranty is secured by the Security Agreement (FiberNet Holdco,
Inc.), dated as of the date hereof, between the Guarantor and the Administrative
Agent, as security for the Obligations.

                                   AGREEMENT
                                   ---------

     NOW, THEREFORE, in consideration of the premises and the mutual agreements
herein contained and for other good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties hereto, intending to
be legally bound, agree as follows:
<PAGE>

                                  ARTICLE I.
                                  DEFINITIONS

Section 1.1  Defined Terms.
             --------------

     The following terms when used in this Guaranty shall have the following
meanings:

     "Administrative Agent" shall have the meaning assigned to such term in the
preamble.
--------

     "Borrower" shall have the meaning assigned to such term in the first
recital.
-------

     "Credit Agreement" shall have the meaning assigned to such term in the
first recital.
-------------

     "Guarantor" shall have the meaning assigned to such term in the preamble.
                                                                     --------

     "Guaranty" shall have the meaning assigned to such term in the preamble.
                                                                    --------

     "Lenders" shall have the meaning assigned to such term in the first
                                                                   -----
recital.

     Section 1.2  Credit Agreement Definitions; Principles of Interpretation.
                  ----------------------------------------------------------

     Unless otherwise defined herein or the context otherwise requires, terms
used in this Guaranty, including its preamble and recitals, have the meanings
provided in the Credit Agreement (including the principles of interpretation set
forth in Section 1.3 of the Credit Agreement). All references herein to any
         -----------
rights or interests granted hereby to the Administrative Agent shall be deemed
to be rights or interests granted to the Administrative Agent for the benefit of
each of the Secured Parties, whether or not specifically so stated.

                                  ARTICLE II.
                              GUARANTY PROVISIONS

     Section 2.1  Guaranty.
                  --------

     The Guarantor hereby, absolutely, unconditionally and irrevocably,
guarantees the full and punctual payment when due, whether at stated maturity,
by required prepayment, declaration, acceleration, demand or otherwise, of all
Obligations, provided, however, that the Guarantor shall be liable under this
             --------- -------
Guaranty only for the maximum amount of such liability that can be hereby
incurred without rendering this Guaranty voidable under applicable law relating
to fraudulent conveyance or fraudulent transfer (or similar concepts under
foreign law), and not for any greater amount.  This Guaranty constitutes a
guaranty of payment when due and not merely of collection, and the Guarantor
specifically agrees that it shall not be necessary or required that any Secured
Party or any holder of any Note exercise any right, assert any claim or demand
or enforce any remedy whatsoever against the Borrower or any other Loan Party
(or any other Person) before or as a condition to the performance of the
obligations of the Guarantor hereunder.

                                       2
<PAGE>

     Section 2.2  Acceleration of Guaranty.
                  ------------------------

     If an Event of Default of the nature set forth in Section 7.4 or 7.5 of the
                                                       -----------    ---
Credit Agreement with respect to the Borrower or any of its Subsidiaries shall
occur and be continuing, all of the Obligations shall, as set forth in Article
                                                                       -------
VII of the Credit Agreement, be immediately due and payable, and, accordingly,
---
the Guarantor shall be required to pay to the Administrative Agent, for the
benefit of the Secured Parties, forthwith an amount equal to all of the
Obligations.

     Section 2.3  Guaranty Absolute, Etc.
                  ----------------------

     This Guaranty shall in all respects be a continuing, absolute,
unconditional and irrevocable guaranty of payment, and shall remain in full
force and effect until all Obligations have been paid in full in cash, all
obligations of the Guarantor hereunder have been paid in full in cash, all
Interest Rate Agreements to which any Secured Party is a party have been
terminated and all Commitments have been terminated.  The Guarantor guarantees
that the Obligations will be paid strictly in accordance with the terms of the
Credit Agreement and each other Loan Document under which they arise.  The
liability of the Guarantor under this Guaranty shall be absolute, unconditional
and irrevocable, irrespective of:

          (a)  any lack of validity, legality or enforceability of the Credit
     Agreement, any Note, or any other Loan Document or any Interest Rate
     Agreement;

          (b)  the failure of any Secured Party or any holder of any Note

               (i)  to assert any claim or demand or to enforce any right or
          remedy against the Borrower, any other Loan Party or any other Person
          (including any other guarantor) under the provisions of the Credit
          Agreement, any Note, any other Loan Document, any Interest Rate
          Agreement or otherwise, or

               (ii) to exercise any right or remedy against any other guarantor
          of, or collateral securing, any of the Obligations;

          (c)  any change in the time, manner or place of payment of, or in any
     other term of, all or any of the Obligations, or any other extension or
     renewal of any Obligation of the Borrower or any other Loan Party;

          (d)  any reduction, limitation, impairment or termination of any of
     the Obligations for any reason other than the written agreement of the
     Secured Parties to terminate the Obligations in full, including any claim
     of waiver, release, surrender, alteration or compromise, and shall not be
     subject to, and the Guarantor hereby waives any right to or claim of, any
     defense or setoff, counterclaim, recoupment or termination whatsoever by
     reason of the invalidity, illegality, nongenuineness, irregularity,
     compromise, unenforceability of, or any other event or occurrence
     affecting, any Obligations of the Borrower, any other Loan Party or
     otherwise;

                                       3
<PAGE>

          (e)  any amendment to, rescission, waiver, or other modification of,
     or any consent to departure from, any of the terms of the Credit Agreement,
     any Note, any other Loan Document or any Interest Rate Agreement;

          (f)  any addition, exchange, release, surrender or non-perfection of
     any collateral, or any amendment to or waiver or release or addition of, or
     consent to departure from, any other guaranty held by any Secured Party or
     any holder of any Note securing any of the Obligations; or

          (g)  any other circumstance which might otherwise constitute a defense
     available to, or a legal or equitable discharge of, the Borrower, any other
     Loan Party, any surety or any guarantor.

     Section 2.4  Reinstatement, Etc.
                  ------------------

     The Guarantor agrees that this Guaranty shall continue to be effective or
be reinstated, as the case may be, if at any time any payment (in whole or in
part) of any of the Obligations is rescinded or must otherwise be restored by
any Secured Party or any holder of any Note upon the insolvency, bankruptcy or
reorganization of the Borrower, any other Loan Party or otherwise, all as though
such payment had not been made.

     Section 2.5  Waiver, Indemnification, Etc.
                  ----------------------------

     The Guarantor hereby waives promptness, diligence, notice of acceptance and
any other notice with respect to any of the Obligations and this Guaranty and
any requirement that the Administrative Agent, any other Secured Party or any
holder of any Note protect, secure, perfect or insure any security interest or
Lien, or any property subject thereto, or exhaust any right or take any action
against the Borrower, any other Loan Party or any other Person (including any
other guarantor) or entity or any collateral securing the Obligations, as the
case may be.  The Guarantor hereby agrees to indemnify and hold harmless the
Administrative Agent and each other Secured Party for any and all costs and
expenses (including reasonable attorney's fees and expenses) incurred in
enforcing any right under this Guaranty.

     Section 2.6  Subordination.
                  -------------

     Except as otherwise specifically provided in, or permitted by, this
Guaranty or the Credit Agreement, all existing and future indebtedness of, or
other obligations owed by, the Borrower to the Guarantor is hereby subordinated
to all obligations and liabilities hereby guaranteed. Without the prior written
consent of the Administrative Agent, such subordinated indebtedness shall not be
paid or withdrawn in whole or in part, nor shall the Guarantor accept any
payment of or on account of any such indebtedness, after the occurrence and
during the continuance of an Event of Default under the Credit Agreement.  Any
payment by the Borrower in violation of this Guaranty shall be received by the
Guarantor in trust for the Administrative Agent, and the Guarantor shall cause
the same to be paid to the Administrative Agent immediately upon demand by the
Administrative Agent on account of the Borrower's obligations and liabilities
hereby guaranteed.  The Guarantor shall not assign all or any portion of such
indebtedness while this Guaranty remains in effect except upon prior written
notice to the Administrative Agent by which the

                                       4
<PAGE>

assignee of any such indebtedness agrees that the assignment is made subject to
the terms of this Guaranty, and that any attempted assignment of such
indebtedness in violation of the provisions hereof shall be void.

