Document:

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                                                                     Exhibit 4.1

     REGISTRATION RIGHTS AGREEMENT dated as of June 30, 2000 (the "Agreement"),
by and among FIBERNET TELECOM GROUP, INC., a Nevada corporation (the "Company"),
and Nortel Networks Inc., a Delaware corporation (the "Stockholder").

     Pursuant to the Securities Purchase Agreement (as defined below), the
Stockholder has the right to purchase or otherwise acquire shares of Common
Stock (as defined below) of the Company. The Company and the Stockholder deem it
to be in their respective best interests to set forth the rights of the
Stockholder in connection with public offerings and sales of the Common Stock.
Accordingly, the Company and the Stockholder agree as follows:

                                   ARTICLE I

                                  DEFINITIONS

     Capitalized terms used and not otherwise defined herein shall have the
meanings ascribed to them in the Securities Purchase Agreement. As used in this
Agreement, the following terms shall have the following meanings:

     "Commission" means the Securities and Exchange Commission or any other
      ----------
Federal agency at the time administering the Securities Act.

     "Common Stock" means the common stock, $.001 par value, of the Company.
      ------------

     "Exchange Act" means the Securities Exchange Act of 1934 or any successor
      ------------
Federal statute, and the rules and regulations of the Commission promulgated
thereunder, all as the same shall be in effect from time to time.

     "Other Shares" means at any time those shares of Common Stock which do not
      ------------
constitute Primary Shares or Registrable Shares but are held by persons who are
entitled to piggyback registration rights.

     "Primary Shares" means at any time the authorized but unissued shares of
      --------------
Common Stock or shares of Common Stock held by the Company in its treasury.

     "Registrable Shares" means at any time the shares of Common Stock, any
      ------------------
securities which by their terms are exercisable or exchangeable for or
convertible into Common Stock, and any securities of the Company received in
respect thereof, which are held by the Stockholder which have not previously
been sold to the public pursuant to a registration statement under the
Securities Act.
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     "Rule 144" means Rule 144 promulgated under the Securities Act or any
      --------
successor rule thereto.

     "Securities Act" means the Securities Act of 1933 or any successor Federal
      --------------
statute, and the rules and regulations of the Commission thereunder, all as the
same shall be in effect from time to time.

     "Securities Purchase Agreement" means the Securities Purchase Agreement
      -----------------------------
dated as of the date hereof among the Company and the Stockholder.

     "Stockholder" means Nortel Networks Inc., or its successors or assigns.
      -----------

     "Transfer" means any disposition of any Registrable Shares or of any
      --------
interest therein which would constitute a sale thereof within the meaning of the
Securities Act, other than any such disposition pursuant to an effective
registration statement under the Securities Act and complying with all
applicable state securities and "blue sky" laws.

                                  ARTICLE II

                            REQUIRED REGISTRATIONS

2.1  Demand Registration.
     -------------------

     If the Company shall be requested by the Stockholder at any time to effect
the registration under the Securities Act of Registrable Shares (a "Demand
                                                                    ------
Registration"), the Company shall promptly use its best efforts to effect such
------------
registration under the Securities Act of the Registrable Shares and to have the
registration statement filed by the Company hereunder declared effective by the
Commission within 90 days of any requested filing (the "Demand Date") and make
any other subsequent filings to keep such registration statement effective;
provided, however, that the Company shall not be obligated to effect any
registration under the Securities Act except in accordance with the following
provisions:

          (a)  the Company shall not be obligated to use its best efforts to
file and cause to become effective (i) more than two registration statements
initiated pursuant to this Section; and (ii) any registration within twelve
months of any other registration required under this Section;

          (b)  A registration statement will not be deemed to have been effected
as a Demand Registration unless it has been declared effective by the Commission
and the Company has complied in all material respects with all of its
obligations under this Agreement with respect thereto.

          (c)  the Company may delay the filing or effectiveness of any
registration statement for a period of up to 90 days after the date of a request
for registration pursuant to this Section if at the time of such request the
Company is engaged, or has fixed plans to engage within 60 days of the time of
such request, in a firm commitment underwritten public offering of Primary
Shares in which the Stockholder may include Registrable Shares pursuant to
Section 2.2; and

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          (d)  with respect to any registration pursuant to this Section, the
Company may include in such registration any Primary Shares or Other Shares;
provided, however, that if any managing underwriter for the public offering
contemplated by such registration advises the Company that the inclusion of
Primary Shares and Other Shares proposed to be included in such registration
would interfere with the successful marketing (including pricing) of all such
securities, then the number of Primary Shares and Other Shares proposed to be
included in such registration shall be included in the following order:

               (i)    first, the Registrable Shares, pro rata based upon the
                      -----
     number of Registrable Shares (based upon Common Stock equivalents) owned by
     each holder thereof at the time of such registration; and

               (ii)   second, the Primary Shares and the Other Shares.
                      ------

     A requested registration under this Section may be rescinded by written
notice to the Company by the Stockholder.  Such rescinded registration shall not
count as a registration statement initiated pursuant to this Section for
purposes of paragraph (a) above.

