<PAGE>

                                                                  EXHIBIT 10.05




                              SECURITY AGREEMENT
                              ------------------



                           Dated as of May 12, 1999



                                    between



                                 DSL.net, Inc.



                                      and



                              Fleet National Bank
<PAGE>

                               TABLE OF CONTENTS
                               -----------------
<TABLE>

<C>        <S>                                                           <C>
    1.      Definitions.................................................  1
    2.      Grant of Security Interest..................................  1
       2.1. Collateral Granted..........................................  1
    3.      Title to Collateral, etc....................................  2
    4.      Continuous Perfection.......................................  2
    5.      No Liens....................................................  2
    6.      No Transfers................................................  2
    7.      Insurance...................................................  2
       7.1. Maintenance of Insurance....................................  2
       7.2. Insurance Proceeds..........................................  3
       7.3. Notice of Cancellation, etc.................................  3
    8.      Maintenance of Collateral; Compliance with Law..............  3
    9.      Collateral Protection Expenses; Preservation of Collateral..  4
       9.1. Expenses Incurred by Bank...................................  4
       9.2. Bank's Obligations and Duties...............................  4
   10.      Deposits....................................................  4
   11.      Intentionally Omitted.......................................  4
   12.      Further Assurances..........................................  4
   13.      Power of Attorney...........................................  5
      13.1  Appointment and Powers of Bank..............................  5
      13.2. Ratification by Company.....................................  5
      13.3. No Duty on Bank.............................................  6
   14.      Remedies....................................................  6
   15.      Regulatory Issues...........................................  6
   16.      No Waiver, etc..............................................  6
   17.      Marshalling.................................................  7
   18.      Proceeds of Dispositions; Expenses..........................  7
   19.      Overdue Amounts.............................................  7
   20.      Governing Law; Consent to Jurisdiction......................  8
   21.      Waiver of Jury Trial........................................  8
   22.      PREJUDGMENT REMEDY WAIVER...................................  8
   23.      Miscellaneous...............................................  9
   24.      Termination; Release of Collateral..........................  9
</TABLE>
<PAGE>

                              SECURITY AGREEMENT
                              ------------------


     SECURITY AGREEMENT, dated as of May 12, 1999, between DSL.NET, INC., a
Delaware corporation (the "Company"), and FLEET NATIONAL BANK, a national
banking association (hereinafter, the "Bank").

     WHEREAS, the Company has entered into a Credit Agreement of even date (as
amended and in effect from time to time, the "Credit Agreement"), with the Bank,
pursuant to which the Bank, subject to the terms and conditions contained
therein, is to make loans or otherwise extend credit to the Company; and

     WHEREAS, it is a condition precedent to the Bank's making any loans or
otherwise extending credit to the Company under the Credit Agreement that the
Company execute and deliver to the Bank a security agreement in substantially
the form hereof; and

     WHEREAS, the Company wishes to grant security interests in favor of the
Bank as herein provided;

     NOW, THEREFORE, in consideration of the promises contained herein and for
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the parties hereto agree as follows:

    1.   Definitions.  All capitalized terms used herein without definitions
         -----------
shall have the respective meanings provided therefor in the Credit Agreement.

    2.   Grant of Security Interest.
         --------------------------

         2.1.  Collateral Granted.  The Company hereby grants to the Bank, to
               ------------------
     secure the payment and performance in full of all of the Obligations, a
     security interest in and so pledges and assigns to the Bank the following
     properties, assets and rights of the Company, wherever located, whether now
     owned or hereafter acquired or arising, and all proceeds and products
     thereof (all of the same being hereinafter called the "Collateral"):

               All furniture, equipment (including, without limitation, all
         distribution systems and all components thereof, including, without
         limitation, hardware, cables, fiber optic or other cables, switches,
         routers, computer equipment, amplifiers and associated devices), raw
         materials, motor vehicles, including, without limitation, the motor
         vehicles described on Schedule 1 attached hereto and made a part
                               ----------
         hereof and other goods, and all recorded data of any kind or nature
         relating to the foregoing, regardless of the medium of recording
         relating to the foregoing including, without limitation, all software,
         writings, plans, specifications and schematics and all insurance refund
         claims, other insurance claims and proceeds relating to or arising from
         any of the foregoing, but specifically excluding any and all inventory,
         raw materials, parts and/or
<PAGE>

                                      -2-


         equipment which is now owned or hereafter acquired and reconfigured or
         processed and installed at any local exchange carrier, any customer
         premises or any other location other than the Company's principal place
         of business (whether such locations are located within or outside of
         the State of Connecticut), intellectual property and intangibles, and
         further excluding all insurance refund claims, other insurance claims,
         tort claims and proceeds relating to or arising from any of such
         excluded items.

