Exhibit 10.80

This AMENDMENT, dated as of December 19, 2005 (this “Nineteenth Amendment”), is
among FIBERNET TELECOM GROUP, INC., a Delaware corporation (the “Parent”),
FIBERNET OPERATIONS, INC., a Delaware corporation (“FiberNet”), DEVNET L.L.C., a
Delaware limited liability company (together with FiberNet, the “Borrowers”),
the financial institutions party to the Credit Agreement (as defined below) as
lenders (collectively, the “Lenders”), and DEUTSCHE BANK AG NEW YORK BRANCH, as
administrative agent for the Lenders (in such capacity, the “Administrative
Agent”), and relates to (1) the Amended and Restated Credit Agreement, dated as
of February 9, 2001 (as amended, amended and restated, supplemented or otherwise
modified from time to time, the “Credit Agreement”), among the Borrowers, the
Lenders, the Administrative Agent, TD Securities. (USA) Inc., as syndication
agent for the Lenders, and Wachovia Investors, Inc., as documentation agent for
the Lenders, and (2) the Amended and Restated Parent Guaranty Agreement, dated
as of February 9, 2001 (as amended, amended and restated, supplemented or
otherwise modified from time to time, the “Parent Guaranty Agreement”), by the
Parent in favor of the Administrative Agent for the benefit of each of the
Secured Parties. Capitalized terms used but not defined herein shall have the
meanings given to such terms in the Credit Agreement.

RECITALS

WHEREAS, Parent has advised the Administrative Agent and the Lenders that it has
formed a new wholly-owned subsidiary named Availius, LLC, a New York limited
liability company;

WHEREAS, Parent and the Borrowers have requested certain amendments to the
Credit Agreement and a limited consent in respect of Availius, LLC, all as
provided herein, and the Administrative Agent and the Lenders have agreed, on
the terms and conditions contained herein, to amend the Credit Agreement and to
grant a limited consent on a one time basis on the terms set forth below.

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:

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ARTICLE I

AMENDMENTS TO CREDIT AGREEMENT

Section 1.01 Amendments to the Credit Agreement. The Credit Agreement is hereby
amended as follows:

(a) Section 1.1 of the Credit Agreement shall be amended by adding the following
new defined terms in the appropriate alphabetical order:

“Availius” means Availius, LLC, a New York limited liability company and a
wholly owned subsidiary of Parent.

“LOC Revolving Loan” has the meaning assigned to that term in Section 2.2.A.

“Nineteenth Amendment” means the Amendment, dated as of December 19, 2005, among
the Parent, the Borrowers, the Lenders and the Administrative Agent, which
amends the Credit Agreement.

“Nineteenth Amendment Effective Date” means the date on which the conditions
precedent to the effectiveness of the Nineteenth Amendment have been satisfied.

(b) Section 1.1 of the Credit Agreement shall be further amended by deleting the
definition of “Business Day” and adding the following new definition of
“Business Day” in the appropriate alphabetical order:

“Business Day” means (i) for all purposes other than as covered by clauses
(ii) below, any day excluding Saturday, Sunday and any day which is a legal
holiday under the laws of the State of New York or is a day on which banking
institutions located in the State of New York are authorized or required by law
or other governmental action to close, and (ii) with respect to all notices,
determinations, fundings and payments in connection with Letters of Credit, this
definition also excludes any day on which banking institutions are authorized or
required by law or other governmental action to close in the jurisdiction in
which the Issuing Bank for such Letter of Credit is located.

(c) Article V of the Credit Agreement is hereby amended by adding the following
paragraph immediately at the end of Section 5.12:

“Notwithstanding anything to the contrary contained in this Agreement or any
Collateral Documents, (a) no proceeds of the Loans under the Credit Agreement
may be provided to, used by or used for or on behalf of, Availius; and (b) no
Letters of Credit may be issued to or for the benefit of, Availius, renewed or
extended for the benefit of Availius, or assigned to any Person for the benefit
of Availius.”