          (e)  The Stockholder may, in the notice delivered pursuant to
paragraph (a) above, elect that such registration cover an underwritten
offering. Upon such election, the Stockholder shall select one or more
nationally recognized firms of investment banks to act as the managing
underwriters and shall select any additional investment banks to be used in
connection with such offering, provided that such investment banks must be
reasonably satisfactory to the Company. The Company shall, together with the
Stockholder and any other parties proposing to sell shares in such offering,
enter into a customary underwriting agreement with such underwriters.

2.2  Piggyback Registration.
     ----------------------

     If the Company proposes for any reason to register Primary Shares or Other
Shares under the Securities Act (other than on Form S-4 or Form S-8 promulgated
under the Securities Act or any successor forms thereto), it shall promptly give
written notice to the Stockholder of its intention so to register the Primary
Shares or Other Shares.  Upon the written request, given within 30 days after
delivery of any such notice by the Company, of the Stockholder (which request
shall specify the number of Registrable Shares proposed to be included in such
registration), the Company shall use its best efforts to cause all such
Registrable Shares to be included in such registration on the same terms and
conditions as the securities otherwise being sold in such registration;
provided, however, that if any managing underwriter for the public offering
contemplated by such registration advises the Company that the inclusion of all
Registrable Shares or Other Shares proposed to be included in such registration
would interfere with the successful marketing (including pricing) of the Primary
Shares proposed to be registered by the Company, then the number of Primary
Shares, Registrable Shares and Other Shares proposed to be included in such
registration shall be included in the following order:

          (a)  first, the Primary Shares; and
               -----

          (b)  second, the Registrable Shares and Other Shares, pro rata based
               ------
upon the number of shares of Common Stock (based upon Common Stock equivalents)
respectively owned by each holder thereof at the time of such registration.

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2.3  Registration on Form S-3.
     ------------------------

     Anything contained in Section 2.1 to the contrary notwithstanding, at such
time as the Company shall have qualified for the use of Form S-3 promulgated
under the Securities Act or any successor form thereto, the Stockholder shall
have the right to request in writing an unlimited number of registrations on
Form S-3 or such successor form of Registrable Shares, which request or requests
shall (i) specify the number of Registrable Shares intended to be sold or
disposed of, (ii) state the intended method of disposition of such Registrable
Shares, and (iii) relate to Registrable Shares having an anticipated aggregate
offering price of at least $1,000,000; provided, however, that the Stockholder
                                       --------  -------
shall not make any request under this Section 2.3 if at such time all
Registrable Shares held by the Stockholder are freely tradeable under Rule 144.
A requested registration on Form S-3 or any such successor form in compliance
with this Section shall not count as a registration statement demanded pursuant
to Section 2.1, but shall otherwise be treated as a registration initiated
pursuant to and shall, except as otherwise expressly provided in this Section,
be subject to Section 2.1.  The Stockholder shall not exercise its rights under
this Section 2.3 more than two times in any twelve month period.

2.4  "Stand-Off" Agreement.
      --------------------

     In consideration for the Company performing its obligations under this
Agreement, the Purchaser agrees, upon request of the Company or the managing
underwriters, to enter into an agreement providing that, for a period of time
(not to exceed one hundred eighty (180) days) from the effective date of a
Qualified IPO, the Purchaser shall not sell, make any short sale of, loan, grant
any option for the purchase of, or otherwise dispose of any Registrable
Securities, other than shares of Registrable Securities included in the
registration, without the prior written consent of the Company or such
underwriters, as the case may be; provided, however, the Purchaser shall not be
obligated to enter into such agreement unless all executive officers and
directors of the Company and each holder of more than 5% of the outstanding
Common Stock (or derivatives thereof), shall have entered into similar
agreements.

                                  ARTICLE III

                          REGISTRATION OF SECURITIES

3.1  Preparation and Filing.
     ----------------------

     If and whenever the Company is under an obligation pursuant to the
provisions of this Agreement to use its best efforts to effect the registration
of any Registrable Shares, the Company shall, as expeditiously as practicable:

          (a)  use its commercially reasonable efforts to cause a registration
statement that registers such Registrable Shares to become and remain effective
for a period of 180 days or until all of such Registrable Shares have been
disposed of (if earlier);

          (b)  furnish, at least five business days before filing a registration
statement that registers such Registrable Shares, a prospectus relating thereto
or any amendments or supplements relating to such a registration statement or
prospectus, to one counsel selected by the Stockholder (the "Selling Purchaser's
Counsel"), copies of all such documents proposed to be

