Sec Form 4 Filing - SUMMIT PARTNERS L P @ A10 Networks, Inc. - 2018-11-07

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person SUMMIT PARTNERS L P 2. Issuer Name and Ticker or Trading Symbol A10 Networks, Inc. [ ATEN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director X __ 10% Owner _____ Officer (give title below) X __ Other (specify below) Manager of GP of 10% owner (Last) (First) (Middle) 222 BERKELEY STREET, 18TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 11/07/2018 (Street) BOSTON, MA02116 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 11/07/2018 A 23,112 ( 1 ) A $ 0 9,515,529 ( 2 ) I See Footnotes. ( 3 ) ( 4 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other SUMMIT PARTNERS L P 222 BERKELEY STREET 18TH FLOOR BOSTON, MA02116 X X Manager of GP of 10% owner SUMMIT MASTER COMPANY, LLC 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X Manager of Manager of 10% owne SUMMIT PARTNERS GE VIII, L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X GP of 10% owner Summit Partners GE VIII, LLC 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X GP of 10% owner Summit Partners Growth Equity Fund VIII-A, L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X X Summit Partners Growth Equity Fund VIII-B, L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X X Summit Investors Management, LLC 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X Manager and GP of 10% Owner Summit Investors I, LLC 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X X Summit Investors I (UK), L.P. 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X X MANNION MARTIN J C/O SUMMIT PARTNERS 222 BERKELEY STREET, 18TH FLOOR BOSTON, MA02116 X X Signatures Summit Partners, L.P., by Summit Master Company, LLC, its GP, by /s/ Robin W. Devereux, Member 11/09/2018 Signature of Reporting Person Date Summit Master Company, LLC, by Robin W. Devereux, Member 11/09/2018 Signature of Reporting Person Date Summit Partners GE VIII, L.P., by Summit Partners GE VIII, LLC, its General Partner, by Summit Partners, L.P., its Manager, by Summit Master Company, LLC, its General Partner, by /s/ Robin W. Devereux, Member 11/09/2018 Signature of Reporting Person Date Summit Partners GE VIII, LLC, by Summit Partners, L.P., its Manager, by Summit Master Company, LLC, its General Partner, by /s/ Robin W. Devereux, Member 11/09/2018 Signature of Reporting Person Date Summit Partners Growth Equity Fund VIII-A, L.P., by Summit Partners GE VIII, L.P., its GP, by Summit Partners GE VIII, LLC, its GP, by Summit Partners, L.P., its Manager, by Summit Master Company, LLC, its GP, by /s/ Robin W. Devereux, Member 11/09/2018 Signature of Reporting Person Date Summit Partners Growth Equity Fund VIII-B, L.P., by Summit Partners GE VIII, L.P., its GP, by Summit Partners GE VIII, LLC, its GP, by Summit Partners, L.P., its Manager, by Summit Master Company, LLC, its GP, by /s/ Robin W. Devereux, Member 11/09/2018 Signature of Reporting Person Date Summit Investors Management, LLC, by Summit Master Company, LLC, its Managing Member, by Robin W. Devereux, Member 11/09/2018 Signature of Reporting Person Date Summit Investors I, LLC, by Summit Investors Management, LLC, its Manager, by Summit Master Company, LLC, its Managing Member, by Robin W. Devereux, Member 11/09/2018 Signature of Reporting Person Date Summit Investors I (UK), L.P., by Summit Investors Management, LLC, its General Partner, by Summit Master Company, LLC, its Managing Member, by Robin W. Devereux, Member 11/09/2018 Signature of Reporting Person Date Robin W. Devereux, POA for Martin J. Mannion 11/09/2018 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Represents restricted stock units granted to Mr. Chung as part of the director compensation program. The restricted stock units will settle solely by delivery of an equal number of shares of Common Stock on the earlier of the first anniversary of the grant date or the date of the Issuer's next Annual Meeting, subject to the Reporting Person's continued service on the board of directors as of such date. Mr. Chung holds any restricted stock units for the benefit of Summit Partners, L.P., which he has empowered to determine when the underlying shares will be sold and which is entitled to the proceeds of any such sales. ( 2 )The Common Stock is held as follows: 6,873,136 shares in the name of Summit Partners Growth Equity Fund VIII-A, L.P.; 2,510,989 shares in the name of Summit Partners Growth Equity Fund VIII-B, L.P.; 40,186 shares in the name of Summit Investors I, LLC; 3,535 shares in the name of Summit Investors I (UK), L.P.; 87,683 shares and restricted stock units in the name of Peter Y. Chung, which are held for the benefit of Summit Partners, L.P. ( 3 )Summit Partners, L.P. is the managing member of Summit Partners GE VIII, LLC, which is the general partner of Summit Partners GE VIII, L.P., which is the general partner of each of Summit Partners Growth Equity Fund VIII-A, L.P. and Summit Partners Growth Equity Fund VIII-B, L.P. Summit Master Company, LLC is the managing member of Summit Investors Management, LLC, which is the manager of Summit Investors I, LLC and the general partner of Summit Investors I (UK), L.P. Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC has delegated investment decisions, including voting and dispositive power, to Summit Partners, L.P. and its Investment Committee responsible for voting and investment decisions with respect to the Issuer. ( 4 )Summit Partners, L.P., through a two-person Investment Committee responsible for voting and investment decisions with respect to the Issuer, currently composed of Martin J. Mannion and Peter Y. Chung, has voting and dispositive authority over the shares held by each of these entities and therefore may be deemed to beneficially own such shares. In addition, Mr. Chung is a member of Summit Master Company, LLC. Each of the Summit entities mentioned herein, Summit Partners, L.P., Summit Master Company, LLC, Mr. Mannion and Mr. Chung disclaims beneficial ownership of the shares of Common Stock and the restricted stock units, except to the extent of their pecuniary interest therein.

Remarks: Form 2 of 2

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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