    3.   Title to Collateral, etc.  The Company is the owner of the Collateral
         ------------------------
free from any adverse lien, security interest or other encumbrance, except for
the security interest created by this Agreement and other liens permitted by the
Credit Agreement.  None of the Collateral constitutes, or is the proceeds of,
"farm products" as defined in (S)9-109(3) of the Massachusetts UCC.

    4.   Continuous Perfection.  The Company's place of business or, if more
         ---------------------
than one, chief executive office is indicated on the Perfection Certificate
delivered to the Bank herewith (the "Perfection Certificate"). The Company will
not change the same, or the name, identity or corporate structure of the Company
in any manner, without providing at least 30 days prior written notice to the
Bank except as permitted under the terms of the Credit Agreement. The
Collateral, to the extent not delivered to the Bank pursuant to (S)2.2, will be
kept at those locations listed on the Perfection Certificate and the Company
will not remove the Collateral from such locations, without providing at least
30 days prior written notice to the Bank.

    5.   No Liens.  Except for the security interest herein granted and as
         --------
permitted under the Credit Agreement, the Company shall be the owner of the
Collateral free from any lien, security interest or other encumbrance, and the
Company shall defend the same against all claims and demands of all persons at
any time claiming the same or any interests therein adverse to the Bank. The
Company shall not pledge, mortgage or create, or suffer to exist a security
interest in the Collateral in favor of any person other than the Bank or as
permitted under the Credit Agreement.

    6.   No Transfers.  The Company will not sell or offer to sell or
         ------------
otherwise transfer the Collateral or any interest therein except for
(i) licenses of general intangibles in the ordinary course of business and
(ii) sales or other dispositions of obsolescent items of equipment in the
ordinary course of business consistent with past practices.

    7.   Insurance.
         ---------

         7.1.  Maintenance of Insurance.  The Company will maintain with
               ------------------------
financially sound and reputable insurers insurance with respect to its
properties and business against such casualties and contingencies as shall be in
accordance with general practices of businesses engaged in similar activities in
similar geographic areas. Such insurance shall be in such minimum amounts that
the Company will not be deemed a coinsurer under applicable insurance laws,
regulations and policies and otherwise shall be in such amounts, contain such
terms, be in such forms and be for such periods as may be reasonably
satisfactory to the Bank. In addition, all such insurance shall be payable to
the
<PAGE>

                                      -3-

Bank as loss payee under a "standard" or "New York" loss payee clause. Without
limiting the foregoing, the Company will (i) keep all of its physical property
insured with casualty or physical hazard insurance on an "all risks" basis, with
electronic data processing coverage, with a full replacement cost endorsement
and an "agreed amount" clause in an amount equal to 100% of the full replacement
cost of such property, (ii) maintain all such workers' compensation or similar
insurance as may be required by law and (iii) maintain, in amounts and with
deductibles equal to those generally maintained by businesses engaged in similar
activities in similar geographic areas, general public liability insurance
against claims of bodily injury, death or property damage occurring, on, in or
about the properties of the Company; business interruption insurance; and
product liability insurance.

         7.2.  Insurance Proceeds.  The proceeds of any casualty insurance in
               ------------------
respect of any casualty loss of any of the Collateral shall, subject to the
rights, if any, of other parties with a prior interest in the property covered
thereby, (i) so long as no Default or Event of Default has occurred and is
continuing and to the extent that the amount of such proceeds is less than
$100,000.00, be disbursed to the Company for direct application by the Company
solely to the repair or replacement of the Company's property so damaged or
destroyed and (ii) in all other circumstances, be held by the Bank as cash
collateral for the Obligations. The Bank may, at its sole option, disburse from
time to time all or any part of such proceeds so held as cash collateral, upon
such terms and conditions as the Bank may reasonably prescribe, for direct
application by the Company solely to the repair or replacement of the Company's
property so damaged or destroyed, or the Bank may apply all or any part of such
proceeds to the Obligations with the Revolving Credit Commitment (if not then
terminated) being reduced by the amount so applied to the Obligations.