     Section 2.7  Postponement of Subrogation.
                  ---------------------------

     The Guarantor agrees that it will not exercise any rights which it may
acquire by way of rights of subrogation under this Guaranty, by any payment made
hereunder or otherwise, until the prior payment in full in cash of all of the
Obligations, the termination of all Interest Rate Agreements to which any
Secured Party is a party and the termination of all Commitments.  Any amount
paid to the Guarantor on account of any such subrogation rights prior to the
payment in full in cash of all of the Obligations shall be held in trust for the
benefit of the Secured Parties and each holder of a Note and shall immediately
be paid to the Administrative Agent for the benefit of the Secured Parties and
each holder of a Note and credited and applied against the Obligations of the
Borrower and each other Loan Party, whether matured or unmatured, such order as
the Administrative Agent shall elect; provided,  however, that if
                                      --------   -------

          (a) the Guarantor has made payment to the Secured Parties and each
     holder of a Note of all or any part of the Obligations, and

          (b) all Obligations have been paid in full in cash, all Interest Rate
     Agreements to which any Secured Party is a party have been terminated and
     all Commitments have been permanently terminated,

then, at the Guarantor's request, the Administrative Agent, on behalf of the
Secured Parties and the holders of the Notes, will execute and deliver to the
Guarantor appropriate documents (without recourse and without representation or
warranty) necessary to evidence the transfer by subrogation to the Guarantor of
an interest in the Obligations resulting from such payment by the Guarantor.  In
furtherance of the foregoing, for so long as any Obligations or Commitments
remain outstanding, the Guarantor shall refrain from taking any action or
commencing any proceeding against the Borrower or any other Loan Party (or any
of its or their successors or assigns, whether in connection with a bankruptcy
proceeding or otherwise) to recover any amounts in respect of payments made
under this Guaranty to any Secured Party or any holder of a Note, except that
the Guarantor may file a proof of claim in a bankruptcy proceeding with respect
to the Borrower or any other Loan Party in connection with any obligations owed
by such Loan Party to the Guarantor in the event that the Administrative Agent
has failed to file a proof of claim on the Guarantor's behalf by the second
business day before the due date for such filing.

     Section 2.8  Successors, Transferees and Assigns; Transfers of Notes, Etc.
                  ------------------------------------------------------------

     This Guaranty shall:

          (a) be binding upon the Guarantor, and the Guarantor's successors,
     transferees and assigns; and

          (b) be enforceable by the Administrative Agent and its successors and
     assigns, for the benefit of the Secured Parties.

                                       5
<PAGE>

Without limiting the generality of the foregoing clause (b), any Lender may
                                                 ----------
assign or otherwise transfer (in whole or in part) any Loan, Note or Commitment
held by it to any other Person or entity as permitted by, and in accordance with
the terms of, the Credit Agreement, and such other Person or entity shall
thereupon become vested with all rights and benefits in respect thereof granted
to such Lender under any Loan Document (including this Guaranty) or otherwise,
subject, however, to any contrary provisions in such assignment or transfer, and
to the provisions of Sections 9.1 and 9.16 of the Credit Agreement.
                     ------------     ----

     Section 2.9  Payments Free and Clear of Taxes, Etc.
                  -------------------------------------

          (a) All payments made by the Guarantor hereunder shall be made in
     accordance with Section 2.7 of the Credit Agreement as if such Section were
                     -----------
     incorporated herein substituting "Guarantor" in the place of "Borrower" in
     each place it appears in that Section.

          (b) Without prejudice to the survival of any other agreement of the
     Guarantor hereunder, the agreements and obligations of the Guarantor
     contained in this Section shall survive the payment in full in cash of the
     principal of and interest on the Loans and all other Obligations.

                                 ARTICLE III.
                                COVENANTS, ETC.

     The Guarantor covenants and agrees that, so long as any portion of the
Obligations shall remain unpaid, any Interest Rate Agreements to which any
Secured Party is a party shall remain in full force and effect or any Lender
shall have any outstanding Commitment:

     Section 3.1  Contracts.
                  ---------

     The Guarantor shall not, unless the Majority Lenders shall otherwise
consent in writing, enter into any revenue contract or Material Contract with
respect to the business, assets or operations of the Borrower, Devnet or any of
their respective Subsidiaries unless (a) any of the Borrower, Devnet or any of
their respective Subsidiaries (each an "Operating Company") is also a party to
such contract and such contract provides that all rights and benefits (including
the right to receive payments) provided by the non-Loan Party party thereto
shall run in favor of only the Operating Company party thereto (and that the
Guarantor shall have no right, title or interest in or to any such right or
benefit) or (b) such Material Contract is a guarantee of the obligations of any
Operating Company under any contract to which it is a party;

     Section 3.2  Transaction Related Matters.
                  ---------------------------

     The Guarantor shall promptly, but in any event not later than five Business
Days, following the consummation of the Transaction, (a) take all actions
necessary to effectuate the assumption by the Guarantor of the Borrower's
obligations to issue securities deliverable upon

                                       6
<PAGE>

the exercise of any then outstanding warrants to acquire any Capital Stock of
the Borrower such that the due exercise of any such warrant will entitle the
holder thereof to acquire a number of shares of the Guarantors' Capital Stock
identical to the class and number of the Borrower's Capital Stock that were
subject to the corresponding Borrower warrant, including, at the request of any
Lender or any Affiliate thereof which holds any Borrower warrants, executing and
delivering to the Administrative Agent new Guarantor warrant agreements in the
form of the Warrant Agreement and the Warrant Escrow Agreement, and new
Guarantor warrants in the form of the forms of warrant certificates attached
thereto, (b) cause Devnet to execute and deliver to the Administrative Agent the
Devnet Security Agreement and a Control Agreement with respect to Devnet's bank
accounts, thereby granting the Administrative Agent, for the benefit of the
Secured Parties, a first priority lien on the assets purported to be covered
thereby, and the Devnet Guaranty Agreement, (c) execute and deliver to the
Administrative Agent the Parent Acknowledgement of Obligations, a Control
Agreement with respect to the Guarantor's bank accounts, a registration rights
agreement in the form of the Registration Rights Agreement referred to in the
Equity Registration Rights Joinder Agreement for the benefit of the
Beneficiaries referred to in the Equity Registration Rights Joinder Agreement if
both parties deem it reasonably necessary, the Parent Pledge Agreements, a
solvency certificate in the form of Exhibit C hereto, thereby granting the
                                    ---------
Administrative Agent, for the benefit of the Secured Parties, a first priority
lien in the assets purported to be governed by the Control Agreement and the
Parent Pledge Agreements, (d) take all actions necessary to (1) amend the
governing documents of Devnet to provide (x) that the membership interests of
Devnet are securities governed by Article 8 of the New York UCC and (y) for the
issuance of such membership interests to the members of Devnet in certificated
form and (2) cause certificated membership interests of Devnet to be issued to
the members thereof evidencing, in the case of the Guarantor, 96.386% of the
outstanding membership interests of Devnet and, in the case of the Borrower,
3.614% of the outstanding membership interests of Devnet and (e) deliver and
cause its Subsidiaries to deliver corporate documentation ancillary to the
foregoing reasonably requested by the Administrative Agent (such as secretary's
resolutions and incumbency certificates).

                                  ARTICLE IV.
                           MISCELLANEOUS PROVISIONS

     Section 4.1  Binding on Successors, Transferees and Assigns; Assignment.
                  ----------------------------------------------------------

     In addition to, and not in limitation of, Section 2.8, this Guaranty shall
                                               -----------
be binding upon the Guarantor and its successors, transferees and assigns and
shall inure to the benefit of and be enforceable by the Administrative Agent and
its successors, transferees and assigns for the benefit of the Secured Parties
(to the full extent provided pursuant to Section 2.8); provided, however, that
                                         -----------   --------  -------
the Guarantor may not assign any of its obligations or rights hereunder without
the prior written consent of the Majority Lenders.

     Section 4.2  Delay and Waiver.
                  ----------------

     No failure or delay by the Administrative Agent in exercising any right or
power hereunder shall operate as a waiver thereof, nor shall any single or
partial exercise of any such

                                       7
<PAGE>

right or power, or any abandonment or discontinuance of steps to enforce such a
right or power, preclude any other or further exercise thereof or the exercise
of any other right or power.

     Section 4.3  Setoff.
                  ------

     In addition to, and not in limitation of, any rights of any Secured Party
or any holder of a Note under applicable law, each Secured Party and each such
holder shall, upon the occurrence of any Event of Default, have the right to
appropriate and apply amounts to the payment of the obligations of the Guarantor
owing to it hereunder, whether or not then due; provided, however, that any such
                                                --------  -------
appropriation and application shall be subject to the provisions of Section 9.5
                                                                    -----------
of the Credit Agreement.

     Section 4.4  Pari Passu Obligation.
                  ---------------------

     The obligations of the Guarantor hereunder shall be at least pari passu
with its obligations in connection with any other senior indebtedness or
obligation incurred by the Guarantor.

     Section 4.5  Notices.
                  -------

     Except in the case of notices and other communications expressly permitted
to be given by telephone, all notices and other communications provided for
herein shall be in writing and shall be delivered by hand or overnight courier
service, mailed by certified or registered mail or sent by telecopy, as follows:

The Guarantor:                FiberNet Holdco, Inc.
                              570 Lexington Avenue
                              3/rd/ Floor
                              New York, New York  10022
                              Attn.: President
                              Telecopy: (212) 421-8920

The Administrative Agent:     Deutsche Bank AG New York Branch
                              31 West 52nd Street,
                              New York, New York 10019
                              Attention: Jon D. Storck
                              Telecopy: (212) 469-3713

Any party hereto may change its address or telecopy number for notices and other
communications hereunder by notice to the other parties hereto.  All notices and
other communications given (a) to the Guarantor in accordance with the terms of
this Guaranty shall be deemed to have been given when sent (answerback received)
in the case of telecopy, when delivered, in the case of hand or overnight
courier service, and 5 days after mailing, in the case of certified or
registered mail, or (b) to any party hereto in accordance with the terms of this
Guaranty other than for purposes of the immediately preceding clause (a), shall
be deemed to have been given on the date of receipt.