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(d) Article V of the Credit Agreement is hereby amended by deleting Section 5.15
and Section 5.16 in their entirety and substituting in lieu thereof in the
appropriate numerical order the following new Section 5.15 and Section 5.16:

“Section 5.15 [RESERVED]”

“Section 5.16 [RESERVED]”

(e) Article VI of the Credit Agreement is hereby amended by adding the following
new Section 6.20 in the appropriate numerical order:

“Section 6.20 Availius. Availius shall not, nor shall Parent permit Availius to,
(i) own any assets in excess of $500,000 in the aggregate at any time; (ii) open
any deposit account or securities account (each within the meaning of the UCC);
(iii) directly or indirectly, create, incur, assume or guaranty, or otherwise
become or remain directly or indirectly liable with respect to, any
Indebtedness; (iv) directly or indirectly, create, incur, assume or permit to
exist any Lien on or with respect to any property or asset of any kind
(including any document or instrument in respect of goods or accounts
receivable), whether now owned or hereafter acquired, or any income or profits
therefrom, or file or permit the filing of, or permit to remain in effect, any
financing statement or other similar notice of any Lien with respect to such
property, asset, income or profits; (v) enter into, assume or become subject to
agreement prohibiting or otherwise restricting the creation or assumption of any
Lien upon its properties or assets for purpose of securing the Obligations,
whether now or hereafter acquired, or requiring the grant of any security for
obligations relating to such agreement if security is given for the Obligations;
(vi) directly or indirectly, make or own any Investment in any Person, including
any Joint Venture, or otherwise form or create any Subsidiary; (vii) enter into
any transaction of merger or consolidation, or liquidate, wind-up or dissolve
itself (or suffer any liquidation or dissolution), or convey, sell, lease or
sublease (as lessor or sublessor), transfer or otherwise dispose of, in one or
more transaction or a series of transactions, all or any part of its business,
property or assets, whether now owned or hereafter acquired, or acquire by
purchase or otherwise all or substantially all of the business, property or
fixed assets of, or stock or other evidence of beneficial ownership of, any
Person or any division or line of business of any Person; (viii) directly or
indirectly, sell with recourse, or discount or otherwise sell for less than the
face value thereof, any of its notes or accounts receivable; (ix) directly or
indirectly enter into or permit to exist any transaction with any holder of 5%
or more of any class of equity Securities of any Borrower, Parent or with any
Affiliate of any Borrower or Parent; and (x) declare or make Restricted Payment;
in each case, except with the prior written consent of the Administrative Agent
and the Required Lenders.”

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Section 1.02 Amendments to Parent Guaranty. Section 4.04 of the Parent Guaranty
is hereby amended by deleting the reference to “[Reserved]” therein and
substituting in lieu thereof the following new Section 4.04 in the appropriate
alphabetical order:

“Section 4.04. Availius. Parent shall not, nor shall Parent permit any
Subsidiary of Parent to, make any capital contribution to Availius (other than a
capital contribution made by Parent in such amounts as is necessary to maintain
its corporate existence), except with the prior written consent of the
Administrative Agent and the Required Lenders. In addition to, and not in lieu
of, the obligations of Parent under this Guaranty and the other Loan Documents,
Parent shall perform, comply with and be bound by all of the agreements,
covenants and obligations contained in Section 6.20 of the Credit Agreement.”

ARTICLE II

LIMITED CONSENT

Subject to the conditions, and only to the extent set forth below, the Lenders
hereby consent to the following:

(a) Notwithstanding the requirements of Section 6.5.B to the Credit Agreement to
the contrary, Borrower shall not be obligated to cause (i) Availius to execute
any Collateral Document and (ii) Parent to execute a Pledge Agreement in respect
of the equity interests of Availius, in each case, so long as Availius owns no
material assets and maintains no deposit accounts and Accounts, in each case, as
determined by the Administrative Agent and the Lenders in their sole discretion.
Upon the request of the Administrative Agent and the Lenders, the Borrower shall
cause each of Availius and Parent to comply with the requirements set forth in
Section 6.5.B to the Credit Agreement.

(b) Except for the limitations set forth in Section 5.12 and Section 6.20 of the
Credit Agreement, Availius is a “Subsidiary” of Parent for all purposes under
the Credit Agreement and the other Loan Documents, including all
representations, warranties and covenants.

ARTICLE III

MISCELLANEOUS

Section 3.01 Execution of this Nineteenth Amendment.

This Nineteenth Amendment is executed and shall be construed as an amendment to
the Credit Agreement, and, as provided in the Credit Agreement, this Nineteenth
Amendment forms a part thereof. This Nineteenth Amendment shall become effective
upon the satisfaction of each of the following conditions:

(a) the Administrative Agent shall have received a counterpart signature page to
this Amendment duly executed and delivered by the Borrowers, the Parent and each
Lender;

(b) the payment of all fees and expenses then due and payable to the
Administrative Agent; and

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(c) the payment of all unpaid fees and expenses of the Administrative Agent’s
counsel, Bingham McCutchen LLP, to the extent that copies of invoices for such
fees and expenses have been delivered to the Borrowers.

Section 3.02 Representations and Warranties.