         7.3.  Notice of Cancellation, etc.  All policies of insurance shall
               ---------------------------
promise provide for at least thirty (30) days prior written cancellation notice
to the Bank. In the event of failure by the Company to provide and maintain
insurance as herein provided, the Bank may, at its option, provide such
insurance and charge the amount thereof to the Company. The Company shall
furnish the Bank with certificates of insurance and policies evidencing
compliance with the foregoing insurance provision.

    8.   Maintenance of Collateral; Compliance with Law.  The Company will
         ----------------------------------------------
keep the Collateral in good order and repair, ordinary wear and tear excepted,
and will not use the same in violation of law or any policy of insurance
thereon. The Bank, or its designee, may inspect the Collateral at any reasonable
time, wherever located in accordance with the terms of the Credit Agreement. The
Company will pay promptly when due all taxes, assessments, governmental charges
and levies upon the Collateral or incurred in connection with the use or
operation of such Collateral or incurred in connection with this Agreement. The
Company has at all times operated, and the Company will continue to operate, its
business in compliance in all material respects with all applicable provisions
of the federal Fair Labor Standards Act, as amended, and with all applicable
provisions of federal, state and local statutes and ordinances dealing with the
control, shipment, storage or disposal of hazardous materials or substances.
<PAGE>

                                      -4-

    9.   Collateral Protection Expenses; Preservation of Collateral.
         ----------------------------------------------------------

         9.1.  Expenses Incurred by Bank.  In its discretion, the Bank may
               -------------------------
discharge taxes and other encumbrances at any time levied or placed on any of
the Collateral, make repairs thereto and pay any necessary filing fees. The
Company agrees to reimburse the Bank on demand for any and all expenditures so
made. The Bank shall have no obligation to the Company to make any such
expenditures, nor shall the making thereof relieve the Company of any default.

         9.2.  Bank's Obligations and Duties.  Anything herein to the contrary
               -----------------------------
notwithstanding, the Company shall remain liable under each contract or
agreement comprised in the Collateral to be observed or performed by the Company
thereunder. The Bank shall not have any obligation or liability under any such
contract or agreement by reason of or arising out of this Agreement or the
receipt by the Bank of any payment relating to any of the Collateral, nor shall
the Bank be obligated in any manner to perform any of the obligations of the
Company under or pursuant to any such contract or agreement, to make inquiry as
to the nature or sufficiency of any payment received by the Bank in respect of
the Collateral or as to the sufficiency of any performance by any party under
any such contract or agreement, to present or file any claim, to take any action
to enforce any performance or to collect the payment of any amounts which may
have been assigned to the Bank or to which the Bank may be entitled at any time
or times. The Bank's sole duty with respect to the custody, safe keeping and
physical preservation of the Collateral in its possession, under (S)9-207 of the
Massachusetts UCC or otherwise, shall be to deal with such Collateral in a
commercially reasonable manner.

    10.  Deposits.  During the existence of an Event of Default, whether or
         --------
not any Obligations are due, the Bank may demand, sue for, collect, or make any
settlement or compromise which it deems desirable with respect to the
Collateral. Regardless of the adequacy of Collateral or any other security for
the Obligations, any deposits or other sums at any time credited by or due from
the Bank to the Company may at any time be applied to or set off against any of
the Obligations.

    11.  Intentionally Omitted.
         ---------------------

    12.  Further Assurances.  The Company, at its own expense, shall do, make,
         ------------------
execute and deliver all such additional and further acts, things, deeds,
assurances and instruments as the Bank may reasonably require more completely to
vest in and assure to the Bank its rights hereunder or in any of the Collateral,
including, without limitation, (i) executing, delivering and, where appropriate,
filing financing statements and continuation statements under the Uniform
Commercial Code, and (ii) obtaining governmental and other third party consents
and approvals.