                                       8
<PAGE>

     Section 4.6  Amendments and Waivers.
                  ----------------------

     No amendment, modification, termination or waiver of any provision of this
Guaranty or consent to any departure by the Guarantor therefrom shall be
effective unless the same shall be in writing and signed by the Administrative
Agent and the Guarantor and shall comply with the provisions set forth in
Section 9.6 of the Credit Agreement.  Each amendment, modification, termination
-----------
or waiver shall be effective only in the specific instance and for the specific
purpose for which it was given.

     Section 4.7  Headings.
                  --------

     Section and subsection headings contained in this Guaranty are inserted for
convenience of reference only, shall not be deemed to be a part of this Guaranty
for any purpose, and shall not in any way define or affect the meaning,
construction or scope of any of the provisions hereof.

     Section 4.8  Applicable Law; Entire Agreement.
                  --------------------------------

     This Guaranty shall be governed by, and shall be construed and enforced in
accordance with, the internal laws of the State of New York, without regard to
conflicts of laws principles (other than Sections 5-1401 and 5-1402 of the
General Obligations Law of the State of New York).  This Guaranty and the other
Loan Documents constitute the entire understanding among the parties hereto with
respect to the subject matter hereof and supersede any prior agreements, written
or oral, with respect thereto.

     Section 4.9  Severability.
                  ------------

     The invalidity, illegality or unenforceability in any jurisdiction of any
provision in or obligation under this Guaranty shall not affect or impair the
validity, legality or enforceability of the remaining provisions or obligations
under this Guaranty or of such provision or obligation in any other
jurisdiction.

     Section 4.10  Consent to Jurisdiction.
                   -----------------------

     The Guarantor hereby irrevocably and unconditionally submits, for itself
and its property, to the nonexclusive jurisdiction of the Supreme Court of the
State of New York sitting in New York County and of the United States District
Court of the Southern District of New York, and any appellate court in respect
thereof, in any action or proceeding arising out of or relating to this
Guaranty, or for recognition or enforcement of any judgment, and hereby
irrevocably and unconditionally agrees that all claims in respect of any such
action or proceeding may be heard and determined in such New York State court
or, to the extent permitted by law, in such federal court.  The Guarantor agrees
that a final judgment in any such action or proceeding shall be conclusive and
may be enforced in other jurisdictions by suit on the judgment or in any other
manner provided by law.  Nothing in this Section 4.11 shall affect any right
                                         ------------
that the Administrative Agent may otherwise have to bring any action or
proceeding relating to this Guaranty against the Guarantor or any of its
properties in the courts of any jurisdiction.  The Guarantor hereby irrevocably
and unconditionally waives, to the fullest extent it may legally and effectively
do so, any objection which it may now or hereafter have to the laying of venue
of any suit, action or proceeding arising out of or relating to this Guaranty in
any court referred to in this

                                       9
<PAGE>

Section 4.11. The Guarantor irrevocably waives, to the fullest extent permitted
------------
by law, the defense of an inconvenient forum to the maintenance of such action
or proceeding in any such court. The Guarantor irrevocably consents to service
of process in the manner provided for notices in Section 4.5. Nothing in this
                                                 ------------
Guaranty will affect the right of any party hereto to serve process in any other
manner permitted by law.

     Section 4.11  Waiver of Jury Trial.
                   --------------------

     THE GUARANTOR HEREBY WAIVES, TO THE FULLEST EXTENT PERMITTED BY APPLICABLE
LAW, ANY RIGHT IT MAY HAVE TO A TRIAL BY JURY IN ANY LEGAL PROCEEDING DIRECTLY
OR INDIRECTLY ARISING OUT OF OR RELATING TO THIS GUARANTY OR THE TRANSACTIONS
CONTEMPLATED HEREBY (WHETHER BASED ON CONTRACT, TORT OR ANY OTHER THEORY).

     Section 4.12  Construction.
                   ------------

     The Guarantor and the Administrative Agent each acknowledges that it has
had the benefit of legal counsel of its own choice and has been afforded an
opportunity to review this Guaranty with its legal counsel and that this
Guaranty shall be construed as if jointly drafted by the Guarantor and the
Administrative Agent.

     Section 4.13  Counterparts; Effectiveness.
                   ---------------------------

     This Guaranty and any amendments, waivers, consents, or supplements may be
executed in any number of counterparts and by different parties hereto in
separate counterparts, each of which when so executed and delivered shall be
deemed an original, but all of which counterparts together shall constitute but
one and the same instrument.  This Guaranty shall become effective upon the
execution of a counterpart hereof by each of the parties hereto.  Delivery of an
executed counterpart of a signature page to this Guaranty or to any amendments,
waivers, consents or supplements hereof or thereof by telecopier shall be as
effective as delivery of a manually executed counterpart thereof.

     Section 4.14  Devnet Transaction.
                   ------------------

     Upon consummation of the Transaction, all references herein to the
"Guarantor" shall be deemed to be references to FiberNet Telecom Group, Inc
(formerly known as FiberNet Holdco, Inc.).

                 [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

                                       10
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Guaranty to be duly
executed and delivered by their officers thereunto duly authorized as of the
date first above written.

                              FIBERNET HOLDCO, INC.,
                              as the Guarantor

                              By: /s/ Michael S. Liss
                                  _______________________________
                                  Name:  Michael S. Liss
                                  Title: Chief Executive Officer
                                         and President

                              DEUTSCHE BANK AG NEW YORK
                              BRANCH, as Administrative Agent

                              By: /s/ Alexander Richarz
                                  _______________________________
                                  Name:  Alexander Richarz
                                  Title: Assistant Vice President

                              By: /s/ Kelvin Cheng
                                  _______________________________
                                  Name:  Kelvin Cheng
                                  Title: Associate

                                    S-1<PAGE>

                                                                   Exhibit 10.48

                              SECURITY AGREEMENT
                            (FIBERNET HOLDCO, INC.)

     This SECURITY AGREEMENT (as amended, supplemented, amended and restated or
otherwise modified from time to time, this "Security Agreement"), is entered
into as of July 14, 2000, by FIBERNET HOLDCO, INC., a Delaware corporation (the
"Grantor"), in favor of DEUTSCHE BANK AG NEW YORK BRANCH, as administrative
agent (together with any successor(s) thereto in such capacity, the
"Administrative Agent") for each of the Secured Parties.

                                R E C I T A L S
                                ---------------

     A.   Pursuant to the Credit Agreement, dated as of April 11, 2000 and
amended as of July 14, 2000 (as amended, supplemented, amended and restated or
otherwise modified from time to time, the "Credit Agreement"), among FiberNet
Telecom Group, Inc., a Delaware corporation (the "Borrower"), the financial
institutions from time to time parties thereto as Lenders (collectively, the
"Lenders"), the Administrative Agent, Deutsche Bank Securities Inc., as co-
syndications agent, and Toronto Dominion (Texas), Inc., as co-syndications
agent, the Lenders have extended commitments to make Loans or issue Letters of
Credit (each as defined in the Credit Agreement) to, and for the benefit of, the
Borrower;

     B.   The Grantor has duly authorized the execution, delivery and
performance of this Security Agreement; and

     C.   It is in the best interests of the Grantor to execute this
Security Agreement as the Grantor will derive substantial direct and indirect
benefits from the Loans made to the Borrower, and the Letters of Credit issued
for the benefit of the Borrower, by the Lenders from time to time pursuant to
the Credit Agreement.