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filed (it being understood that such five-business-day period need not apply to
successive drafts of the same document proposed to be filed so long as such
successive drafts are supplied to such counsel in advance of the proposed filing
by a period of time that is customary and reasonable under the circumstances);

          (c)  prepare and file with the Commission such amendments and
supplements to such registration statement and the prospectus used in connection
therewith as may be necessary to keep such registration statement effective for
at least a period of 180 days or until all of such Registrable Shares have been
disposed of (if earlier) and to comply with the provisions of the Securities Act
with respect to the sale or other disposition of such Registrable Shares;

          (d)  notify in writing the Selling Purchaser's Counsel promptly (i) of
the receipt by the Company of any notification with respect to any comments by
the Commission with respect to such registration statement or prospectus or any
amendment or supplement thereto or any request by the Commission for the
amending or supplementing thereof or for additional information with respect
thereto, (ii) of the receipt by the Company of any notification with respect to
the issuance by the Commission of any stop order suspending the effectiveness of
such registration statement or prospectus or any amendment or supplement thereto
or the initiation or threatening of any proceeding for that purpose and (iii) of
the receipt by the Company of any notification with respect to the suspension of
the qualification of such Registrable Shares for sale in any jurisdiction or the
initiation or threatening of any proceeding for such purposes;

          (e)  use its commercially reasonable efforts to register or qualify
such Registrable Shares under such other securities or blue sky laws of such
jurisdictions as any seller of Registrable Shares reasonably requests and do any
and all other acts and things which may be reasonably necessary or advisable to
enable such seller of Registrable Shares to consummate the disposition in such
jurisdictions of the Registrable Shares owned by such seller; provided, however,
that the Company will not be required to qualify generally to do business,
subject itself to general taxation or consent to general service of process in
any jurisdiction where it would not otherwise be required so to do but for this
paragraph (e);

          (f)  furnish to each seller of such Registrable Shares such number of
copies of a summary prospectus or other prospectus, including a preliminary
prospectus, in conformity with the requirements of the Securities Act, and such
other documents as such seller of Registrable Shares may reasonably request in
order to facilitate the public sale or other disposition of such Registrable
Shares;

          (g)  use its commercially reasonable efforts to cause such Registrable
Shares to be registered with or approved by such other governmental agencies or
authorities as may be necessary by virtue of the business and operations of the
Company to enable the seller or sellers thereof to consummate the disposition of
such Registrable Shares;

          (h)  notify on a timely basis each seller of such Registrable Shares
at any time when a prospectus relating to such Registrable Shares is required to
be delivered under the Securities Act within the appropriate period mentioned in
paragraph (a) of this Section, of the happening of any event as a result of
which the prospectus included in such registration statement, as then in effect,
includes an untrue statement of a material fact or omits to state a

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material fact required to be stated therein or necessary to make the statements
therein not misleading in light of the circumstances then existing and, at the
request of such seller, prepare and furnish to such seller a reasonable number
of copies of a supplement to or an amendment of such prospectus as may be
necessary so that, as thereafter delivered to the offerees of such shares, such
prospectus shall not include an untrue statement of a material fact or omit to
state a material fact required to be stated therein or necessary to make the
statements therein not misleading in light of the circumstances then existing;

          (i)  make available for inspection by any seller of such Registrable
Shares, any underwriter participating in any disposition pursuant to such
registration statement and any attorney, accountant or other agent retained by
any such seller or underwriter (collectively, the "Inspectors"), all pertinent
financial and other records, pertinent corporate documents and properties of the
Company (collectively, the "Records"), as shall be reasonably necessary to
enable them to exercise their due diligence responsibility, and cause the
Company's officers, directors and employees to supply all information (together
with the Records, the "Information") reasonably requested by any such Inspector
in connection with such registration statement. Any of the Information which the
Company determines in good faith to be confidential, and of which determination
the Inspectors are so notified, shall not be disclosed by the Inspectors unless
(i) the disclosure of such Information is necessary to avoid or correct a
misstatement or omission in the registration statement, (ii) the release of such
Information is ordered pursuant to a subpoena or other order from a court of
competent jurisdiction or (iii) such Information has been made generally
available to the public. The seller of Registrable Shares agrees that it will,
upon learning that disclosure of such Information is sought in a court of
competent jurisdiction, give notice to the Company and allow the Company, at the
Company's expense, to undertake appropriate action to prevent disclosure of the
Information deemed confidential;

          (j)  use its commercially reasonable efforts to obtain from its
independent certified public accountants "cold comfort" letters in customary
form and at customary times and covering matters of the type customarily covered
by cold comfort letters;

          (k)  use its commercially reasonable efforts to obtain from its
counsel an opinion or opinions in customary form;

