He states: "Today's Internet uses IPv4, the 4th version of the Internet Protocol. (Versions 1 through 3 never made it out of the lab. Neither, for that matter, did Version 5.)"
From http://tldp.org/HOWTO/Linux+IPv6-HOWTO/x419.html:
"2.4.1. Why is the name IPv6 and not IPv5 as successor for IPv4?
On any IP header, the first 4 bits are reserved for protocol version. So theoretically a protocol number between 0 and 15 is possible:
* 4: is already used for IPv4
* 5: is reserved for the Stream Protocol (STP, RFC 1819 / Internet Stream Protocol Version 2) (which never really made it to the public)
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I'll continue:
He states: "Today's Internet uses IPv4, the 4th version of the Internet Protocol. (Versions 1 through 3 never made it out of the lab. Neither, for that matter, did Version 5.)"
From http://tldp.org/HOWTO/Linux+IPv6-HOWTO/x419.html:
"2.4.1. Why is the name IPv6 and not IPv5 as successor for IPv4?
On any IP header, the first 4 bits are reserved for protocol version. So theoretically a protocol number between 0 and 15 is possible:
* 4: is already used for IPv4
* 5: is reserved for the Stream Protocol (STP, RFC 1819 / Internet Stream Protocol Version 2) (which never really made it to the public)
The next free number was 6. Hence IPv6 was born!"
So now you know.