                                   AGREEMENT
                                   ---------

     For good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, and in order to induce the Lenders to make Loans and
issue Letters of Credit to or for the benefit of the Borrower pursuant to the
Credit Agreement, and to induce the Secured Parties to enter into any Interest
Rate Agreements, the Grantor agrees, for the benefit of each Secured Party, as
follows:
<PAGE>

                                  ARTICLE I.
                                  DEFINITIONS

     Section 1.1    Defined Terms.
                    --------------

     The following terms when used in this Security Agreement shall have the
following meanings:

          "Administrative Agent" shall have the meaning set forth in the
preamble.
--------

          "Borrower" shall have the meaning set forth in the first recital.
                                                             --------------

          "Collateral" shall have the meaning set forth in Section 2.1.
                                                           -----------

          "Credit Agreement" shall have the meaning set forth in the first
                                                                     -----
recital.
-------

          "Grantor" shall have the meaning set forth in the preamble.
                                                            --------

          "Lenders" shall have the meaning set forth in the first recital.
                                                            -------------

          "Permits" shall have the meaning set forth in Section 2.1.
                                                        -----------

          "Receivables" shall have the meaning set forth in Section 2.1.
                                                            -----------

          "Related Contracts" shall have the meaning set forth in Section 2.1.
                                                                  -----------

          "Security Agreement" shall have the meaning set forth in the preamble.
                                                                       --------

     Section 1.2    Credit Agreement Definitions; Principles of Interpretation.
                   ------------------------------------------------------------

     Unless otherwise defined herein or unless the context otherwise requires,
terms used in this Security Agreement, including its preamble and recitals, have
the meanings provided in the Credit Agreement (including the principles of
interpretation set forth in Section 1.3 of the Credit Agreement).  All
                            -----------
references herein to the security interest granted to, assignment or pledge to
or other rights or interests granted hereby to the Administrative Agent shall be
deemed to be rights or interests granted to the Administrative Agent for the
benefit of each of the Secured Parties, whether or not specifically so stated.

     Section 1.3    UCC Definitions.
                    ----------------

     Unless otherwise defined herein or in the Credit Agreement or unless the
context otherwise requires, terms for which meanings are provided in the UCC are
used in this Security Agreement, including its preamble and recitals, with such
meanings.

                                       2
<PAGE>

                                  ARTICLE II.

     Section 2.1    Creation of Security Interest.
                    ------------------------------

     As security for the due and punctual payment in full in cash and
performance in full of the Obligations, the Grantor hereby assigns and pledges
to the Administrative Agent for the benefit of the Secured Parties, and grants
to the Administrative Agent for the benefit of the Secured Parties a security
interest in and lien on, all of the Grantor's right, title and interest in and
to the following, whether now owned or hereafter existing or acquired by the
Grantor (collectively, the "Collateral"):

                    (a)  all "accounts" (as defined in the UCC), contracts
               (including the Contracts) and contract rights (including (i)
               rights of the Grantor to receive moneys due and to become due
               under or pursuant to any contract (whether as contractual
               obligations, damages or otherwise), (ii) all rights of the
               Grantor to receive any proceeds of any insurance, indemnity,
               warranty, or guarantee with respect to any contract, (iii) all
               rights of the Grantor with respect to claims, rights, powers, or
               privileges under any contract, (iv) all rights of the Grantor to
               terminate, amend, supplement or modify any contract, to perform
               thereunder and to compel performance and otherwise exercise all
               remedies thereunder, (v) all rights of the Grantor under each
               contract to make determinations, to exercise any election
               (including, but not limited to, the election of remedies) or
               option or to give or receive any notice, consent, waiver, or
               approval, together with full power and authority with respect to
               any contract to demand, receive, enforce, collect or provide
               receipt for any of the foregoing rights or any property the
               subject of any of the contracts, to enforce or execute any
               checks, or other instruments or orders, to file any claims and to
               take any action which may be necessary or advisable in connection
               with any of the foregoing, and (vi) the rights of the Grantor to
               payment for goods or other property (including, the sale of
               capacity or any other use of the System or a portion thereof)
               sold or leased or services performed by the Grantor) (the
               "Contracts"), chattel paper, documents, and instruments of the
               Grantor, in all cases whether or not arising out of or in
               connection with the sale or lease of goods or the rendering of
               services, and all of the Grantor's right, title and interest in
               and to any goods, services or property represented by the
               foregoing prior to the sale thereof, and all rights of the
               Grantor now or hereafter existing in and to all security
               agreements, guaranties, leases, letters of credit, guarantees and
               other contracts securing or otherwise relating to any such
               accounts, contracts, contract rights, chattel paper, documents,
               and instruments (any and all such accounts, contracts, contract
               rights, chattel paper, documents and instruments being the
               "Receivables," and any and all such security agreements,
               guaranties, leases and other contracts being the "Related
               Contracts");

                    (b)  all "general intangibles" (as defined in the UCC),
               including, to the extent assignable, all rights relating to
               design, development, operation, and use of

                                       3
<PAGE>

          the System, all certificates of occupancy, zoning variances, building,
          use or other permits, approvals, authorizations, licenses and consents
          obtained from any governmental agency in connection with the
          development, use, operation or management of the System (the
          "Permits"), all construction, service, engineering, consulting,
          architectural and other similar contracts concerning the design,
          construction, operation, occupancy and/or use of the System, all
          architectural drawings, plans, specifications, soil tests, appraisals,
          route surveys, engineering reports and similar materials relating to
          all or any portion of the System, and all payment and performance
          bonds or warranties or guarantees relating to the System; all rights
          under and in patents, patent licenses, rights in intellectual
          property, trademarks, trade names, corporate names, company names,
          business names, fictitious business names, trade styles, trade
          secrets, service marks, logos, other source and business identifiers,
          trademark registrations and applications for registration used
          exclusively at or relating exclusively to any part of the Grantor's
          business; all renewals, extensions and continuations-in-part of the
          items referred to above; any written agreements granting to the
          Grantor any right to use any trademark or trademark registration at or
          in connection with the Grantor's business; and the right of the
          Grantor to sue for past, present and future infringements of the
          foregoing; and the right in the name and on behalf of the Grantor to
          appear in and defend any action or proceeding brought with respect to
          any part of the Grantor's real or personal property and to commence
          any action or proceeding to protect the interest of the Grantor in
          such Collateral;

               (c)  all books, records, writings, design documents, computer
          programs, printouts and other computer materials and records, data
          bases, software, information and other property relating to, used or
          useful in connection with, the Grantor's business;

               (d)  to the extent not otherwise included in any of the classes
          or categories enumerated above, all equipment, inventory, documents,
          instruments, securities and chattel paper (as each of such terms is
          defined in the UCC) relating to the Collateral;

               (e)  all personal property of whatever kind or nature whatsoever,
          including personal property used in the operation of the Grantor's
          business, or in any way related to the land on which the System is
          located, the System or any other improvements on such land, whether
          located on or in, affixed to, or attached to such land or improvements
          or otherwise related thereto or arising therefrom, and whether
          tangible or intangible, direct or indirect, fully matured or
          contingent, and all extensions, additions, improvements, betterments,
          renewals, substitutions, and replacements to or of any of the
          foregoing;

               (f)  to the extent not otherwise included in any of the foregoing
          classes or categories of personal property, all proceeds (including
          all proceeds as defined in the UCC and all cash and non-cash proceeds
          as referred to in Section 552 of the

                                       4
<PAGE>

          United States Bankruptcy Code), products, offspring and profits of or
          from any of the foregoing;

               (g)  all motor vehicles and all rights under equipment leases and
          all bills of lading and warehouse receipts relating to the Collateral;
          and

               (h)  any and all additions and accessions to the Collateral, and
          all proceeds thereof, including proceeds of the conversion, voluntary
          or involuntary, of any of the foregoing into cash or liquidated
          claims, including all awards, all insurance proceeds, including any
          unearned premiums or refunds of premiums on any insurance policies
          covering all or any part of the Collateral and the right to receive
          and apply the proceeds of any insurance, or of any judgments or
          settlements made in lieu thereof for damage to or diminution of the
          Collateral;

     provided, however, that with respect to the Contracts, any such Contract
shall only be deemed to be and treated as Collateral if (i) such Contract or
Related Contract (other than Receivables) may lawfully be assigned (whether as
an outright assignment or as collateral security) to the Administrative Agent,
for the benefit of the Secured Parties, and (ii) the granting of a security
interest in and lien on, all of the Grantor's right, title and interest in such
Contract or Related Contract (other than Receivables) will not conflict with,
result in a breach of or constitute (with due notice or lapse of time or both) a
default under such Contract or Related Contract (other than Receivables); and
provided, further, that with respect to the Permits, any such Permit shall only
be deemed to be and treated as Collateral if such Permit may lawfully be
assigned (whether as an outright assignment or as collateral security) to the
Administrative Agent, for the benefit of the Secured Parties.

     Section 2.2    Financing Statements.
                    ---------------------

     The Grantor shall execute all financing statements, continuation
statements, assignments, certificates, and other documents and instruments with
respect to the Collateral pursuant to the UCC and otherwise as may be necessary
or reasonably requested by the Administrative Agent to perfect or from time to
time to publish notice of, or continue or renew the security interests granted
hereby (including, such financing statements, continuation statements,
certificates, and other documents as may be necessary or reasonably requested to
perfect a security interest in any additional property or rights hereafter
acquired by the Grantor or in any replacements, products or proceeds thereof),
in each case in form and substance satisfactory to the Administrative Agent. The
Grantor will pay the cost of filing the same in all public offices where filing
is necessary or reasonably requested by the Administrative Agent and will pay
any and all recording, transfer or filing taxes that may be due in connection
with any such filing. The Grantor grants the Administrative Agent the right, at
any time and at the Administrative Agent's option, and at the Grantor's expense,
to file any or all such financing statements, continuation statements, and other
documents pursuant to the UCC and otherwise as the Administrative Agent
reasonably may deem necessary or desirable.