          (l)  provide a transfer agent and registrar (which may be the same
entity and which may be the Company) for such Registrable Shares;

          (m)  issue to any underwriter, which underwriter shall be chosen by
the Company subject to the approval of the Stockholder, to which any seller of
Registrable Shares may sell shares in such offering certificates evidencing such
Registrable Shares;

          (n)  list such Registrable Shares on any national securities exchange
or national automated quotation system on which any shares of the Common Stock
are listed or, if the Common Stock is not so listed, use its commercially
reasonable efforts to qualify such Registrable Shares for inclusion on the
automated quotation system of the National Association of Securities Dealers,
Inc. (the "NASD") or such national securities exchange as the Company shall
reasonably determine;

                                                                               6
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          (o)  otherwise use its commercially reasonable efforts to comply with
all applicable rules and regulations of the Commission and make available to its
securityholders, as soon as reasonably practicable, earnings statements (which
need not be audited) covering a period of 12 months beginning within three
months after the effective date of the registration statement, which earnings
statements shall satisfy the provisions of Section 11(a) of the Securities Act;
and

          (p)  use its commercially reasonable efforts to take all other steps
necessary to effect the registration of such Registrable Shares contemplated
hereby.

3.2  Information by Holder.
     ---------------------

     Each holder of Registrable Shares to be included in any registration shall
furnish to the Company such written information regarding such holder and the
distribution proposed by such holder as the Company may reasonably request in
writing and as shall be reasonably required in connection with any registration,
qualification or compliance referred to in this Agreement.

3.3  Exchange Act Compliance.
     -----------------------

     The Company shall comply with all of the reporting requirements of the
Exchange Act and shall comply with all other public information reporting
requirements of the Commission which are conditions to the availability of Rule
144 for the sale of shares of Common Stock.  The Company shall cooperate with
the Stockholder in supplying such information as may be necessary for the
Stockholder to complete and file any information reporting forms presently or
hereafter required by the Commission as a condition to the availability of Rule
144.

3.4  Expenses.
     --------

     All expenses incurred by the Company in complying with this Agreement,
including, without limitation, all registration and filing fees (including all
expenses incident to filing with the NASD), fees and expenses of complying with
securities and blue sky laws, printing expenses, fees and expenses of the
Company's counsel and accountants, and reasonable fees and expenses of the
Selling Purchaser's Counsel, shall be paid by the Company; provided, however,
that all underwriting discounts and selling commissions applicable to the
Registrable Shares shall be borne by the sellers thereof, in proportion to the
number of Registrable Shares sold by such sellers.

                                  ARTICLE IV

                                INDEMNIFICATION

4.1  Indemnification.
     ---------------

          (a)  In connection with any registration of any Registrable Shares
under the Securities Act pursuant to this Agreement, the Company shall indemnify
and hold harmless the seller of such Registrable Shares, each underwriter,
broker or any other person acting on behalf of such seller and each other
person, if any, who controls any of the foregoing persons within the meaning of
the Securities Act against any losses, claims, damages or liabilities, joint or
several, (or actions in respect thereof) to which any of the foregoing persons
may become subject under

                                                                               7
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the Securities Act or otherwise, insofar as such losses, claims, damages or
liabilities (or actions in respect thereof) arise out of or are based upon an
untrue statement or alleged untrue statement of a material fact contained in the
registration statement under which such Registrable Shares were registered under
the Securities Act, any preliminary prospectus or final prospectus contained
therein or otherwise filed with the Commission, any amendment or supplement
thereto or any document incident to registration or qualification of any
Registrable Shares, or arise out of or are based upon the omission or alleged
omission to state therein a material fact required to be stated therein or
necessary to make the statements therein not misleading or, with respect to any
prospectus, necessary to make the statements therein in light of the
circumstances under which they were made not misleading, or any violation by the
Company of the Securities Act or state securities or blue sky laws applicable to
the Company and relating to action or inaction required of the Company in
connection with such registration or qualification under such state securities
or blue sky laws; and shall reimburse such seller, such underwriter, such broker
or such other person acting on behalf of such seller and each such controlling
person for any legal or other expenses reasonably incurred by any of them in
connection with investigating or defending any such loss, claim, damage,
liability or action; provided, however, that the Company shall not be liable in
any such case to the extent that any such loss, claim, damage, liability or
action arises out of or is based upon an untrue statement or alleged untrue
statement or omission or alleged omission made in said registration statement,
preliminary prospectus, final prospectus, amendment, supplement or document
incident to registration or qualification of any Registrable Shares in reliance
upon and in conformity with written information furnished to the Company through
an instrument duly executed by such seller or underwriter specifically for use
in the preparation thereof.

