AMENDMENT NO. 1 TO AGREEMENT AND PLAN OF MERGER

Exhibit 2.1

AMENDMENT NO. 1 TO AGREEMENT AND PLAN OF MERGER

This AMENDMENT NO. 1 TO AGREEMENT AND PLAN OF MERGER (this “ Amendment ”) is made as of October 1, 2007 by and among NetScout Systems, Inc., a Delaware corporation (“ NetScout ”), Bradley Merger Sub LLC, a Delaware limited liability company (“ Merger Sub ”), Network General Central Corporation, a Delaware corporation (“ NetGen ”), Network General Corporation, a Delaware corporation (“ NetGen Opco ”), and Silver Lake Partners, L.P., a Delaware limited partnership, and TPG Starburst IV, LLC, a Delaware limited liability company, in each case solely in its capacity as a representative for the stockholders of NetGen (each, in such capacity, a “ Stockholders Representative ”), and this Amendment amends that certain Agreement and Plan of Merger (the “ Merger Agreement ”), dated as of September 19, 2007, by and among NetScout, Merger Sub, NetGen, NetGen Opco and the Stockholders Representatives.

WHEREAS, in accordance with Section 10.04 of the Merger Agreement, the parties hereto wish to amend the Merger Agreement as specified herein.

NOW, THEREFORE, the parties hereto agree as follows:

1. Amendments .

(a) Section 1.01(a) of the Merger Agreement is hereby amended by deleting the definition of “Accredited Holder” and inserting the following definition in lieu thereof:

““ Accredited Holder ” shall mean any holder of NetGen Stock who no later than 25 days after the date hereof has delivered to NetGen and NetScout representations and warranties in the form letter set forth as Exhibit B hereto (the “ Accredited Investor Representations Letter ”).”

(b) Section 1.01(a) of the Merger Agreement is hereby amended by adding the following definition:

““ Bring-Down Date ” shall mean (a) if the waiting period applicable to the Merger under the HSR Act has not been terminated prior to October 26, 2007 and has not have been extended on or prior to October 26, 2007, in either case by the FTC or DOJ, then October 30, 2007 or (b) if the waiting period applicable to the Merger under the HSR Act has been terminated prior to October 26, 2007 or has been extended on or prior to October 26, 2007, in either case by the FTC or DOJ, then the Closing Date.”

(c) Section 1.01(a) of the Merger Agreement is hereby amended by deleting the definition of “Fully Diluted Basis” and inserting the following definition in lieu thereof:

““ Fully Diluted Basis ” shall mean, when used with respect to the outstanding number of shares of NetGen Stock as of the Effective Time, the sum of (i) all shares of NetGen Stock outstanding immediately prior to the Effective Time (including, as applicable at such time, the

shares of NetGen Common Stock that are reserved for issuance under any Deferred Compensation Agreement and that have not been issued at such time or the shares of NetGen Common Stock issued to the NetGen Deferred Compensation Trust pursuant to such Deferred Compensation Agreement); (ii) all shares of NetGen Common Stock issuable upon the exercise of all of the vested portions of in-the-money NetGen Stock Options outstanding immediately prior to the Effective Time; and (iii) all shares of NetGen Common Stock underlying all of the vested portions of in-the-money NetGen SARs outstanding immediately prior to the Effective Time.”

(d) Section 1.01(a) of the Merger Agreement is hereby amended by deleting the definition of “NetGen Stockholders” and inserting the following definition in lieu thereof:

““ NetGen Stockholders ” means each of the stockholders of NetGen immediately prior to the Effective Time.”

(e) Sections 2.06(b) and (c) of the Merger Agreement are hereby amended in their entirety and the following text is hereby inserted in lieu thereof:

“(b) At least three (3) Business Days prior to the Closing, NetGen will deliver to NetScout a schedule (the “ Distribution Schedule ”) setting forth for (i) each holder of NetGen Shares that is an Accredited Holder (based solely on the delivery by Accredited Holders of the Accredited Investor Representations Letter), the portion and mix of the Merger Consideration payable or issuable, as the case may be, to such holder on or about the Effective Time and the portion of the Escrow Cash and Escrow Shares corresponding to such holder, and (ii) each holder of NetGen Shares that is a Non-Accredited Holder, the portion of the Merger Consideration payable to such holder in cash on or about the Effective Time and the portion of the Escrow Cash corresponding to such holder, in each case, in accordance with NetGen’ certificate of incorporation in effect as of the date hereof. The Distribution Schedule will also list all holders of vested, in-the-money NetGen Stock Options and the amount payable to each such holder under Section 2.10 and all holders of vested, in-the-money NetGen SARs and the amount payable to each such holder under Section 2.10. NetScout shall be entitled to rely exclusively on the Distribution Schedule in making distributions of Merger Consideration pursuant to this Section 2.06.

(c) For purposes of this Agreement, each of the following terms shall have the meaning set forth below:

“ Accredited Cash Consideration Per Share ” shall mean an amount in cash equal to (a) the Total Consideration Per Share minus (b) the Accredited Stock Consideration Per Share.

“ Accredited Stock Consideration Per Share ” shall mean such number of shares of NetScout Common Stock equal in value to the quotient of (a) the Total Stock Consideration divided by (b) the number of shares of outstanding NetGen Stock held by the Accredited Holders as of the Effective Time.

“ Escrow Cash ” shall mean $10,000,000.

“ Escrow Consideration ” shall mean the Escrow Cash and the Escrow Shares.

“ Escrow Shares ” shall mean a number of shares of NetScout Common Stock equal to $5,000,000 divided by the NetScout Common Stock Value.

“ in-the-money ” shall mean (a) in respect of the NetGen Stock Options, each NetGen Stock Option that has an exercise price per share that is less than the Total Consideration Per Share, and (b) in respect of the NetGen SARs, each NetGen SAR that has a base price per share that is less than the Total Consideration Per Share.

“ Merger Consideration ” shall mean cash and/or shares of NetScout Common Stock, as applicable, payable to NetGen Stockholders and holders of NetGen Stock Options and NetGen SARs pursuant to this ARTICLE II .

“ NetScout Common Stock Value ” shall mean the average of the closing price of one share of NetScout Common Stock on the Nasdaq