     Section 2.3    Injury to Collateral.
                    ---------------------

                                       5
<PAGE>

     No injury to, or loss or destruction of, the Collateral or any part thereof
shall relieve the Grantor of any of the Obligations.

     Section 2.4    Continuing Security Interest; Transfer of Notes.
                    ------------------------------------------------

     This Security Agreement shall create a continuing security interest in the
Collateral and shall

                    (a)  remain in full force and effect until payment in full
          in cash of all Obligations, the termination of all Interest Rate
          Agreements to which any Secured Party is a party and the termination
          of all Commitments,

                    (b)  be binding upon the Grantor, its successors,
          transferees and assigns, and

                    (c)  inure, together with the rights and remedies of the
          Administrative Agent hereunder, to the benefit of the Administrative
          Agent and each other Secured Party.

Without limiting the generality of the foregoing clause (c), any Lender may
assign or otherwise transfer (in whole or in part) any Note, Loan or Commitment
held by it as well as any interest in any other Obligations to any other Person
or entity as permitted by, and  in accordance with the terms of the Credit
Agreement, and such other Person or entity shall thereupon become vested with
all the rights and benefits in respect thereof granted to such Lender under any
Loan Document (including this Security Agreement) or otherwise, subject,
however, to any contrary provisions in such assignment or transfer, and to the
provisions of Sections 9.1 and 9.16 of the Credit Agreement.
              ------------     ----

     Section 2.5    Grantor Remains Liable.
                    -------------------------

     Anything herein to the contrary notwithstanding:

                    (a)  the Grantor shall remain liable under the contracts and
          agreements included in the Collateral to the extent set forth therein,
          and shall perform all of its duties and obligations under such
          contracts and agreements to the same extent as if this Security
          Agreement had not been executed,

                    (b)  the exercise by the Administrative Agent of any of its
          rights hereunder shall not release the Grantor from any of its duties
          or obligations under any such contracts or agreements included in the
          Collateral, and

                    (c)  neither the Administrative Agent nor any other Secured
          Party shall have any obligation or liability under any such contracts
          or agreements included in the Collateral by reason of this Security
          Agreement, nor shall the Administrative Agent or any other Secured
          Party be obligated to perform any of

                                       6
<PAGE>

          the obligations or duties of the Grantor thereunder or to take any
          action to collect or enforce any claim for payment assigned hereunder.

     Section 2.6    Security Interest Absolute.
                    ---------------------------

     All rights of the Administrative Agent and the security interests granted
to the Administrative Agent hereunder, and all obligations of the Grantor
hereunder, shall be absolute and unconditional irrespective of:

               (a)  any lack of validity, legality or enforceability of the
     Credit Agreement, any Note, or any other Loan Document or any Interest Rate
     Agreement;

               (b)  the failure of any Secured Party or any holder of any Note

                         (i)  to assert any claim or demand or to enforce any
                    right or remedy against the Borrower, any other Loan Party
                    or any other Person (including any other grantor) under the
                    provisions of the Credit Agreement, any Note, any other Loan
                    Document, any Interest Rate Agreement or otherwise, or

                         (ii) to exercise any right or remedy against any other
                    grantor of, or collateral securing, any of the Obligations;

          (c)  any change in the time, manner or place of payment of, or in any
     other term of, all or any of the Obligations, or any other extension or
     renewal of any Obligation of the Borrower or any other Loan Party;

          (d)  any reduction, limitation, impairment or termination of any of
     the Obligations for any reason other than the written agreement of the
     Secured Parties to terminate the Obligations in full, including any claim
     of waiver, release, surrender, alteration or compromise, and shall not be
     subject to, and the Grantor hereby waives any right to or claim of, any
     defense or setoff, counterclaim, recoupment or termination whatsoever by
     reason of the invalidity, illegality, nongenuineness, irregularity,
     compromise, unenforceability of, or any other event or occurrence
     affecting, any Obligations of the Borrower, any other Loan Party or
     otherwise;

          (e)  any amendment to, rescission, waiver, or other modification of,
     or any consent to departure from, any of the terms of the Credit Agreement,
     any Note, any other Loan Document or any Interest Rate Agreement;

          (f)  any addition, exchange, release, surrender or non-perfection of
     any collateral, or any amendment to or waiver or release or addition of, or
     consent to departure from, any other security interest held by any Secured
     Party or any holder of any Note securing any of the Obligations; or

                                       7
<PAGE>

          (g)  any other circumstance which might otherwise constitute a defense
     available to, or a legal or equitable discharge of, the Borrower, any other
     Loan Party, any surety or any grantor.

     Section 2.7    Postponement of Subrogation.
                    ----------------------------

     The Grantor agrees that it will not exercise any rights which it may
acquire by way of rights of subrogation under this Security Agreement, by any
payment made hereunder or otherwise, until the prior payment in full in cash of
all of the Obligations, the termination of all Interest Rate Agreements to which
any Secured Party is a party and the termination of all Commitments.  Any amount
paid to the Grantor on account of any such subrogation rights prior to the
payment in full in cash of all of the Obligations shall be held in trust for the
benefit of the Secured Parties and each holder of a Note and shall immediately
be paid to the Administrative Agent for the benefit of the Secured Parties and
each holder of a Note and credited and applied against the Obligations of the
Borrower and each other Loan Party, whether matured or unmatured, such order as
the Administrative Agent shall elect; provided,  however, that if
                                      --------   -------

          (a)  the Grantor has made payment to the Secured Parties and each
     holder of a Note of all or any part of the Obligations, and

          (b)  all Obligations have been paid in full in cash, all Interest Rate
     Agreements to which any Secured Party is a party have been terminated and
     all Commitments have been permanently terminated,

then, at the Grantor's request, the Administrative Agent, on behalf of the
--------------------
Secured Parties and the holders of the Notes, will execute and deliver to the
Grantor appropriate documents (without recourse and without representation or
warranty) necessary to evidence the transfer by subrogation to the Grantor of an
interest in the Obligations resulting from such payment by the Grantor.  In
furtherance of the foregoing, for so long as any Obligations or Commitments
remain outstanding, the Grantor shall refrain from taking any action or
commencing any proceeding against the Borrower or any other Loan Party (or any
of its or their successors or assigns, whether in connection with a bankruptcy
proceeding or otherwise) to recover any amounts in respect of payments made
under this Security Agreement to any Secured Party or any holder of a Note,
except that the Grantor may file a proof of claim in a bankruptcy proceeding
with respect to the Borrower or any other Loan Party in connection with any
obligations owed by such Loan Party to the Grantor in the event that the
Administrative Agent has failed to file a proof of claim on the Grantor's behalf
by the second business day before the due date for such filing.

                                 ARTICLE III.
                   REPRESENTATIONS AND WARRANTIES OF GRANTOR

     The Grantor hereby represents and warrants to the Administrative Agent and
the other Secured Parties that:

     Section 3.1    Title to Collateral.
                    -------------------

                                       8
<PAGE>

     The Grantor is the sole owner of, and has good, valid, and marketable title
to, the Collateral, free from all Liens other than Permitted Liens, and the
Grantor has full right and power to grant the Administrative Agent for the
benefit of the Secured Parties a lien thereon and a security interest therein.

     Section 3.2    Security Interest.
                    ------------------

     The execution and delivery of this Security Agreement creates a good and
valid lien on and security interest in the Collateral.

     Section 3.3    Negotiable Documents, Instruments and Chattel Paper.
                    ----------------------------------------------------

     The Grantor has, contemporaneously with the execution and delivery of this
Security Agreement, delivered to the Administrative Agent possession of all
originals of all negotiable documents, instruments and chattel paper, including
all negotiable documents, instruments and chattel paper evidencing Receivables,
currently owned or held by the Grantor (duly endorsed in blank, if requested by
the Administrative Agent).

     Section 3.4    Authorization, Approval, Etc.
                    -----------------------------

     Except as have been obtained or made and are in full force and effect, no
authorization, approval or other action by, and no notice to or filing with, any
Governmental Instrumentality, regulatory body or any other Person is required
for the grant by the Grantor of the security interest granted hereby or for the
execution, delivery and performance of this Security Agreement by the Grantor or
for the exercise by the Administrative Agent of the rights provided for in this
Security Agreement, other than those authorizations, approvals, actions, notices
or filings set forth in Schedule 3.4 hereto.
                        ------------

     Section 3.5    Bankruptcy Matters.
                    -------------------

     The Grantor has not made a general assignment for the benefit of creditors,
filed any voluntary petition in bankruptcy or suffered the filing of an
involuntary petition by its creditors, suffered the appointment of a receiver to
take possession of all or substantially all of its assets, suffered the
attachment or other judicial seizure of all or substantially all of its assets,
admitted its inability to pay its debts as they come due, or made an offer of
settlement, extension or composition to its creditors generally.

