Document:

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                                                                     EXHIBIT 4.3

[Front]

              Incorporated under the laws of the State of Delaware

                                    NETSCOUT

NUMBER                                                 SHARES
NET

                                                       Common Stock See
                                                       Reverse for
                                                       Certain Definitions

                             NetScout Systems, Inc.

                   FULLY PAID AND NONASSESSABLE SHARES OF THE
                 PAR VALUE OF $.001 EACH OF THE COMMON STOCK OF

NetScout Systems, Inc. transferable upon the books of the Corporation in person
or by attorney upon surrender of this certificate duly endorsed or assigned.
This certificate and the shares represented hereby are subject to the laws of
the State of Delaware and to the Certificate of Incorporation and By-Laws of the
Corporation as from time to time amended.

       This certificate is not valid unless countersigned by the Transfer Agent
and registered by the Registrar.

       IN WITNESS WHEREOF, NetScout Systems, Inc. has caused its facsimile
corporate seal and the facsimile signatures of its duly authorized officers to
be hereunto affixed.

Dated:

         Anil Singhal                           Anil Singhal
         Treasurer                              President

Countersigned and Registered: Mellon Investor Services, LLC
                              Transfer Agent and Registrar

                               By Authorized Signatory

[Reverse Side]

<PAGE>

       THE CORPORATION IS AUTHORIZED TO ISSUE MORE THAN ONE CLASS OR SERIES OF
STOCK. A COPY OF THE PREFERENCES, POWERS, QUALIFICATIONS AND RIGHTS OF EACH
CLASS AND SERIES WILL BE FURNISHED BY THE CORPORATION UPON WRITTEN REQUEST AND
WITHOUT CHARGE.

       The following abbreviations, when used in the inscription on the face of
this certificate, shall be construed as though they were written out in full
according to applicable laws or regulations:

TEN COM - as tenants in common           UNIF GIFT MIN ACT-____Custodian_____
TEN ENT - as tenants by the entireties                    (Cust)       (Minor)
JT TEN  - as joint tenants with right           under Uniform Gift to Minors
          of survivorship and not as            Act__________
          tenants in common                         (State)

       Additional abbreviations may also be used though not in the above list.

For value received, __________________ hereby sell, assign, and transfer unto
[PLEASE INSERT] SOCIAL SECURITY OR OTHER IDENTIFYING NUMBER OF ASSIGNEE
________________________________________________________________________________

[PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS OF ASSIGNEE]

________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________

Shares of the capital stock represented by the within Certificate, and does
hereby irrevocably constitute and appoint ______________________________________
Attorney to transfer the said stock on the books of the within-named Corporation
with full power of substitution in the premises. Dated,_______________

             ---------------------------------------------------------------
             NOTICE: The signature to this assignment must correspond with
             the name as written upon the face of the certificate, in every
             particular, without alteration or enlargement or any change
             whatever.

SIGNATURE(S) GUARANTEED:

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The signature(s) should be guaranteed by an eligible
guarantor institution (banks, stockbrokers, savings
and loan associations and credit unions with
membership in an approved signature guarantee
medallion program), pursuant to SEC Rule 17Ad-15.<PAGE>

                                                                   EXHIBIT 10.22

                           LOAN MODIFICATION AGREEMENT

       This LOAN MODIFICATION AGREEMENT is entered into as of June 27, 2000, by
and between SILICON VALLEY BANK, a California-chartered bank with its principal
place of business at 3003 Tasman Drive, Santa Clara, CA 95054 and with a loan
production office located at Wellesley Office Park, 40 William Street, Suite
350, Wellesley, MA 02481, doing business under the name "Silicon Valley East"
("Bank"), and NETSCOUT SYSTEMS, INC., a Delaware corporation with its principal
place of business at 4 Technology Park Drive, Westford, MA 01886 ("Borrower").

                                    RECITALS

       Borrower has borrowed money from Bank pursuant to certain Existing Loan
Documents, as defined below. In consideration of certain financial
accommodations from Bank, and Borrower's continuing obligations under the
Existing Loan Documents, Borrower and Bank agree as follows:

       1. DESCRIPTION OF EXISTING INDEBTEDNESS. Among other indebtedness which
may be owing by Borrower to Bank, Borrower is indebted to Bank pursuant to,
among other documents, an Amended and Restated Loan and Security Agreement dated
as of March 12, 1998, between Borrower and Bank, as amended by Loan Modification
Agreements dated as of March 11, 1999 and March 10, 2000, providing for a
revolving credit facility up to a maximum principal amount of FIVE MILLION AND
NO/100THS DOLLARS ($5,000,000), as such Loan and Security Agreement may be
further amended from time to time (the "Loan Agreement").

       Hereinafter, all Indebtedness owing by Borrower to Bank shall be referred
to as the "Indebtedness."

       2. DESCRIPTION OF COLLATERAL. Repayment of the Indebtedness is secured
pursuant to the Loan Agreement. Hereinafter, the Loan Agreement, together with
all other documents securing payment of the Indebtedness, shall be referred to
as the "Existing Loan Documents."

       3. DESCRIPTION OF CHANGES IN TERMS.

       3.1 MODIFICATIONS TO REVOLVING ADVANCE PROVISIONS. Section 2.1.1(a) of
the Loan Agreement is hereby replaced in its entirety with the following:

       (a) Bank will make Advances not exceeding (i) the lesser of (A) the
       Committed Revolving Line minus the Cash Management Services Sublimit or
       (B) the Borrowing Base, whichever is less, minus (ii) the amount of all
       outstanding Letters of Credit (including drawn but unreimbursed Letters
       of Credit), minus (iii) immediately upon the closing of the acquisition
       by Borrower of NextPoint Networks, Inc., all outstanding principal and
       interest under that certain equipment line of credit originally extended
       by Bank to NextPoint Networks, Inc. Amounts borrowed under this Section
       may be repaid and reborrowed during the term of this Agreement.

<PAGE>

       4. CONSENT TO ACQUISITION. Bank hereby consents to Borrower's acquisition
of NextPoint Networks, Inc., and waives Borrower's compliance with Sections 7.2,
7.3 and 7.6 of the Loan Agreement for the sole purpose of closing that
acquisition, provided that no Event of Default would exist after giving effect
to that acquisition.

       5. FEES. Borrower shall pay to Bank any out-of-pocket expenses incurred
by the Bank through the date hereof, including reasonable attorneys' fees and
expenses, and after the date hereof, all Bank Expenses, including reasonable
attorneys' fees and expenses, as and when they become due.

       6. CONDITIONS TO FURTHER ADVANCES. The obligation of Bank to make further
advances to Borrower under this line is subject to the condition precedent that
Bank shall have received, in form and substance satisfactory to Bank, the
following:

       (i)    this Loan Modification Agreement duly executed by Borrower;

       (ii)   payment of the fees and Bank Expenses then due specified in
              Section 6 hereof;

       (iii)  such other documents, and competition of such other matters, as
              Bank may reasonably deem necessary or appropriate.

       7. CONSISTENT CHANGES. The Existing Loan Documents are hereby amended
wherever necessary to retain the changes described in this Loan Modification
Agreement.

       8. NO DEFENSES OF BORROWER. Borrower agrees that as of this date, it has
no defenses against any of the obligations to pay any amounts under the
indebtedness.

       9. CONTINUING VALIDITY. Borrower understands and agrees that (i) in
modifying the Existing Loan Documents, Bank is relying upon Borrower's
representations, warranties and agreements, as set forth in the Existing Loan
Documents, (ii) except as expressly modified pursuant to this Loan Modification
Agreement (including the effects of Section 7 hereof), the Existing Loan
Documents remain unchanged and in full force and effect, (iii) Bank's agreement
to modify the Existing Loan Documents pursuant to this Loan Modification
Agreement shall in no way obligate Bank to make any future modifications to the
Existing Loan Documents, (iv) it is the intention of Bank and Borrower to retain
as liable parties all makers and endorsers of the Existing Loan Documents,
unless a party is expressly released by Bank in writing, (v) no maker, endorser
or guarantor will be released by virtue of this Loan Modification Agreement, and
(vi) the terms of this Section 9 apply not only to this Loan Modification
Agreement but also to all subsequent loan modification agreements, if any.

<PAGE>

       10. EFFECTIVENESS. This Agreement shall become effective only when it
shall have been executed by Borrower and Bank (provided, however, in no event
shall this Agreement become effective until signed by an officer of Bank in
California).

       IN WITNESS WHEREOF, the parties hereto have caused this Loan Modification
Agreement to be executed as a sealed investment as of the date first set forth
above.

"Borrower":  NETSCOUT SYSTEMS, INC.     "Bank":  SILICON VALLEY BANK,  doing
                                        business as SILICON VALLEY EAST

By: /s/ LISA FIORENTINO                 By: /s/ DALE EDMUNDS
   -------------------------------          -----------------------------------
   Lisa Fiorentino, VP of Finance           Dale C. Edmunds

                                        SILICON VALLEY BANK

                                        By: /s/ MAGGIE GARCIA
                                            -----------------------------------

                                        Title: DOCUMENTATION OFFICER
                                              ---------------------------------
                                      (Signed in Santa Clara County, California)

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