Which of the following is not a standardized Internet Protocol (IP) version?

Concept

Internet Protocol versions are identified by standardized specifications and by the version field carried in an IP packet.

A historical or experimental protocol label is not automatically a standardized Internet Protocol version; the specification status must be checked.

Application

The IANA IP Version Numbers registry marks decimal values 2–3 as unassigned, value 4 as Internet Protocol with RFC 791, and value 6 as Internet Protocol version 6 with RFC 8200.

Cross-check and contrast

• IPv3: its numeric version value, 3, is unassigned in the IANA registry.

• IPv4: a standardized Internet Protocol version with 32-bit addresses.

• IPv6: a standardized Internet Protocol version with 128-bit addresses.

• None of these: would require every listed named version to be standardized, which the specification check does not support.

Result

Therefore, IPv3 is the version name that is not valid in the standardized IP-version sense used by the question.