IP version numbers
As the version number is carried in a 4-bit field, only numbers 0–15 can be assigned.History
During the development of the first version of the Internet Protocol in the 1970s, the initial experimental versions 1 to 3 were not standardized. The first working version that was widely deployed was assigned version number 4. A separate protocol based on reliable connections was developed and assigned version 5. IP version 7 was chosen in 1988 by R. Ullmann as the next IP version because he incorrectly assumed that version 6 was in use for ST-II. However, ST-II had reused version 5 of the original ST protocol.{{Ref RFC, 1475, repeat=yes, rp=7 In the early 1990s, when it became apparent that IPv4 could not sustain routing in a growing Internet, several new Internet Protocols were proposed. The Internet Protocol that finally emerged was assigned version number 6, being the lowest free number greater than 4. The PIP protocol and TUBA protocol used versions 8 and 9, following version 7 for TP/IX. In 2004, an IPv9 protocol was developed in China using 256-bit addresses.References