Sec Form 4 Filing - Singer Eric @ A10 Networks, Inc. - 2019-07-26

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 Singer Eric 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 _____ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O A10 NETWORKS, INC., 3 WEST PLUMERIA DRIVE 3. Date of Earliest Transaction (MM/DD/YY) 07/26/2019 (Street) SAN JOSE, CA95134 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, par value $0.00001 07/26/2019 A 30,120 A $ 0 30,120 ( 2 ) D ( 1 ) Common Stock, par value $0.00001 1,658,360 I By Viex Opportunities Fund, LP - Series One ( 1 ) ( 3 ) ( 7 ) Common Stock, par value $0.00001 331,977 I By Viex Opportunities Fund, LP - Series Two ( 1 ) ( 4 ) ( 7 ) Common Stock, par value $0.00001 3,531,222 I By Viex Special Opportunities Fund II, LP ( 1 ) ( 5 ) ( 7 ) Common Stock, par value $0.00001 271,116 I By Viex Special Opportunities Fund III, LP ( 1 ) ( 6 ) ( 7 )

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 Call Option (Right to Buy) $ 2.5 07/30/2019 11/15/2019 Common Stock, par value $0.00001 150,900 1,509 I By Viex Opportunities Fund, LP - Series Two ( 1 ) ( 4 ) ( 7 ) Call Option (Right to Buy) $ 2.5 07/30/2019 11/15/2019 Common Stock, par value $0.00001 150,000 1,500 I By Viex Special Opportunities Fund II, LP ( 1 ) ( 5 ) ( 7 ) Call Option (Right to Buy) $ 5 07/30/2019 11/15/2019 Common Stock, par value $0.00001 5,900 59 I By Viex Special Opportunities Fund II, LP ( 1 ) ( 5 ) ( 7 ) Put Option (Obligation to Buy) $ 5 07/30/2019 08/16/2019 Common Stock, par value $0.00001 111,700 1,117 I By Viex Special Opportunities Fund II, LP ( 1 ) ( 5 ) ( 7 ) Put Option (Obligation to Buy) $ 7.5 07/30/2019 08/16/2019 Common Stock, par value $0.00001 1,367,900 13,679 I By Viex Special Opportunities Fund II, LP ( 1 ) ( 5 ) ( 7 ) Put Option (Obligation to Buy) $ 7.5 07/30/2019 11/15/2019 Common Stock, par value $0.00001 200,000 2,000 I By Viex Special Opportunities Fund II, LP ( 1 ) ( 5 ) ( 7 ) Call Option (Right to Buy) $ 2.5 07/30/2019 11/15/2019 Common Stock, par value $0.00001 210,500 2,105 I By Viex Special Opportunities Fund III, LP ( 1 ) ( 6 ) ( 7 ) Call Option (Right to Buy) $ 5 07/30/2019 11/15/2019 Common Stock, par value $0.00001 5,800 58 I By Viex Special Opportunities Fund III, LP ( 1 ) ( 6 ) ( 7 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Singer Eric C/O A10 NETWORKS, INC. 3 WEST PLUMERIA DRIVE SAN JOSE, CA95134 X VIEX Opportunities Fund, LP Series One C/O A10 NETWORKS, INC. 3 WEST PLUMERIA DRIVE SAN JOSE, CA95134 See Explanation of Responses VIEX Opportunities Fund, LP - Series Two C/O A10 NETWORKS, INC. 3 WEST PLUMERIA DRIVE SAN JOSE, CA95134 See Explanation of Responses VIEX Special Opportunities Fund II, LP C/O A10 NETWORKS, INC. 3 WEST PLUMERIA DRIVE SAN JOSE, CA95134 See Explanation of Responses VIEX Special Opportunities Fund III, LP C/O A10 NETWORKS, INC. 3 WEST PLUMERIA DRIVE SAN JOSE, CA95134 See Explanation of Responses VIEX GP, LLC C/O A10 NETWORKS, INC. 3 WEST PLUMERIA DRIVE SAN JOSE, CA95134 See Explanation of Responses VIEX Special Opportunities GP II, LLC C/O A10 NETWORKS, INC. 3 WEST PLUMERIA DRIVE SAN JOSE, CA95134 See Explanation of Responses VIEX Special Opportunities GP III, LLC C/O A10 NETWORKS, INC. 3 WEST PLUMERIA DRIVE SAN JOSE, CA95134 See Explanation of Responses VIEX Capital Advisors, LLC C/O A10 NETWORKS, INC. 3 WEST PLUMERIA DRIVE SAN JOSE, CA95134 See Explanation of Responses Signatures ERIC SINGER, Name: /s/ Eric Singer 07/30/2019 Signature of Reporting Person Date VIEX OPPORTUNITIES FUND, LP - SERIES ONE, By: VIEX GP, LLC, its General Partner, Name: /s/ Eric Singer, Title: Managing Member 07/30/2019 Signature of Reporting Person Date VIEX OPPORTUNITIES FUND, LP - SERIES TWO, By: VIEX GP, LLC, its General Partner, Name: /s/ Eric Singer, Title: Managing Member 07/30/2019 Signature of Reporting Person Date VIEX GP, LLC, Name: /s/ Eric Singer, Title: Managing Member 07/30/2019 Signature of Reporting Person Date VIEX SPECIAL OPPORTUNITIES FUND II, LP, By: VIEX Special Opportunities GP II, LLC, its General Partner, Name: /s/ Eric Singer, Title: Managing Member 07/30/2019 Signature of Reporting Person Date VIEX SPECIAL OPPORTUNITIES GP II, LLC, Name: /s/ Eric Singer, Title: Managing Member 07/30/2019 Signature of Reporting Person Date VIEX SPECIAL OPPORTUNITIES FUND III, LP, By: VIEX Special Opportunities GP III, LLC, its General Partner, Name: /s/ Eric Singer, Title: Managing Member 07/30/2019 Signature of Reporting Person Date VIEX SPECIAL OPPORTUNITIES GP III, LLC, Name: /s/ Eric Singer, Title: Managing Member 07/30/2019 Signature of Reporting Person Date VIEX CAPITAL ADVISORS, LLC, Name: /s/ Eric Singer, Title: Managing Member 07/30/2019 Signature of Reporting Person Date

