- The Internet Protocol version 6 or IPv6 (other name is Internet Protocol Next Generation or IPng) specification has been defined with the RFC 2460.
- The Internet Protocol version 4 or IPv4 (it is sometimes called as legacy Internet address protocol) specification has been defined with the RFC 791 which, the United States Department of Defense also standardized it as MIL-STD-1777.
There are other IP versions not popular as the two above but it has been part of the IETF discussions in the past. Search through the Internet particularly IETF list and you will see.