    13.  Power of Attorney.
         -----------------

         13.1  Appointment and Powers of Bank.  The Company hereby irrevocably
               ------------------------------
constitutes and appoints the Bank and any officer or agent thereof, with full
power of substitution, as its true and lawful attorneys-in-fact with full
irrevocable power and authority in the place and stead of the Company or in the
Bank's own name, for the
<PAGE>

                                      -5-

purpose of carrying out the terms of this Agreement, to take any and all
appropriate action and to execute any and all documents and instruments that may
be necessary or desirable to accomplish the purposes of this Agreement and,
without limiting the generality of the foregoing, hereby gives said attorneys
the power and right, on behalf of the Company, without notice to or assent by
the Company, to do the following:

               (a)  upon the occurrence and during the continuance of an Event
         of Default, generally to sell, transfer, pledge, make any agreement
         with respect to or otherwise deal with any of the Collateral in such
         manner as is consistent with the Massachusetts UCC and as fully and
         completely as though the Bank were the absolute owner thereof for all
         purposes, and to do at the Company's expense, at any time, or from time
         to time, all acts and things which the Bank deems necessary to protect,
         preserve or realize upon the Collateral and the Bank's security
         interest therein, in order to effect the intent of this Agreement, all
         as fully and effectively as the Company might do, including, without
         limitation, (i) the filing and prosecuting of registration and transfer
         applications with the appropriate federal or local agencies or
         authorities with respect to trademarks, copyrights and patentable
         inventions and processes, (ii) upon written notice to the Company, the
         exercise of voting rights with respect to voting securities, which
         rights may be exercised, if the Bank so elects, with a view to causing
         the liquidation in a commercially reasonable manner of assets of the
         issuer of any such securities and (iii) the execution, delivery and
         recording, in connection with any sale or other disposition of any
         Collateral, of the endorsements, assignments or other instruments of
         conveyance or transfer with respect to such Collateral; and

               (b)  to file such financing statements with respect hereto, with
         or without the Company's signature, or a photocopy of this Agreement in
         substitution for a financing statement, as the Bank may deem
         appropriate and to execute in the Company's name such financing
         statements and amendments thereto and continuation statements which may
         require the Company's signature.

         13.2. Ratification by Company.  To the extent permitted by law, the
               -----------------------
    Company hereby ratifies all that said attorneys shall lawfully do or cause
    to be done by virtue hereof This power of attorney is a power coupled with
    an interest and shall be irrevocable.

         13.3. No Duty on Bank.  The powers conferred on the Bank hereunder are
               ---------------
    solely to protect its interests in the Collateral and shall not impose any
    duty upon it to exercise any such powers. The Bank shall be accountable only
    for the amounts that it actually receives as a result of the exercise of
    such powers and neither it nor any of its officers, directors, employees or
    agents shall be responsible to the Company for any act or failure to act,
    except for the Bank's own gross negligence or willful misconduct.

    14.  Remedies.  If an Event of Default shall have occurred and be
         --------
continuing, the Bank may, without notice to or demand upon the Company, declare
this Agreement to be in default, and the Bank shall thereafter have in any
jurisdiction in which enforcement hereof is
<PAGE>

                                      -6-

sought, in addition to all other rights and remedies, the rights and remedies of
a secured party under the Uniform Commercial Code, including, without
limitation, the right to take possession of the Collateral, and for that purpose
the Bank may, so far as the Company can give authority therefor, enter upon any
premises on which the Collateral may be situated and remove the same therefrom.
The Bank may in its discretion require the Company to assemble all or any part
of the Collateral at such location or locations within the state(s) of the
Company's principal office(s) or at such other locations as the Bank may
designate. Unless the Collateral is perishable or threatens to decline speedily
in value or is of a type customarily sold on a recognized market, the Bank shall
give to the Company at least ten (10) Business Days prior written notice of the
time and place of any public sale of Collateral or of the time after which any
private sale or any other intended disposition is to be made. The Company hereby
acknowledges that ten (10) Business Days prior written notice of such sale or
sales shall be reasonable notice. In addition, the Company waives any and all
rights that it may have to a judicial hearing in advance of the enforcement of
any of the Bank's rights hereunder, including, without limitation, its right
following an Event of Default to take immediate possession of the Collateral and
to exercise its rights with respect thereto.

