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

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