Explanation of Responses:

( 1 )This Form 4 is filed jointly by VIEX Opportunities Fund, LP - Series One ("Series One"), a series of VIEX Opportunities Fund, LP, VIEX Opportunities Fund, LP - Series Two ("Series Two"), a series of VIEX Opportunities Fund, LP, VIEX Special Opportunities Fund II, LP ("VSO II"), Viex Special Opportunities Fund III, LP ("VSO III"), VIEX GP, LLC ("VIEX GP"), VIEX Special Opportunities GP II, LLC ("VSO GP II"), Viex Special Opportunities GP III, LLC ("VSO GP III"), VIEX Capital Advisors, LLC ("VIEX Capital"), and Eric Singer (collectively, the "Reporting Persons"). ( 2 )Each share is represented by a restricted stock unit, which vests in three equal annual installments on the anniversary of the grant date of 07/26/2019, provided that the Reporting Person continues to serve through each vesting date. ( 3 )Securities of A10 Networks, Inc. (the "Issuer") beneficially owned directly by Series One. VIEX GP, as the general partner of Series One, may be deemed the beneficial owner of the securities of the Issuer beneficially owned by Series One. VIEX Capital, as the investment manager of Series One, may be deemed the beneficial owner of the securities of the Issuer beneficially owned by Series One. Mr. Singer, as the managing member of each of VIEX GP and VIEX Capital, may be deemed the beneficial owner of the securities of the Issuer beneficially owned by Series One. ( 4 )Securities of the Issuer beneficially owned directly by Series Two. VIEX GP, as the general partner of Series Two, may be deemed the beneficial owner of the securities of the Issuer beneficially owned by Series Two. VIEX Capital, as the investment manager of Series Two, may be deemed the beneficial owner of the securities of the Issuer beneficially owned by Series Two. Mr. Singer, as the managing member of each of VIEX GP and VIEX Capital, may be deemed the beneficial owner of the securities of the Issuer beneficially owned by Series Two. ( 5 )Securities of the Issuer beneficially owned directly by VSO II. VSO GP II, as the general partner of VSO II, may be deemed the beneficial owner of the securities of the Issuer beneficially owned by VSO II. VIEX Capital, as the investment manager of VSO II, may be deemed the beneficial owner of the securities of the Issuer beneficially owned by VSO II. Mr. Singer, as the managing member of each of VSO GP II and VIEX Capital, may be deemed the beneficial owner of the securities of the Issuer beneficially owned by VSO II. ( 6 )Securities of the Issuer beneficially owned directly by VSO III. VSO GP III, as the general partner of VSO III, may be deemed the beneficial owner of the securities of the Issuer beneficially owned by VSO III. VIEX Capital, as the investment manager of VSO III, may be deemed the beneficial owner of the securities of the Issuer beneficially owned by VSO III. Mr. Singer, as the managing member of each of VSO GP III and VIEX Capital, may be deemed the beneficial owner of the securities of the Issuer beneficially owned by VSO III. ( 7 )Each Reporting Person disclaims beneficial ownership of the securities of the Issuer reported herein except to the extent of his or its pecuniary interest therein, and this report shall not be deemed to be an admission that any Reporting Person is the beneficial owner of such securities for purposes of Section 16 or for any other purpose.

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).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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