    15.  Regulatory Issues.  Upon the occurrence of an Event of Default, the
         -----------------
Bank shall receive, to the fullest extent permitted by applicable law and
governmental policy (including, without limitation, the rules, regulations and
policies of the FCC), all rights necessary or desirable to obtain, use or sell
the Collateral, and to exercise all remedies available to it under the Loan
Documents, the UCC or other applicable law. The parties further acknowledge and
agree that, in the event of changes in law or governmental policy occurring
subsequent to the date hereof that affect in any manner the Bank's right of
access to, or use or sale of, any of the Collateral, or the procedures necessary
to enable the Bank to obtain such rights of access, use or sale, the Bank and
the Company shall amend the Loan Documents, in such manner as the Bank shall
request in order to provide the Bank such rights to the greatest extent possible
consistent with then applicable law and governmental policy. The Company agrees
to take any action that the Bank may request in order to obtain from the FCC
such approval as may be necessary to enable the Bank to exercise and enjoy the
full rights and benefits granted to the Bank by this Agreement and each other
agreement, instrument and document delivered to the Bank in connection herewith.

    16.  No Waiver, etc.  The Company waives demand, notice, protest, notice of
         --------------
acceptance of this Agreement, notice of loans made, credit extended, Collateral
received or delivered or other action taken in reliance hereon and all other
demands and notices of any description. With respect to both the Obligations and
the Collateral, the Company assents to any extension or postponement of the time
of payment or any other indulgence, to any substitution, exchange or release of
or failure to perfect any security interest in any Collateral, to the addition
or release of any party or person primarily or secondarily liable, to the
acceptance of partial payment thereon and the settlement, Compromising or
adjusting of any thereof, all in such manner and at such time or times as the
Bank may deem advisable. The Bank shall have no duty as to the collection or
protection of the Collateral or any income thereon, nor as to the preservation
of rights against prior parties, nor as to the preservation of any rights
pertaining thereto beyond the safe custody thereof as set forth in (S)9.2. The
Bank shall not be deemed to have waived any of its rights upon or under the
Obligations or the Collateral unless such waiver
<PAGE>

                                      -7-

shall be in writing and signed by the Bank. No delay or omission on the part of
the Bank in exercising any right shall operate as a waiver of such right or any
other right. A waiver on any one occasion shall not be construed as a bar to or
waiver of any right on any future occasion. All rights and remedies of the Bank
with respect to the Obligations or the Collateral, whether evidenced hereby or
by any other instrument or papers, shall be cumulative and may be exercised
singularly, alternatively, successively or concurrently at such time or at such
times as the Bank deems expedient.

    17.  Marshalling.  The Bank shall not be required to marshal any present
         -----------
or future collateral security (including but not limited to this Agreement and
the Collateral) for, or other assurances of payment of, the Obligations or any
of them or to resort to such collateral security or other assurances of payment
in any particular order, and all of its rights hereunder and in respect of such
collateral security and other assurances of payment shall be cumulative and in
addition to all other rights, however existing or arising.  To the extent that
it lawfully may, the Company hereby agrees that it will not invoke any law
relating to the marshalling of collateral which might cause delay in or impede
the enforcement of the Bank's rights under this Agreement or under any other
instrument creating or evidencing any of the Obligations or under which any of
the Obligations is outstanding or by which any of the Obligations is secured or
payment thereof is otherwise assured, and, to the extent that it lawfully may,
the Company hereby irrevocably waives the benefits of all such laws.

    18.  Proceeds of Dispositions; Expenses.  The Company shall pay to the
         ----------------------------------
Bank on demand any and all expenses, including reasonable attorneys' fees and
disbursements, incurred or paid by the Bank in protecting, preserving or
enforcing the Bank's rights under or in respect of any of the Obligations or any
of the Collateral.  After deducting all of said expenses, the residue of any
proceeds of collection or sale of the Obligations or Collateral shall, to the
extent actually received in cash, be applied to the payment of the Obligations
in such order or preference as the Bank may determine, proper allowance and
provision being made for any Obligations not then due.  Upon the final payment
and satisfaction in full of all of the Obligations and after making any payments
required by Section 9-504(l)(c) of the Massachusetts UCC, any excess shall be
returned to the Company, and the Company shall remain liable for any deficiency
in the payment of the Obligations.