     Section 3.6    Representations and Warranties under the Credit Agreement.
                    ----------------------------------------------------------

     The representations and warranties of or pertaining to the Grantor
contained in the Credit Agreement and each of the other Loan Documents are true
and correct as of the date hereof to the same extent as if set forth in full
herein.

     Section 3.7    Location of Collateral.
                    -----------------------

     The Grantor's principal place of business and chief executive office is
located at 570 Lexington Avenue, New York, New York 10022.

                                       9
<PAGE>

                                  ARTICLE IV.
                             COVENANTS OF GRANTOR

     Section 4.1    General Covenants Relating to Collateral.
                    -----------------------------------------

     Until all Obligations have been paid and performed in full, all Commitments
have been terminated and all Interest Rate Agreements to which any Secured Party
is a party have been terminated, the Grantor hereby covenants that, unless the
Administrative Agent, acting pursuant to the Credit Agreement, otherwise
consents in advance in writing:

     Section 4.1.1  Collateral.
                    ----------

     The Grantor shall (a) execute and deliver any and all documents, or cause
the execution and delivery of any and all documents, necessary to create,
perfect, preserve, validate or otherwise protect the Administrative Agent's lien
on and security interest in the Collateral and the priority thereof, (b)
maintain, or cause to be maintained, at all times the Administrative Agent's
lien on and security interest in the Collateral and the priority thereof, (c)
promptly upon learning thereof, report to the Administrative Agent any matters
that could reasonably be expected to materially and adversely affect the value
or enforceability or collectibility of any of the Collateral, (d) defend the
Collateral and the Administrative Agent's interests therein against all claims
and demands of all persons at any time claiming the same or any interest therein
adverse to the Administrative Agent and pay all costs and expenses (including,
reasonable attorneys' fees and charges) incurred in connection with such
defense, and (e) at the Grantor's sole cost and expense, settle any and all such
claims and disputes and indemnify and protect the Administrative Agent against
any liability, loss, cost or expense (including, reasonable attorneys' fees and
charges), arising therefrom or out of any matter affecting any of the Collateral
(provided, however, that if the Administrative Agent shall so elect after the
 --------  -------
occurrence and during the continuation of an Event of Default hereunder, the
Administrative Agent shall have the right at all times to settle, compromise,
adjust or liquidate all claims or disputes directly with the Grantor or any
obligor of the Grantor upon such terms and conditions as the Administrative
Agent reasonably deems advisable, and to charge all costs and expenses thereof
(including, reasonable attorneys' fees and charges) to the Grantor's account and
to add them to the Obligations, whereupon such costs and expenses shall be and
become part of the Obligations).

     Section 4.1.2  No Change in Places of Business or Collateral.
                    ---------------------------------------------

     The Grantor shall (a) maintain its places of business and its chief
executive office only in the locations set forth in Section 3.7, and in such
                                                    -----------
other places as the Grantor may select, provided that the Grantor shall provide
to the Administrative Agent at least twenty (20) days' prior written notice of
any changes in or additions to the locations of the Grantor's places of business
or of the Grantor's principal office and shall make all filings in the
applicable jurisdictions necessary to maintain the Administrative Agent's lien
on and security interest in the Collateral and the priority thereof in
connection with any such changes or additions, (b) keep, store and maintain the
Collateral only at the foregoing locations, and (c) keep and maintain the
records and books of account relating to the Collateral only at the Grantor's
chief executive office.

                                       10
<PAGE>

     Section 4.1.3  No Impairment.
                    -------------

     The Grantor shall not take or permit to be taken any action in connection
with the Collateral which would impair in any material respect the value of the
interests or rights of the Grantor therein or which would impair the interests
or rights of the Administrative Agent therein or with respect thereto.

     Section 4.2    Sale of Assets.
                    ---------------

     Sell, lease, assign, pledge, transfer or otherwise dispose of any of the
Collateral, whether now owned or hereafter acquired, except as expressly
permitted by the Credit Agreement.

                                  ARTICLE V.
                RIGHTS AND REMEDIES OF THE ADMINISTRATIVE AGENT

     Section 5.1    Miscellaneous Rights of the Administrative Agent.
                    -------------------------------------------------

     Upon the occurrence and during the continuation of an Event of Default, the
Administrative Agent shall have the right: (i) to declare all of the monetary
Obligations to be immediately due and payable, whereupon all such Obligations
shall become immediately due and payable without presentment, demand, notice of
dishonor, protest or further notice of any kind, all of which are hereby
expressly waived by the Grantor, anything contained herein to the contrary
notwithstanding; (ii) to exercise any one or more of the rights and remedies
exercisable by the Administrative Agent under any other provisions of this
Security Agreement, or any other related agreement, or exercisable by a secured
party under the UCC or under any other applicable law; and (iii) to exercise, in
the name of the Grantor or in the name of the Administrative Agent, such rights
and powers with respect to the Collateral as the Grantor might exercise,
including, the right to:

               (a)  enter into any extension, reorganization, deposit, merger,
          consolidation or other agreement pertaining to, or deposit, surrender,
          accept, hold or apply other property in exchange for, the Collateral
          or any part thereof;

               (b)  insure, process, and preserve the Collateral;

               (c)  transfer the Collateral or any part thereof to the name of
          the Administrative Agent or to the name of the Administrative Agent's
          nominee;

               (d)  receive, open, and dispose of mail addressed to the Grantor
          relating to the Collateral or any part thereof;

               (e)  collect and endorse, receive, and give receipts for all
          dividends, interest, rent, payments, proceeds, and other sums and
          property now or hereafter payable on or on account of the Collateral
          or any part thereof or on account of its sale or lease;

                                       11
<PAGE>

               (f)  initiate, pursue, compromise, settle or withdraw any claims,
          suits or proceedings pertaining to the Collateral or any part thereof
          or to any interest, rent or other payment on or on account of the
          Collateral or any part thereof or on account of its sale or lease;

               (g)  take possession of and endorse in the name of the Grantor or
          in the name of the Administrative Agent, for the account of the
          Grantor, any bills of exchange, checks, drafts, money orders, notes or
          any other chattel paper, documents or instruments constituting all or
          any part of the Collateral or received as interest, rent or other
          payment on or on account of the Collateral or any part thereof or on
          account of its sale or lease;

               (h)  appoint another (who may be an employee, officer or other
          representative of the Administrative Agent) to do any of the foregoing
          on behalf of the Administrative Agent;

               (i)  execute (in the name, place and stead of the Grantor)
          endorsements, assignments and other instruments of conveyance or
          transfer with respect to all or any of the Collateral; and

               (j)  take any other action which the Administrative Agent deems
          necessary or desirable to protect or realize upon its security
          interest in the Collateral or any part thereof, and the Grantor hereby
          irrevocably appoints the Administrative Agent as the Grantor's
          attorney-in-fact to take any such action, including the execution and
          delivery of any and all documents or instruments related to the
          Collateral or any part thereof in the Grantor's name, and said
          appointment shall create in the Administrative Agent a power coupled
          with an interest which shall be irrevocable.

     Section 5.2    Right of the Administrative Agent to Take Possession and
                    --------------------------------------------------------
Foreclose.
----------

     Upon the occurrence and during the continuation of any Event of Default,
the Administrative Agent shall have the right and power to take possession of
the Collateral and of any and all books of account and records of the Grantor
relating to any of the Collateral, the right to place the Administrative Agent's
representatives upon any premises on which the Collateral or any part thereof or
any such books of account or records may be situated with full power to remove
the same therefrom, and the right to exclude the Grantor and all persons
claiming under the Grantor from any access to the Collateral or to any part
thereof, and the Administrative Agent and such representatives are hereby
granted the irrevocable license to enter upon such premises for such purpose.
The Administrative Agent may require the Grantor to assemble the Collateral or
any part thereof and to make the same (to the extent the same is moveable)
available to the Administrative Agent at a place to be designated by the
Administrative Agent which is reasonably convenient to the Grantor and the
Administrative Agent.  The Administrative Agent may render the Collateral or any
part thereof unusable without removing the same from the premises on which it
may be situated, and may sell the same on the premises of the Grantor if such
Collateral or part thereof is situated thereon.  The Administrative Agent may
make formal