          (b)  In connection with any registration of Registrable Shares under
the Securities Act pursuant to this Agreement, each seller of Registrable Shares
shall indemnify and hold harmless (in the same manner and to the same extent as
set forth in the preceding paragraph of this Section) the Company, each director
of the Company, each officer of the Company who shall sign such registration
statement, each underwriter, broker or other person acting on behalf of such
seller, each person who controls any of the foregoing persons within the meaning
of the Securities Act and each other seller of Registrable Shares under such
registration statement with respect to any statement or omission from such
registration statement, any preliminary prospectus or final prospectus contained
therein or otherwise filed with the Commission, any amendment or supplement
thereto or any document incident to registration or qualification of any
Registrable Shares, if such statement or omission was made in reliance upon and
in conformity with written information furnished to the Company or such
underwriter through an instrument duly executed by such seller specifically for
use in connection with the preparation of such registration statement,
preliminary prospectus, final prospectus, amendment, supplement or document;
provided, however, that the maximum amount of liability in respect of such
indemnification shall be limited, in the case of each seller of Registrable
Shares, to an amount equal to the net proceeds actually received by such seller
from the sale of Registrable Shares effected pursuant to such registration.

          (c)  Promptly after receipt by an indemnified party of notice of the
commencement of any action involving a claim referred to in the preceding
paragraphs of this Section, such indemnified party will, if a claim in respect
thereof is made against an indemnifying party, give written notice to the latter
of the commencement of such action.  In case any such action is

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brought against an indemnified party, the indemnifying party will be entitled to
participate in and to assume the defense thereof, jointly with any other
indemnifying party similarly notified to the extent that it may wish, with
counsel reasonably satisfactory to such indemnified party, and after notice from
the indemnifying party to such indemnified party of its election so to assume
the defense thereof, the indemnifying party shall not be responsible for any
legal or other expenses subsequently incurred by the latter in connection with
the defense thereof; provided, however, that if any indemnified party shall have
reasonably concluded that there may be one or more legal or equitable defenses
available to such indemnified party which are additional to or conflict with
those available to the indemnifying party, or that such claim or litigation
involves or could have an effect upon matters beyond the scope of the indemnity
agreement provided in this Section, the indemnifying party shall not have the
right to assume the defense of such action on behalf of such indemnified party
and such indemnifying party shall reimburse such indemnified party and any
person controlling such indemnified party for that portion of the fees and
expenses of any counsel retained by the indemnified party which is reasonably
related to the matters covered by the indemnity agreement provided in this
Section.

          (d)  If the indemnification provided for in this Section is held by a
court of competent jurisdiction to be unavailable to an indemnified party with
respect to any loss, claim, damage, liability or action referred to herein, then
the indemnifying party, in lieu of indemnifying such indemnified party
hereunder, shall contribute to the amounts paid or payable by such indemnified
party as a result of such loss, claim, damage, liability or action in such
proportion as is appropriate to reflect the relative fault of the indemnifying
party on the one hand and of the indemnified party on the other in connection
with the statements or omissions which resulted in such loss, claim, damage or
liability as well as any other relevant equitable considerations. The relative
fault of the indemnifying party and of the indemnified party shall be determined
by reference to, among other things, whether the untrue or alleged untrue
statement of a material fact or the omission or alleged omission to state a
material fact relates to information supplied by the indemnifying party or by
the indemnified party and the parties' relative intent, knowledge, access to
information and opportunity to correct or prevent such statement or omission.

4.2  Underwriting Agreement.
     ----------------------

     Notwithstanding the provisions of Sections 3.1, 3.4, and 4.1, to the extent
that the Stockholder selling Registrable Shares, in a proposed registration
shall enter into an underwriting or similar agreement, and such agreement
contains provisions covering one or more issues addressed in such Sections, the
provisions contained in such agreement shall supersede the provisions of
Sections 3.1, 3.4, and 4.1.

                                   ARTICLE V

                                 MISCELLANEOUS

5.1  Termination.
     -----------

     This Agreement shall terminate and be of no further force or effect when
the Stockholder ceases to hold or to have the rights to obtain any Registrable
Shares.

                                                                               9
<PAGE>

5.2  Successors and Assigns.
     ----------------------

     This Agreement shall bind and inure to the benefit of the Company and the
Stockholder and, subject to Section 5.3, their respective successors and
assigns.

5.3  Assignment.
     ----------

     The Stockholder may assign its respective rights hereunder to any other
person or entity to whom Registrable Shares of such Stockholder have been sold
or otherwise transferred.

5.4  Entire Agreement.
     ----------------

     This Agreement contains the entire agreement among the parties with respect
to the subject matter hereof and supersedes all prior arrangements or
understandings with respect hereto.

5.5  No Conflict of Rights.
     ---------------------

     The Company represents and warrants to the Stockholder that the
registration rights granted to the Stockholder hereby do not conflict with any
other registration rights granted by the Company.  The Company shall not, after
the date hereof, grant any registration rights which conflict with or impair the
registration rights granted hereby.