(a) The Borrowers hereby represent and warrant to the Administrative Agent and
the Lenders that (i) all consents, approvals and authorizations necessary for
the Borrowers’ execution, delivery and performance of this Nineteenth Amendment
have been obtained or made, (ii) this Nineteenth Amendment has been duly
executed and delivered by the Borrowers and constitutes a legal, valid and
binding obligation of each Borrower, enforceable against such Borrower in
accordance with its terms, (iii) the representations and warranties of the
Borrowers set forth in Article II of the Credit Agreement are true and correct
in all material respects as of the date hereof and (iv) no Event of Default or
Potential Event of Default has occurred and is continuing as of the date hereof.

(b) The Parent hereby represents and warrants to the Administrative Agent and
the Lenders that (i) all consents, approvals and authorizations necessary for
the Parent’s execution, delivery and performance of this Nineteenth Amendment
have been obtained or made, (ii) this Nineteenth Amendment has been duly
executed and delivered by the Parent and constitutes a legal, valid and binding
obligation of the Parent, enforceable against such Parent in accordance with its
terms and (iii) the representations and warranties of the Parent set forth in
Article III of the Parent Guaranty Agreement are true and correct in all
material respects as of the date hereof.

Section 3.03 No Waiver.

This Nineteenth Amendment is made in amendment and modification of, but not
extinguishment of, the obligations set forth in the Credit Agreement, the Parent
Guaranty Agreement and the other Loan Documents and, except as specifically
modified pursuant to the terms of this Nineteenth Amendment, the terms and
conditions of the Credit Agreement, the Parent Guaranty Agreement and the other
Loan Documents remain in full force and effect. Nothing herein shall limit in
any way the rights and remedies of Administrative Agent and the Lenders under
the Credit Agreement, the Parent Guaranty Agreement and the other Loan
Documents. The execution and delivery by the Lenders of this Nineteenth
Amendment shall not constitute a waiver, forbearance or other indulgence with
respect to any Potential Event of Default or Event of Default now existing or
hereafter arising.

Section 3.04 Counterparts, Integration; Effectiveness.

This Nineteenth Amendment may be executed in counterparts (and by different
parties hereto on different counterparts), each of which shall constitute an
original, but all of which when taken together shall constitute a single
contract. This Nineteenth Amendment and any agreements referred to herein
constitute the entire contract among the parties hereto relating to the subject
matter hereof and supersede any and all previous

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agreements and understandings, oral or written, relating to the subject matter
hereof. On the Nineteenth Amendment Effective Date and thereafter this
Nineteenth Amendment shall be binding upon and inure to the benefit of the
parties hereto and, subject to and in accordance with Section 9.16 of the Credit
Agreement, their respective successors and assigns. Delivery of an executed
counterpart of a signature page of this Nineteenth Amendment by telecopy shall
be as effective as delivery of a manually executed counterpart of this
Nineteenth Amendment.

Section 3.05 Severability.

Any provision of this Nineteenth Amendment held to be invalid, illegal or
unenforceable in any jurisdiction shall, as to such jurisdiction, be ineffective
to the extent of such invalidity, illegality or unenforceability without
affecting the validity, legality or enforceability of the remaining provisions
hereof, and the invalidity of a particular provision in a particular
jurisdiction shall not invalidate such provision in any other jurisdiction.

Section 3.06 Governing Law.

This Nineteenth Amendment shall be construed in accordance with and governed by
the laws of the State of New York without regard to the conflicts of law
provisions thereof, other than Sections 5-1401 and 5-1402 of the General
Obligations Law of the State of New York.

Section 3.07 Headings.

Article and Section headings used herein are for convenience of reference only,
are not part of this Nineteenth Amendment and shall not affect the construction
of, or be taken into consideration in interpreting, this Nineteenth Amendment.

[REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

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IN WITNESS WHEREOF, the parties hereto have caused this Nineteenth Amendment to
be duly executed by their respective authorized officers as of the day and year
first above written.

 

FIBERNET TELECOM GROUP, INC.

By: 

    

Name: 

 

Title: 

  FIBERNET OPERATIONS, INC.

By: 

    

Name: 

 

Title: 

  DEVNET L.L.C.

By: 

    

Name: 

 

Title: 

 

DEUTSCHE BANK AG NEW YORK

BRANCH, as Administrative Agent and as

a Lender

By: 

    

Name: 

 

Title: 

 

By: 

    

Name: 

 

Title: 

 

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WACHOVIA INVESTMENT HOLDINGS, LLC, as a Lender

By: 

    

Name: 

 

Title: 

 

IBM CREDIT LLC, as a Lender

By: 

    

Name: 

 

Title: