Registration No. 333-58362

Filed Pursuant to Rule 424(b)(3)

SUPPLEMENT DATED AUGUST 13, 2002

TO PROSPECTUS DATED SEPTEMBER 10, 2001

5,402,250 Shares of Common Stock

COM21, INC.

SUMMARY OF RIGHTS TO PURCHASE COMMON STOCK OF COM21, INC.

On July 17, 2002, the Board of Directors of Com21, Inc. (the “Company”) declared a dividend of one preferred share purchase right (a “Right”) for each outstanding share of common stock, par value $0.001 per share, of the Company (the “Common Stock”). The dividend is payable on August 7, 2002 (the “Record Date”) to the stockholders of record on that date. Each Right entitles the registered holder to purchase from the Company one one-thousandth of a share of Series A Junior Participating Preferred Stock, par value $0.001 per share, of the Company (the “Preferred Stock”) at a price of $5.00 per one one-thousandth of a share of Preferred Stock (the “Purchase Price”), subject to adjustment. The description and terms of the Rights are set forth in a Rights Agreement dated as of July 19, 2002, as the same may be amended from time to time (the “Rights Agreement”), between the Company and EquiServe Trust Company, N.A., as Rights Agent (the “Rights Agent”).

Until the earlier to occur of (1) 10 days following a public announcement that a person or group of affiliated or associated persons (with certain exceptions, an “Acquiring Person”) has acquired beneficial ownership of 15% or more of the outstanding shares of Common Stock or (2) 10 business days (or such later date as may be determined by action of the Board of Directors prior to such time as any person or group of affiliated persons becomes an Acquiring Person) following the commencement of, or announcement of an intention to make, a tender offer or exchange offer the consummation of which would result in the beneficial ownership by a person or group of 15% or more of the outstanding shares of Common Stock (the earlier of such dates being called the “Distribution Date”), the Rights will be evidenced, with respect to any of the Common Stock certificates outstanding as of the Record Date, by such Common Stock certificate together with this Summary of Rights.

The Rights Agreement provides that, until the Distribution Date (or earlier expiration of the Rights), the Rights will be transferred with and only with the Common Stock. Until the Distribution Date (or earlier expiration of the Rights), new Common Stock certificates issued after the Record Date upon transfer or new issuances of Common Stock will contain a notation incorporating the Rights Agreement by reference. Until the Distribution Date (or earlier expiration of the Rights), the surrender for transfer of any certificates for shares of Common Stock outstanding as of the Record Date, even without such notation or a copy of this Summary

of Rights, will also constitute the transfer of the Rights associated with the shares of Common Stock represented by such certificate. As soon as practicable following the Distribution Date, separate certificates evidencing the Rights (“Right Certificates”) will be mailed to holders of record of the Common Stock as of the close of business on the Distribution Date and such separate Right Certificates alone will evidence the Rights.

The Rights are not exercisable until the Distribution Date. The Rights will expire on August 7, 2012 (the “Final Expiration Date”), unless the Final Expiration Date is advanced or extended or unless the Rights are earlier redeemed or exchanged by the Company, in each case as described below.

The Purchase Price payable, and the number of shares of Preferred Stock or other securities or property issuable, upon exercise of the Rights is subject to adjustment from time to time to prevent dilution (1) in the event of a stock dividend on, or a subdivision, combination or reclassification of, the Preferred Stock, (2) upon the grant to holders of the Preferred Stock of certain rights or warrants to subscribe for or purchase Preferred Stock at a price, or securities convertible into Preferred Stock with a conversion price, less than the then-current market price of the Preferred Stock or (3) upon the distribution to holders of the Preferred Stock of evidences of indebtedness or assets (excluding regular periodic cash dividends or dividends payable in Preferred Stock) or of subscription rights or warrants (other than those referred to above).

The number of outstanding Rights is subject to adjustment in the event of a stock dividend on the Common Stock payable in shares of Common Stock or subdivisions, consolidations or combinations of the Common Stock occurring, in any such case, prior to the Distribution Date.

Shares of Preferred Stock purchasable upon exercise of the Rights will not be redeemable. Each share of Preferred Stock will be entitled, when, as and if declared, to a minimum preferential quarterly dividend payment of the greater of (1) $0.001 per share, and (2) an amount equal to 1,000 times the dividend declared per share of Common Stock. In the event of liquidation, dissolution or winding up of the Company, the holders of the Preferred Stock will be entitled to a minimum preferential payment of the greater of (a) $0.001 per share (plus any accrued but unpaid dividends), and (b) an amount equal to 1,000 times the payment made per share of Common Stock. Each share of Preferred Stock will have 1,000 votes, voting together with the Common Stock. Finally, in the event of any merger, consolidation or other transaction in which outstanding shares of Common Stock are converted or exchanged, each share of Preferred Stock will be entitled to receive 1,000 times the amount received per share of Common Stock. These rights are protected by customary antidilution provisions.

Because of the nature of the Preferred Stock’s dividend, liquidation and voting rights, the value of the one one-thousandth interest in a share of Preferred Stock purchasable upon exercise of each Right should approximate the value of one share of Common Stock.

In the event that any person or group of affiliated or associated persons becomes an Acquiring Person, each holder of a Right, other than Rights beneficially owned by the Acquiring Person (which will thereupon become void), will thereafter have the right to receive upon

exercise of a Right that number of shares of Common Stock having a market value of two times the exercise price of the Right.

In the event that, after a person or group has become an Acquiring Person, the Company is acquired in a merger or other business combination transaction or 50% or more of its consolidated assets or earning power are sold, proper provisions will be made so that each holder of a Right (other than Rights beneficially owned by an Acquiring Person which will have become void) will thereafter have the right to receive upon the exercise of a Right that number of shares of common stock of the person with whom the Company has engaged in the foregoing transaction (or its parent) that at the time of such transaction have a market value of two times the exercise price of the Right.

At any time after any person or group becomes an Acquiring Person and prior to the earlier of one of the events described in the previous paragraph or the acquisition by such Acquiring Person of 50% or more of the outstanding shares of Common Stock, the Board of Directors of the Company may exchange the Rights (other than Rights owned by such Acquiring Person which will have become void), in whole or in part, for shares of Common Stock or Preferred Stock (or a series of the Company’s preferred stock having equivalent rights, preferences and privileges), at an exchange ratio of one share of Common Stock, or a fractional share of Preferred Stock (or other preferred stock) equivalent in value thereto, per Right.

With certain exceptions, no adjustment in the Purchase Price will be required until cumulative adjustments require an adjustment of at least 1% in such Purchase Price. No fractional shares of Preferred Stock or Common Stock will be issued (other than fractions of Preferred Stock which are integral multiples of one one-thousandth of a share of Preferred Stock, which may, at the election of the Company, be evidenced by depositary receipts), and in lieu thereof an adjustment in cash will be made based on the current market price of the Preferred Stock or the Common Stock.

At any time prior to the time an Acquiring Person becomes such, the Board of Directors of the Company may redeem the Rights in whole, but not in part, at a price of $0.001 per Right (the “Redemption Price”) payable, at the option of the Company, in cash, shares of Common Stock or such other form of consideration as the Board of Directors of the Company shall determine. The redemption of the Rights may be made effective at such time, on such basis and with such conditions as the Board of Directors in its sole discretion may establish. Immediately upon any redemption of the Rights, the right to exercise the Rights will terminate and the only right of the holders of Rights will be to receive the Redemption Price.

For so long as the Rights are then redeemable, the Company may, except with respect to the Redemption Price, amend the Rights Agreement in any manner. After the Rights are no longer redeemable, the Company may, except with respect to the Redemption Price, amend the Rights Agreement in any manner that does not adversely affect the interests of holders of the Rights.

Until a Right is exercised or exchanged, the holder thereof, as such, will have no rights as a stockholder of the Company, including, without limitation, the right to vote or to receive dividends.

The Rights have certain anti-takeover effects. The Rights will cause substantial dilution to a person or group that attempts to acquire the Company in certain circumstances. Accordingly, the existence of the Rights may deter certain acquirors from making takeover proposals or tender offers. The Rights should not interfere with any merger or other business combination approved by the Board of Directors of the Company since the Board of Directors may, at its option, at any time prior to the time an Acquiring Person becomes such, redeem all but not less than all the then outstanding Rights at $0.001 per Right.

For the purposes of the Rights Agreement, the Board of Directors has established the Series A Junior Participating Preferred Stock with 100,000 authorized shares, none of which were outstanding as of the date of this Prospectus, from the shares of preferred stock the Board of Directors is authorized pursuant to the Company’s Certificate of Incorporation to establish and issue by resolution without any further stockholder approval. Shares of the Series A Junior Participating Preferred Stock may be purchased pursuant to the terms and conditions of the Rights Agreement. Rights and privileges of the Series A Junior Participating Preferred Stock are set forth in the form of Certificate of Designation of Series A Junior Participating Preferred Stock which is included as an exhibit to the Rights Agreement that is an exhibit to the Current Report on Form 8-K of the Company, dated August 6, 2002. The foregoing description of the Rights does not purport to be complete and is qualified in its entirety by reference to the Rights Agreement.

No other change or amendment is made hereby.

This Prospectus Supplement should be read in conjunction with the Prospectus, and this Prospectus Supplement is qualified by reference to the Prospectus except to the extent that the information herein contained supersedes the information contained in the Prospectus.