5.6  Notices.
     -------

     All notices, claims, certificates, requests, demands and other
communications hereunder shall be in writing and shall be deemed to have been
duly given if personally delivered or if sent by nationally-recognized overnight
courier, by telecopy, or by registered or certified mail, return receipt
requested and postage prepaid, addressed as follows:

     if to the Company:

          FiberNet Telecom Group, Inc.
          570 Lexington Avenue
          New York, NY 10022
          Telecopier: (212) 421-4900
          Telephone:  (212) 421-8860

          Attention:  President

     if to the Stockholder:

          Nortel Networks, Inc.
          GMS 991 15 A40
          2221 Lakeside Boulevard
          Richardson, TX 75082-4399
          Attention:  Paul D. Day

                                                                              10
<PAGE>

          Vice President Customer Finance North America
          Telecopy:  (972) 684-3679

     with a copy to:

          Jenkens & Gilchrist, a Professional Corporation
          1445 Ross Avenue
          Suite 3200
          Dallas, Texas  75202
          Attention:  Daryl Robertson, Esq.
          Telephone:  (214) 855-4165
          Telecopy:  (214) 855-4300

or to such other address as the party to whom notice is to be given may have
furnished to the other parties in writing in accordance herewith.  Any such
notice or communication shall be deemed to have been received (i) in the case of
personal delivery, on the date of such delivery, (ii) in the case of nationally-
recognized overnight courier, on the next business day after the date when sent,
(iii) in the case of telecopy transmission, when received, and (iv) in the case
of mailing, on the third business day following that on which the piece of mail
containing such communication is posted.

5.7  Modifications; Amendments; Waivers.
     ----------------------------------

     The terms and provisions of this Agreement may not be modified or amended,
except pursuant to a writing signed by the Company and the Stockholder.

5.8  Counterparts.
     ------------

     This Agreement may be executed in any number of counterparts, and each such
counterpart hereof shall be deemed to be an original instrument, but all such
counterparts together shall constitute but one agreement.

5.9  Headings.
     --------

     The headings of the various sections of this Agreement have been inserted
for convenience of reference only and shall not be deemed to be a part of this
Agreement.

5.10 Severability.
     ------------

     It is the desire and intent of the parties that the provisions of this
Agreement be enforced to the fullest extent permissible under the law and public
policies applied in each jurisdiction in which enforcement is sought.
Accordingly, if any provision of this Agreement would be held in any
jurisdiction to be invalid, prohibited or unenforceable for any reason, such
provision, as to such jurisdiction, shall be ineffective, without invalidating
the remaining provisions of this Agreement or affecting the validity or
enforceability of such provision in any other jurisdiction.  Notwithstanding the
foregoing, if such provision could be more narrowly drawn so as not to be
invalid, prohibited or unenforceable in such jurisdiction, it shall, as to such
jurisdiction, be so

                                                                              11
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narrowly drawn, without invalidating the remaining provisions of this Agreement
or affecting the validity or enforceability of such provision in any other
jurisdiction.

5.11 GOVERNING LAW; SUBMISSION TO JURISDICTION.
     -----------------------------------------

     THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE
SUBSTANTIVE INTERNAL LAWS OF THE STATE OF NEW YORK.  EACH OF THE PARTIES HERETO
IRREVOCABLY CONSENTS TO SERVICE OF PROCESS IN THE MANNER PROVIDED FOR NOTICES IN
SECTION 5.6.  NOTHING IN THIS AGREEMENT WILL AFFECT THE RIGHT OF ANY PARTY TO
THIS AGREEMENT TO SERVE PROCESS IN ANY OTHER MANNER PERMITTED BY LAW.

                                   * * * * *

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<PAGE>

          IN WITNESS WHEREOF, the parties have executed and delivered this
Registration Rights Agreement on the date first above written.

                              FIBERNET TELECOM GROUP, INC.

                              By:
                                 -------------------------------------
                                 Name:
                                 Title:

                              NORTEL NETWORKS INC.

                              By:
                                 -------------------------------------
                                 Name:
                                 Title:<PAGE>

                                                                     EXHIBIT 4.2

               AMENDMENT ("Amendment") dated as of June 30, 2000 between
FIBERNET TELECOM GROUP, INC. ("Company"), SIGNAL EQUITY PARTNERS, L.P. (formerly
known as Signal Capital Partners, L.P.), as the Majority in Interest of the
Purchasers ("Signal"), and NORTEL NETWORKS INC. ("Nortel") (each term as defined
herein), to the Stockholders Agreement dated as of May 7, 1999 ("Stockholders
Agreement") by and among the Company and the Stockholders (as defined in the
Stockholders Agreement) listed therein ("Stockholders").

