Ever ask yourself "What happened to IPv5?"
Or, "Why are we skipping from
IPv4 to
IPv6?"
IPv5 never existed. The version number "5" in the
IP header was assigned to identify
packets carrying an experimental non-IP
real-time stream protocol called
ST. 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