    19.  Overdue Amounts.  Until paid, all amounts due and payable by the
         ---------------
Company hereunder shall be a debt secured by the Collateral and shall bear,
whether before or after judgment, interest at the rate of interest for overdue
principal set forth in the Credit Agreement.

    20.  Governing Law; Consent to Jurisdiction.  THIS AGREEMENT IS INTENDED
         --------------------------------------
TO TAKE EFFECT AS A SEALED INSTRUMENT AND SHALL BE GOVERNED BY, AND CONSTRUED IN
ACCORDANCE WITH, THE LAWS OF THE COMMONWEALTH OF MASSACHUSETTS EXCEPT AS TO
MATTERS RELATING TO PERFECTION OF SECURITY INTERESTS, WHICH ARE GOVERNED BY
CONNECTICUT LAW.  The Company agrees that any suit for the enforcement of this
Agreement may be brought in the courts of the Commonwealth of Massachusetts or
any federal court sitting therein and consents to the non-exclusive jurisdiction
of such court and to service of process in any such suit being made upon the
Company by mail at the address specified in (S)10 of
<PAGE>

                                      -8-

the Credit Agreement. The Company hereby waives any objection that it may now or
hereafter have to the venue of any such suit or any such court or that such suit
is brought in an inconvenient court.

    21.  Waiver of Jury Trial.  THE COMPANY WAIVES ITS RIGHT TO A JURY TRIAL
         --------------------
WITH RESPECT TO ANY ACTION OR CLAIM ARISING OUT OF ANY DISPUTE IN CONNECTION
WITH THIS AGREEMENT, ANY RIGHTS OR OBLIGATIONS HEREUNDER OR THE PERFORMANCE OF
ANY SUCH RIGHTS OR OBLIGATIONS. Except as prohibited by law, the Company waives
any right which it may have to claim or recover in any litigation referred to in
the preceding sentence any special, exemplary, punitive or consequential damages
or any damages other than, or in addition to, actual damages. The Company (i)
certifies that neither the Bank nor any representative, agent or attorney of the
Bank has represented, expressly or otherwise, that the Bank would not, if the
event of litigation, seek to enforce the foregoing waivers and (ii) acknowledges
that, in entering into the Credit Agreement and the other Loan Documents to
which the Bank is a party, the Bank is relying upon, among other things, the
waivers and certifications contained in this (S)21.

    22.  PREJUDGMENT REMEDY WAIVER.  THE COMPANY ACKNOWLEDGES THAT THE
         -------------------------
FINANCING EVIDENCED HEREBY IS A COMMERCIAL TRANSACTION WITHIN THE MEANING OF
CHAPTER 903a OF THE CONNECTICUT GENERAL STATUTES. THE COMPANY HEREBY WAIVES ITS
RIGHT TO NOTICE AND PRIOR COURT HEARING OR COURT ORDER UNDER CONNECTICUT GENERAL
STATUTES SECTIONS 52-278a ET. SEQ. AS AMENDED OR UNDER ANY OTHER STATE OR
FEDERAL LAW WITH RESPECT TO ANY AND ALL PREJUDGMENT REMEDIES THE BANK MAY EMPLOY
TO ENFORCE ITS RIGHTS AND REMEDIES HEREUNDER. MORE SPECIFICALLY, THE COMPANY
ACKNOWLEDGES THAT THE BANK'S ATTORNEY MAY, PURSUANT TO CONN. GEN. STAT. (S)52-
278f, ISSUE A WRIT FOR A PREJUDGMENT REMEDY WITHOUT SECURING A COURT ORDER. THE
COMPANY ACKNOWLEDGES AND RESERVES ITS RIGHT TO NOTICE AND A HEARING SUBSEQUENT
TO THE ISSUANCE OF A WRIT FOR PREJUDGMENT REMEDY AS AFORESAID AND THE BANK
ACKNOWLEDGES THE COMPANY'S RIGHT TO SAID HEARING SUBSEQUENT TO THE ISSUANCE OF
SAID WRIT. THE COMPANY FURTHER WAIVES ITS RIGHTS TO REQUEST THAT BANK POST A
BOND, WITH OR WITHOUT SURETY, TO PROTECT THE COMPANY AGAINST GES THAT MAY BE
CAUSED BY ANY PREJUDGMENT REMEDY SOUGHT OR OBTAINED BY BANK.