               WHEREAS, pursuant to Section 17 of the Stockholders Agreement,
the Company and Signal, as the Majority in Interest, have the right to modify
the Stockholders Agreement;

               WHEREAS, the Company and Signal, as the Majority in Interest,
wish to amend the Stockholders Agreement; and

               WHEREAS, Nortel desires to become subject to certain portions of
the Stockholders Agreement, as amended hereby.

               NOW, THEREFORE, the parties agree as follows:

     Section   1.   Definitions. Capitalized terms used and not otherwise
                    -----------
defined herein shall have the meanings ascribed to such terms in the
Stockholders Agreement.

     Section   2.   Agreement to Amend. Pursuant to Section 17 of the
                    ------------------
Stockholders Agreement, the Company and Signal hereby agree to amend the
Stockholders Agreement as provided herein.

     Section   3.   Amendments.
                    ----------

               (a)  By execution of this Amendment, the undersigned agree that
Nortel shall become a party to the Stockholders Agreement, as amended hereby,
and that Nortel shall have all rights, and shall observe all of the obligations,
applicable to a Stockholder thereunder; provided, however, that Section 3 (Right
                                        --------  -------
of First Refusal), Section 5 (Right of First Offer), Section 6 (Drag Along
Right), Section 8 (Irrevocable Proxy), Section 9 (Voting Agreement) and Section
10 (Covenants) shall not apply to Nortel and Nortel shall have no rights or
obligations under these sections.

               (b)  Section 1 (Definitions) of the Stockholders Agreement shall
be amended as follows:
<PAGE>

               (i)   The definition of the term "Qualified Public Offering"
shall be amended and restated in its entirety as follows:

                    "Qualified Public Offering" shall mean a fully underwritten
                     -------------------------
               public offering (underwritten by an underwriter of national
               reputation) of shares of Common Stock registered pursuant to the
               Securities Act with proceeds to the Company of at least
               $75,000,000 (net of underwriting discounts and expenses) or
               otherwise on terms reasonably acceptable to the Majority in
               Interest and Nortel.

               (ii)  The definition of the term "Stock" shall be amended and
restated in its entirety as follows:

                    "Stock" shall mean (i) the presently issued and outstanding
                     -----
               shares of Common Stock, Series C Preferred Stock, Series D
               Preferred Stock, Series E Preferred Stock, Series F Preferred
               Stock and Series G Preferred Stock and any options, warrants or
               other instruments exercisable therefor (which options, warrants
               or other instruments shall be deemed to be that number of
               outstanding shares of Stock for which they are exercisable), (ii)
               any additional shares of capital stock of the Company hereafter
               issued and outstanding and (iii) any shares of capital stock of
               the Company into which such shares may be converted or for which
               they may be exchanged or exercised.

               (iii) The following definitions shall be added:

                         "Nortel" shall mean Nortel Networks Inc., a Delaware
                          ------
               corporation.

                         "Series D Preferred Stock" shall mean the Series D
                          ------------------------
               Convertible Preferred Stock, par value $.001, of the Company.

                         "Series E Preferred Stock" shall mean the Series E
                          ------------------------
               Convertible Preferred Stock, par value $.001, of the Company.

                         "Series F Preferred Stock" shall mean the Series F
                          ------------------------
               Convertible Preferred Stock, par value $.001, of the Company.

                         "Series G Preferred Stock" shall mean the Series G
                          ------------------------
               Convertible Preferred Stock, par value $.001, of the Company.

          (c) Section 2 (Limitations on Transfers of Stock by Stockholders) of
the Stockholders Agreement  shall be amended to add the following paragraphs
after Section 2 as follows:

               "The foregoing provisions of this Section 2 shall not apply to or
          be binding upon Nortel.  Anything contained herein to the contrary
          notwithstanding,
<PAGE>

          Nortel hereby agrees that it shall not Transfer any Stock for a period
          of one (1) year after the date of the Securities Purchase Agreement
          dated as of June 30, 2000, between the Company and Nortel, other than
          to any member of Nortel's Group, provided that such Group member
          agrees in writing to be bound by this provision.

               Each Stockholder agrees, upon request of the Company or the
          managing underwriters with respect to a Qualified Public Offering, to
          enter into an agreement providing that, for a period of time (not to
          exceed one hundred eighty (180) days) from the effective date of the
          Qualified Public Offering, the Stockholder shall not sell, make any
          short sale of, loan, grant any option for the purchase of, or
          otherwise dispose of any Stock, other than shares of Stock included in
          the registration, without the prior written consent of the Company or
          such underwriters, as the case may be; provided, however, the
          Stockholder shall not be obligated to enter into such agreement unless
          all executive officers and directors of the Company and each holder of
          more than 5% of the outstanding Common Stock (or derivatives thereof),
          shall have entered into similar agreements."