                                       12
<PAGE>

application for the transfer of all of the Grantor's permits, licenses,
approvals, and the like relating to the Collateral or to the Grantor's business
to the Administrative Agent or to any assignee of the Administrative Agent or to
any purchaser of any of the Collateral to the extent the same are assignable in
accordance with their terms and applicable law. Unless the Collateral is
perishable or threatens to decline speedily in value or is of a type customarily
sold on a recognized market, the Administrative Agent will give the Grantor at
least ten (10) days' prior written notice of the time and place of any public
sale thereof or of the time after which any private sale or any other intended
disposition thereof is to be made, which notice shall constitute reasonable
notice. In addition to exercising the foregoing rights, the Administrative Agent
may, to the extent permitted by law, arrange for and conduct the sale of the
Collateral at a public or private sale, as the Administrative Agent may elect,
which sale may be conducted by an employee or representative of the
Administrative Agent, and any such sale shall be considered or deemed to be a
sale made in a commercially reasonable manner. The Administrative Agent may
release, temporarily or otherwise, to the Grantor any item of Collateral of
which the Administrative Agent has taken possession pursuant to any right
granted to the Administrative Agent by this Security Agreement without waiving
any rights granted to the Administrative Agent under this Security Agreement,
the Credit Agreement, or the other Loan Documents or any other agreement related
hereto or thereto. The Grantor, in dealing with or disposing of the Collateral
or any part thereof, hereby waives all rights, legal and equitable, it may now
or hereafter have to require marshaling of assets or to require, upon
foreclosure, sales of assets in a particular order. Each successor and assign of
the Grantor, including a holder of a lien subordinate to the lien created hereby
(without implying that the Grantor has, except as expressly provided herein, a
right to grant an interest in, or a subordinate lien on, any of the Collateral),
by acceptance of its interest or lien agrees that it shall be bound by the above
waiver, to the same extent as if such holder gave the waiver itself. The Grantor
also hereby waives, to the full extent it may lawfully do so, the benefit of all
laws providing for rights of appraisal, valuation, stay or extension or of
redemption after foreclosure now or hereafter in force.

     Section 5.3    Right of the Administrative Agent to Collect and Service
                    --------------------------------------------------------
Accounts.
---------

     Upon the occurrence and during the continuation of any Event of Default,
and the acceleration of any Loan in accordance with Article VII of the Credit
                                                    -----------
Agreement, the Administrative Agent may notify or may require the Grantor to
notify any person or entity obligated to the Grantor under any account for
monies due or to become due forming all or any part of the Collateral, whether
now existing or hereafter acquired, that the same has been assigned to the
Administrative Agent and that such obligor should make payment or performance of
its obligations under such account directly to the Administrative Agent, and the
Administrative Agent may take possession of and exercise control over all
proceeds of any such account in the Grantor's possession or otherwise, and may
take any other action which the Administrative Agent deems necessary or
desirable to collect any such account or the proceeds thereof.  To evidence the
Administrative Agent's rights hereunder, the Grantor shall, at the Grantor's
expense, execute such assignments or endorsements of any such account, or of the
proceeds thereof, as the Administrative Agent may request.

                                       13
<PAGE>

     Section 5.4    Right of the Administrative Agent to Use, Operate and
                    -----------------------------------------------------
Maintain Collateral.
--------------------

     Section 5.4.1  Rights of the Administrative Agent.
                    ----------------------------------

     Upon the Administrative Agent's taking possession of all or any part of the
Collateral in accordance with the terms of this Security Agreement or otherwise,
the Administrative Agent shall have the right to hold, store, and/or use,
operate, manage, and control the same. Upon any such taking of possession, the
Administrative Agent may (but shall not be obligated to), from time to time, at
the expense of the Grantor, make all such repairs, replacements, alterations,
additions, and improvements to and of all or any of the Collateral as the
Administrative Agent may deem proper. In any such case the Administrative Agent
shall have the right to exercise all rights and powers of the Grantor in respect
of the Collateral or any part thereof as the Administrative Agent shall deem
proper, including the right to enter into any and all such agreements with
respect to the leasing and/or operation of the Collateral or any part thereof as
the Administrative Agent may see fit; and the Administrative Agent shall be
entitled to collect and receive all rents, issues, profits, fees, revenues, and
other income of the same and every part thereof.

     Section 5.4.2  The Administrative Agent Has No Duty.
                    ------------------------------------

     The powers conferred on the Administrative Agent hereunder are solely to
protect its interest in the Collateral and shall not impose any duty on it to
exercise any such powers.  Except for the reasonable care and preservation of
any Collateral in its possession and the accounting for moneys actually received
by it hereunder, the Administrative Agent shall have no duty as to any
Collateral or as to the taking of any necessary steps to preserve rights against
prior parties or any other rights pertaining to any Collateral.

     Section 5.5    Right of the Administrative Agent to Appoint Receiver.
                    ------------------------------------------------------

     Upon the occurrence and during the continuation of any Event of Default,
the Administrative Agent shall, as a matter of right and without any requirement
of notice, to the extent permitted under applicable law, be entitled to appoint
a receiver for all or any part of the Collateral, whether such receivership be
incidental to a proposed sale of the Collateral or otherwise.  All disbursements
made by the receiver under this Section 5.5 and the expenses of receivership
                                -----------
shall be added to and be a part of the Obligations, and, whether or not said
principal sum, including such disbursements and expenses, exceeds the
indebtedness originally intended to be secured hereby, the entire amount of said
sum, including such disbursements and expenses, shall be secured by this
Security Agreement and shall be due and payable upon demand therefor and
thereafter shall bear interest at the rate set forth in Section 2.3.E of the
                                                        -------------
Credit Agreement.

     Section 5.6    Remedies Cumulative; Delay Not Waiver.
                    --------------------------------------

     The rights and remedies of the Administrative Agent under the Credit
Agreement, this Security Agreement, the other Loan Documents, or any other
related agreement are cumulative and shall in no way affect, or deprive the
Administrative Agent of, or be deemed to constitute a

                                       14
<PAGE>

waiver by the Administrative Agent of any other rights or remedies allowed to
the Administrative Agent at law or in equity. No notice to or demand on the
Grantor in any case shall entitle the Grantor to any other notice or demand in
similar or other circumstances and the exercise of any one remedy shall not
impair the Administrative Agent's right simultaneously or at any time or in any
order to exercise any other remedy nor shall the exercise of any remedy in one
case impair or otherwise affect the Administrative Agent's right or ability to
exercise such remedy contemporaneously or again in the same case or in any other
case. No failure or delay by the Administrative Agent in exercising any right or
power hereunder shall operate as a waiver thereof, nor shall any single or
partial exercise of any such right or power, or any abandonment or
discontinuance of steps to enforce such a right or power, preclude any other or
further exercise thereof or the exercise of any other right or power.

     Section 5.7    Waiver of Rights.
                    -----------------

     To the extent permitted under applicable law, the Grantor waives all rights
and remedies of a debtor or grantor under the UCC or other applicable law, and
all formalities prescribed by law relative to the sale or disposition of the
Collateral (other than notice of sale) after the occurrence and during the
continuation of an Event of Default and all other rights and remedies of the
Grantor with respect thereto.  In exercising its right to take possession of the
Collateral upon the occurrence and during the continuation of an Event of
Default hereunder, the Administrative Agent, personally or by its agents or
attorneys, and subject to the rights of any tenant under any lease or sublease
of the Collateral, to the fullest extent permitted by law, may enter upon any
land owned or leased by the Grantor without being guilty of trespass or any
wrongdoing, and without liability for damages thereby occasioned.  In the event
the Administrative Agent elects to proceed with respect to the Collateral,
separately from any real property, the Administrative Agent shall give the
Grantor at least ten (10) days' notice of the sale of the Collateral, which
shall for all purposes be deemed to be commercially reasonable.

     Section 5.8    Compliance with Restrictions.
                    -----------------------------

     The Grantor agrees that in any sale of any of the Collateral whenever an
Event of Default shall have occurred and be continuing, the Administrative Agent
is hereby authorized to comply with any limitation or restriction in connection
with such sale as it may be advised by counsel is necessary in order to avoid
any violation of applicable law, or in order to obtain any required approval of
the sale or of the purchaser by any governmental regulatory authority or
official, and the Grantor further agrees that such compliance shall not result
in such sale being considered or deemed not to have been made in a commercially
reasonable manner, nor shall the Administrative Agent be liable or accountable
to the Grantor for any discount allowed by reason of the fact that such
Collateral is sold in compliance with any such limitation or restriction.

     Section 5.9    Distribution of Proceeds.
                    -------------------------

     All cash proceeds received by the Administrative Agent in respect of any
sale of, collection from, or other realization upon all or any part of the
Collateral may, in the discretion of the Administrative Agent, be held by the
Administrative Agent as collateral for, and/or then or at any time thereafter
applied (after payment of any amounts payable to the Administrative Agent

                                       15
<PAGE>

pursuant to Section 6.2) in whole or in part by the Administrative Agent for the
            -----------
ratable benefit of the Secured Parties against, all or any part of the
Obligations in such order as the Administrative Agent shall elect.  The Grantor
shall remain liable for any deficiency.  Any surplus of such cash or cash
proceeds held by the Administrative Agent and remaining after payment in full in
cash of all the Obligations, the termination of all Interest Rate Agreements to
which a Secured Party is a party and the termination of all Commitments shall be
paid over to the Grantor or to whomsoever may be lawfully entitled to receive
such surplus.

