Ever ask yourself "What happened to IPv5?"
Or, "Why are we skipping from IPv4
IPv5 never existed. The version number "5" in the IP header
was assigned to identify packet
s carrying an experimental non-IP real-time stream protocol
. ST was never widely used, but since the version number 5 had already been allocated the new version of IP was given its own unique identifying number, 6. ST is described in RFC 1819
--ripped from the pages of the TCP/IP FAQ