          (d)  Section 4 (Tag-along Right) shall be amended to add the following
provision after Section 4(c) as follows:

                    "(d) Anything contained herein to the contrary
               notwithstanding, (i) any Transfer by Nortel of any of the shares
               of Stock held by Nortel shall not be subject to the provisions of
               this Section 4 and (ii) Nortel shall be entitled to Transfer
               shares of Stock held by Nortel to a Section 4 Purchaser pursuant
               to a Section 4 Offer only in the event that such Section 4
               Purchaser is purchasing at least twenty percent (20%) of the
               Company's Common Stock, as determined on a fully diluted basis
               assuming the full exercise and conversion of the outstanding
               securities of the Company.

          (e) Subsection (f)(v) of Section 7 (Pre-emptive Rights) of the
Stockholders Agreement shall be amended and restated in its entirety as follows:

                    "(v) Common Stock or other equity securities or instruments
               exercisable for or convertible into equity securities issued as
               part of a strategic arrangement or alliance by the Company or its
               Subsidiaries to building licensors or landlords, carriers, joint
               venture partners, vendors (other than equipment vendors and
               software vendors), consultants, lessors or lenders, and
               securities or instruments issued in connection with acquisitions,
               as each such transaction is approved by the Board of the Company;
               provided, that such issuances do not, in the aggregate, involve
               --------  ----
               the issuance of securities accounting for in excess of five
               percent (5%) of the fully diluted outstanding Common Stock of the
               Company; and
<PAGE>

                    (vi) two million shares of Series G Preferred Stock issued
               or to be issued to Nortel Networks Inc."

          (f) Section 17 (Modification) of the Stockholders Agreement shall be
amended and restated in its entirety as follows:

               "Section 17.  Modification. Except as otherwise provided herein,
                             ------------
          neither this Agreement nor any provisions hereof can be modified,
          changed, discharged or terminated except by an instrument in writing
          signed by the Company and a Majority in Interest of the Purchasers;
          provided, however, that any modification or amendment which relates to
          --------  -------
          the rights and obligations of Nortel under this Stockholders Agreement
          shall also require Nortel to execute such instrument in writing;
          provided, further, that no modification or amendment shall
          --------  -------
          discriminate against any Stockholder without the consent of such
          Stockholder;

          (g) Section 16 (Notices) of the Stockholders Agreement shall be
amended to add the following provision at the end thereof:

               "In lieu of the foregoing addresses, the address for notice to
          Nortel shall be as follows:

               Nortel Networks, Inc.
               GMS 991 15 A40
               2221 Lakeside Boulevard
               Richardson, TX  75082-4399
               Attention:  Paul D. Day, Vice President
               Customer Finance North America
               Facsimile:  (972) 684-3679

          with a copy to:

               Jenkens & Gilchrist, a Professional Corporation
               1445 Ross Avenue
               Suite 3200
               Dallas, Texas  75202
               Attention:  Daryl B. Robertson, Esq.
               Telephone:  (214) 855-4165
               Telecopy:  (214) 855-4300

          Section 4.  Governing Law.  This Amendment shall be governed by and
                      -------------
construed in accordance with the laws of the State of New York (without regard
to principles of conflicts of laws).

          Section 5.  Successors and Assigns.  This Amendment shall bind and
                      ----------------------
inure to the benefit of the parties and their respective successors and assigns,
transferees, legal representatives and heirs.
<PAGE>

          Section 6.  Headings. The headings of this Amendment have been
                      --------
inserted for convenience of reference only and shall not be deemed to be a part
of this Amendment.

          Section 7.  Entire Agreement. This Amendment and the other writings
                      ----------------
referred to herein or delivered pursuant hereto contain the entire agreement
among the parties hereto with respect to the subject matter hereof and supersede
all prior and contemporaneous agreements and understandings with respect
thereto.

          Section 8.  Counterparts. This Amendment may be executed in any
                      ------------
number of counterparts, and each such counterpart shall be deemed to be an
original instrument, but all such counterparts together shall constitute but one
agreement.

          Section 9.  Supersession. This Agreement supersedes and replaces the
                      ------------
Amendment to the Stockholders Agreement dated as of June 30, 1999, between the
Company and the Majority in Interest.

                                   * * * * *
<PAGE>

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

                                   FIBERNET TELECOM GROUP, INC.

                                   By: _________________________________
                                       Name:
                                       Title:

                                   SIGNAL EQUITY PARTNERS, L.P.

                                   By:  Signal Equity Advisors, L.P.
                                   Its: General Partner

                                   By:  Signal Equity Advisors, Inc.
                                   Its: General Partner

                                   By: _________________________________
                                       Name:
                                       Title:

                                   NORTEL NETWORKS INC.

                                   By: _________________________________
                                       Name:
                                       Title:

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