                                  ARTICLE VI.
                           MISCELLANEOUS PROVISIONS

     Section 6.1    Additional Actions and Documents.
                    ---------------------------------

     The Grantor agrees that at any time, and from time to time, at the expense
of the Grantor, the Grantor will promptly execute and deliver all further
instruments, and take all further action that the Administrative Agent may
reasonably request, in order to perfect and protect any security interest
granted or purported to be granted hereby or to enable the Administrative Agent
to exercise and enforce its rights and remedies hereunder with respect to any
Collateral. Nothing in this Section 6.1 shall be construed as limiting the
                            -----------
Obligations of the Borrower under Section 5.11 of the Credit Agreement.
                                  ------------

     Section 6.2    Expenses.
                    ---------

     The Grantor agrees to reimburse and save the Administrative Agent harmless
against liability for the payment of all out-of-pocket expenses arising in
connection with the administration or enforcement of, or the preservation or
exercise of, any rights (including the right to collect and dispose of the
Collateral) under, this Security Agreement, including the reasonable fees and
other charges of outside counsel to the Administrative Agent arising in such
connection, and all such fees and other charges shall be added to the Grantor's
obligations secured hereby.

     Section 6.3    Notices.
                    --------

     Any communications between the parties hereto or notices provided herein to
be given shall be sent in accordance with the provisions of, and to the
addresses set forth in, Section 9.8 of the Credit Agreement, and if to the
                        -----------
Grantor, to the following address:

                         FiberNet Holdco, Inc.
                         570 Lexington Avenue
                         3/rd/ Floor
                         New York, New York  10022
                         Attn.: President
                         Telecopy: (212) 421-8920

     Section 6.4    Release and Satisfaction.
                    -------------------------

                                       16
<PAGE>

     Upon the indefeasible payment (whether in cash and/or other consideration
which is satisfactory to the Lenders in their sole discretion) and performance
in full of the Obligations, the termination of all Commitments and the
termination of all Interest Rate Agreements to which any Secured Party is a
party, (i) this Security Agreement and the security interest created hereby
shall terminate, and (ii) upon written request of the Grantor, the
Administrative Agent shall execute and deliver to the Grantor, at the Grantor's
expense and without representation or warranty by or recourse to the
Administrative Agent or the Secured Parties, releases and satisfactions of all
financing statements, mortgages, notices of assignment and other registrations
of security, and the Grantor shall deliver to the Administrative Agent a general
release of all of the Administrative Agent's liabilities and obligations under
all Loan Documents and an acknowledgment that the same have been terminated.

     Section 6.5    Benefit.
                    --------

     This Security Agreement shall be binding upon, and shall inure to the
benefit of, the parties hereto and the Secured Parties and their respective
successors, legal representatives and permitted assigns. The Grantor shall not
assign any of its rights or obligations hereunder without the prior written
consent of the Required Lenders.

     Section 6.6    Amendments and Waivers.
                    -----------------------

     No amendment, modification, termination or waiver of any provision of this
Security Agreement, or consent to any departure by the Administrative Agent
therefrom, shall be effective unless the same shall be in writing and signed by
the Administrative Agent and the Grantor and shall comply with the provisions
set forth in Section 9.6 of the Credit Agreement.  Each amendment, modification,
             -----------
termination or waiver shall be effective only in the specific instance and for
the specific purpose for which it was given.

     Section 6.7    Headings.
                    ---------

     Section and subsection headings contained in this Security Agreement are
inserted for convenience of reference only, shall not be deemed to be a part of
this Security Agreement for any purpose, and shall not in any way define or
affect the meaning, construction or scope of any of the provisions hereof.

     Section 6.8    Applicable Law; Entire Agreement.
                    ---------------------------------

     This Security Agreement shall be governed by, and shall be construed and
enforced in accordance with, the internal laws of the State of New York, without
regard to conflicts of laws principles (other than Sections 5-1401 and 5-1402 of
the General Obligations Law of the State of New York), except to the extent that
the validity or perfection of the security interest hereunder, or exercise of
remedies hereunder, in respect of any particular Collateral are governed by the
laws of a jurisdiction other than the State of New York. This Security Agreement
and the other Loan Documents constitute the entire understanding among the
parties hereto with respect to the subject matter hereof and supersede any prior
agreements, written or oral, with respect thereto.

                                       17
<PAGE>

     Section 6.9    Severability.
                    -------------

     The invalidity, illegality or unenforceability in any jurisdiction of any
provision in or obligation under this Security Agreement shall not affect or
impair the validity, legality or enforceability of the remaining provisions or
obligations under this Security Agreement or of such provision or obligation in
any other jurisdiction.

     Section 6.10   Consent to Jurisdiction.
                    ------------------------

     The Grantor hereby irrevocably and unconditionally submits, for itself and
its property, to the nonexclusive jurisdiction of the Supreme Court of the State
of New York sitting in New York County and of the United States District Court
of the Southern District of New York, and any appellate court in respect
thereof, in any action or proceeding arising out of or relating to this Security
Agreement, or for recognition or enforcement of any judgment, and hereby
irrevocably and unconditionally agrees that all claims in respect of any such
action or proceeding may be heard and determined in such New York State court
or, to the extent permitted by law, in such federal court.  The Grantor agrees
that a final judgment in any such action or proceeding shall be conclusive and
may be enforced in other jurisdictions by suit on the judgment or in any other
manner provided by law. Nothing in this Section 6.10 shall affect any right that
                                        ------------
the Administrative Agent or any Secured Party may otherwise have to bring any
action or proceeding relating to this Security Agreement against the Grantor or
any of its properties in the courts of any jurisdiction.  The Grantor hereby
irrevocably and unconditionally waives, to the fullest extent it may legally and
effectively do so, any objection which it may now or hereafter have to the
laying of venue of any suit, action or proceeding arising out of or relating to
this Security Agreement in any court referred to in this Section 6.10. The
                                                         ------------
Grantor irrevocably waives, to the fullest extent permitted by law, the defense
of an inconvenient forum to the maintenance of such action or proceeding in any
such court. The Grantor irrevocably consents to service of process in the manner
provided for notices in Section 6.3. Nothing in this Security Agreement will
                        -----------
affect the right of any party hereto to serve process in any other manner
permitted by law.

     Section 6.11   Construction.
                    -------------

     The Grantor and the Administrative Agent each acknowledges that it has had
the benefit of legal counsel of its own choice and has been afforded an
opportunity to review this Security Agreement with its legal counsel and that
this Security Agreement shall be construed as if jointly drafted by the Grantor
and the Administrative Agent.

     Section 6.12   Counterparts; Effectiveness.
                    ----------------------------

     This Security Agreement and any amendments, waivers, consents, or
supplements may be executed in any number of counterparts and by different
parties hereto in separate counterparts, each of which when so executed and
delivered shall be deemed an original, but all of which counterparts together
shall constitute but one and the same instrument.  This Security Agreement shall
become effective upon the execution of a counterpart hereof by each of the
parties hereto.  Delivery of an executed counterpart of a signature page to this
Security Agreement or to any

                                       18
<PAGE>

amendments, waivers, consents or supplements hereof by telecopier shall be as
effective as delivery of a manually executed counterpart thereof.

     Section 6.13   Waiver of Jury Trial.
                    -----------------------

     THE GRANTOR HEREBY WAIVES, TO THE FULLEST EXTENT PERMITTED BY APPLICABLE
LAW, ANY RIGHT IT MAY HAVE TO A TRIAL BY JURY IN ANY LEGAL PROCEEDING DIRECTLY
OR INDIRECTLY ARISING OUT OF OR RELATING TO THIS SECURITY AGREEMENT OR THE
TRANSACTIONS CONTEMPLATED HEREBY (WHETHER BASED ON CONTRACT, TORT OR ANY OTHER
THEORY).

     Section 6.14   Certain References.
                    -------------------

     Upon consummation of the Transaction, all references herein to the
"Grantor" shall be deemed to be references to FiberNet Telecom Group, Inc
(formerly known as FiberNet Holdco, Inc.).
OP

                 [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

                                       19
<PAGE>

     IN WITNESS WHEREOF, each of the parties hereto has caused this Subsidiary
Security Agreement to be duly executed and delivered in its name and on its
behalf, all as of the day and year first above written.

                              FIBERNET HOLDCO, INC.,
                              as the Grantor

                              By: /s/ Michael S. Liss
                                  __________________________________
                                  Name:   Michael S. Liss
                                  Title:  Chief Executive Officer and President

                              DEUTSCHE BANK AG NEW YORK
                              BRANCH, as the Administrative Agent

                              By: /s/ Alexander Richarz
                                  __________________________________
                                  Name:   Alexander Richarz
                                  Title:  Assistant Vice President

                              By: /s/ Kelvin Cheng
                                  __________________________________
                                  Name:   Kelvin Cheng
                                  Title:  Associate

                                      S-1
<PAGE>

                                 SCHEDULE 3.4

                             GOVERNMENTAL CONSENTS
                             ---------------------

None.

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