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                                                                    EXHIBIT 99.6

                                    ADDENDUM
                                       TO
                             STOCK OPTION AGREEMENT


               The following provisions are hereby incorporated into, and are
hereby made a part of, that certain Stock Option Agreement dated (the "Option
Agreement") by and between ACT Networks, Inc. (the "Corporation") and _________ 
("Optionee") evidencing the stock option (the "Option") granted on such date to
Optionee under the terms of the Corporation's 1997 Stock Incentive Plan, and
such provisions shall be effective immediately. All capitalized terms in this
Addendum, to the extent not otherwise defined herein, shall have the meanings
assigned to them in the Option Agreement.

                        LIMITED STOCK APPRECIATION RIGHT

               1. Optionee is hereby granted a limited stock appreciation right
in tandem with the Option, exercisable upon the terms set forth below:

                    (i) Optionee shall have the unconditional right, exercisable
        at any time during the thirty (30)-day period following a Hostile
        Take-Over, to surrender the Option to the Corporation, to the extent the
        Option is at the time exercisable for one or more Option Shares. In
        return for the surrendered Option, Optionee shall receive a cash
        distribution from the Corporation in an amount equal to the excess of
        (A) the Take-Over Price of the Option Shares for which the surrendered
        Option (or surrendered portion) is at the time exercisable over (B) the
        aggregate Exercise Price payable for those shares.

                   (ii) To exercise this limited stock appreciation right,
        Optionee must, during the applicable thirty (30)-day exercise period,
        provide the Corporation with written notice of the option surrender in
        which there is specified the number of Option Shares as to which the
        option is being surrendered. Such notice must be accompanied by the
        return of Optionee's copy of the Option Agreement, together with any
        written amendments to such Agreement. The cash distribution shall be
        paid to Optionee within five (5) days following such delivery date. The
        exercise of such limited stock appreciation right in accordance with the
        terms of this Addendum is hereby pre-approved by the Plan Administrator,
        and no further approval of the Plan Administrator or the Board shall be
        required at the time of the actual option surrender or cash
        distribution. Upon receipt of the cash distribution, the Option shall be
        cancelled with respect to the Option Shares for which the Option has
        been surrendered, and Optionee shall cease to have any further right to
        acquire those Option Shares under the Option Agreement. The


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        Option shall, however, remain outstanding and exercisable for the
        balance of the Option Shares (if any) in accordance with the terms of
        the Option Agreement, and the Corporation shall issue a replacement
        stock option agreement (substantially in the same form of the
        surrendered Option Agreement) for those remaining Option Shares.

                  (iii) In no event may this limited stock appreciation right be
        exercised when there is not a positive spread between the Fair Market
        Value of the Option Shares and the aggregate Exercise Price payable for
        such shares. This limited stock appreciation right shall in all events
        terminate upon the expiration or sooner termination of the option term
        and may not be assigned or transferred by Optionee.

               2. For purposes of this Addendum, the following definitions shall
be in effect:

                    (i) A HOSTILE TAKE-OVER shall be deemed to occur in the
        event any person or related group of persons (other than the Corporation
        or a person that directly or indirectly controls, is controlled by, or
        is under common control with, the Corporation) directly or indirectly
        acquires beneficial ownership (within the meaning of Rule 13d-3 of the
        Securities Exchange Act of 1934, as amended) of securities possessing
        more than fifty percent (50%) of the total combined voting power of the
        Corporation's outstanding securities pursuant to a tender or exchange
        offer made directly to the Corporation's stockholders which the Board
        does not recommend such stockholders to accept.

                   (ii) The TAKE-OVER PRICE per share shall be deemed to be
        equal to the greater of (A) the Fair Market Value per Option Share on
        the option surrender date or (B) the highest reported price per share of
        Common Stock paid by the tender offeror in effecting the Hostile
        Take-Over. However, if the surrendered Option is designated as an
        Incentive Option in the Grant Notice, then the Take-Over Price shall not
        exceed the clause (A) price per share.


                                       2.

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               IN WITNESS WHEREOF, ACT Networks, Inc. has caused this Addendum
to be executed by its duly-authorized officer as of the Effective Date specified
below.


                                            ACT NETWORKS, INC.

                                            By:
                                              ----------------------------------

                                            Title:
                                                 -------------------------------




EFFECTIVE DATE:____________________ , 199


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