UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No. 6)*

Ixia

(Name of Issuer)

Common Stock

(Title of Class of Securities)

45071R109

(CUSIP Number)

September 15, 2016

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

¨ Rule 13d-1(b)

x Rule 13d-1(c)

¨ Rule 13d-1(d)

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

The original statement shall be signed by each person on whose behalf the statement is filed or his authorized representative. If the statement is signed on behalf of a person by his authorized representative other than an executive officer or general partner of the filing person, evidence of the representative’s authority to sign on behalf of such person shall be filed with the statement, provided, however, that a power of attorney for this purpose which is already on file with the Commission may be incorporated by reference. The name and any title of each person who signs the statement shall be typed or printed beneath his signature.

NOTE: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See §240.13d-7 for other parties for whom copies are to be sent.

Attention: Intentional misstatements or omissions of fact constitute Federal criminal violations (See 18 U.S.C. 1001)

EXHIBIT INDEX

EXHIBIT 1

AGREEMENT TO FILE JOINT

STATEMENTS ON SCHEDULE 13G

THIS AGREEMENT is entered into as of the 15th day of September, 2016 by and among Addington Hills Ltd., a company organized under the laws of the Bahamas, The Pink Trust, a trust organized under the laws of Jersey (the “Trust”), and PraxisIFM Trustees SA, a company organized under the laws of Switzerland (the “Trustee”).

W I T N E S S E T H

WHEREAS, Addington Hills Ltd., the Trust and the Trustee may each be deemed to hold beneficial ownership of more than five percent of the shares of the Common Stock of Ixia, a California corporation (the “Ixia Common Stock”);

WHEREAS, the Ixia Common Stock has been registered by Ixia under Section 12(g) of the Securities Exchange Act of 1934, as amended (the “Act”);

WHEREAS, pursuant to Rule 13d-1 under the Act, any person who holds more than five percent of such a class of registered equity securities is, under certain circumstances, permitted and/or required to file with the Securities and Exchange Commission a statement on Schedule 13G; and

WHEREAS, Rule 13d-1(k) under the Act provides that, whenever two or more persons are permitted to file a statement on Schedule 13G with respect to the same securities, only one such statement need be filed, provided such persons agree in writing that such statement is filed on behalf of each of them;

NOW, THEREFORE, in consideration of the foregoing premises and the mutual covenants and agreements contained herein, the parties hereby agree as follows:

Addington Hills Ltd., the Trust and the Trustee each hereby agree, in accordance with Rule 13d-1(k) under the Act, to file jointly statements on Schedule 13G and amendments thereto (collectively, the “Statements”) with respect to the Ixia Common Stock beneficially owned or deemed to be beneficially owned by each of them pursuant to Sections 13(d) and 13(g) of the Act and the rules thereunder.

Addington Hills Ltd., the Trust and the Trustee each hereby agree that the Statements shall be filed jointly on behalf of each of them, and that a copy of this Agreement shall be filed as an exhibit thereto when and as required in accordance with Rule 13d-1(k)(iii) under the Act.

This Agreement may be executed in counterparts which together shall constitute one agreement.

IN WITNESS WHEREOF, the parties have executed this Agreement or caused this Agreement to be signed on their behalf by their duly authorized representatives as of the date first above written.