    23.  Miscellaneous.  The headings of each section of this Agreement are
         -------------
for convenience only and shall not define or limit the provisions thereof.  This
Agreement and all rights and obligations hereunder shall be binding upon the
Company and its respective successors and assigns, and shall inure to the
benefit of the Bank and its successors and assigns.  If any term of this
Agreement shall be held to be invalid, illegal or unenforceable, the validity of
all other terms hereof shall in no way be affected thereby, and this Agreement
shall be construed and be enforceable as if such invalid, illegal or
unenforceable term had not been included herein.  The Company acknowledges
receipt of a copy of this Agreement.  This Agreement and any
<PAGE>

                                      -9-

amendment hereto may be executed in several counterparts, each of which shall be
an original, and all of which shall constitute one agreement.

    24.  Termination; Release of Collateral.  This Agreement and the security
         ----------------------------------
interest granted hereunder shall terminate on the date on which all Obligations
have been indefeasibly and fully satisfied, and Company shall have no right to
request further Loans under the Credit Agreement.  Bank shall thereupon reassign
and redeliver (or cause to be reassigned or redelivered) to Company or such
other person(s) as Company shall designate, against due execution and delivery
by Company or such person(s) of a receipt therefor reasonably satisfactory to
Bank in form and substance, such items of the Collateral (if any) as are then
held by Bank or its representatives, together with appropriate instruments of
reassignment and release.

     IN WITNESS WHEREOF, intending to be legally bound, the Company has caused
this Agreement to be duly executed as a sealed instrument as of the date first
above written.


                              DSL.NET, INC.

/s/ George S. Smith
-------------------

/s/ Andrea S. Harrington        By:  /s/ Robert Berlin
------------------------             -----------------
                                     Title:  Vice President
<PAGE>

                                      -10-

Accepted:

FLEET NATIONAL BANK


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


STATE OF CONNECTICUT     )

                         ) ss:    New Haven    May __, 1999

COUNTY OF NEW HAVEN      )


     On this ____ day of May, 1999, before me, ________________, the undersigned
officer, personally appeared ___________________, who acknowledged ____self to
be the ____________ of _____________, a corporation, and that ___ as such
_____________, being authorized so to do, executed the foregoing instrument for
the purposes therein contained, by signing the name of the corporation by
____self as such _____________ and as ____ and its free act and deed.

     In witness whereof I hereunto set my hand.

                              /s/ George S. Smith
                              -------------------
                              George S. Smith
                              Notary Public
                              My Commission Expires:
<PAGE>

                                      -11-

Accepted:

FLEET NATIONAL BANK


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


STATE OF CONNECTICUT     )

                         ) ss:    New Haven    May __, 1999

COUNTY OF NEW HAVEN      )


     On this ____ day of May, 1999, before me, ________________, the undersigned
officer, personally appeared ___________________, who acknowledged ____self to
be the ____________ of _____________, a corporation, and that ___ as such
_____________, being authorized so to do, executed the foregoing instrument for
the purposes therein contained, by signing the name of the corporation by
____self as such _____________ and as ____ and its free act and deed.

     In witness whereof I hereunto set my hand.

                              /s/ George S. Smith
                              -------------------
                              George S. Smith
                              Notary Public
                              My Commission Expires:
<PAGE>

                                   SCHEDULE I

<TABLE>
<CAPTION>
Vehicle I.D. Number              Make        Year        Model         Model Name          Color
-----------------------------------------------------------------------------------------------------
<S>                          <C>            <C>          <C>           <C>                 <C>
IGCDL196WXB125623             Chevrolet      1999         Van           Astro Van          White
-----------------------------------------------------------------------------------------------------
IGCDL19W6XB125461             Chevrolet      1999         Van           Astro Van          White
-----------------------------------------------------------------------------------------------------
</TABLE>