I hear/read much about IPv4 and IPv6 and nothing about IPv5. Where did it get lost in the discussion?
The first four bits in the IP header are used to store the Protocol version (4 or 6). The version 5 was assigned to another protocol, the Internet Steam Protocol and so was not available for the successor of IPv4.
Internet Steam Protocol is defined by RFC1819 in which you will find in section 1.2:
Both ST2 and IP apply the same addressing schemes to identify different hosts. ST2 and IP packets differ in the first four bits, which contain the internetwork protocol version number: number 5 is reserved for ST2 (IP itself has version number 4). As a network layer protocol, like IP, ST2 operates independently of its underlying subnets. Existing implementations use ARP for address resolution, and use the same Layer 2 SAPs as IP.
Quoted from O'Reilly archive:
But, what ever happened to IPv5?
IPng, Internet Protocol next generation, was conceived in 1994 with a goal for implementations to start flooding out by 1996 (yeah, like that ever happened). IPv6 was supposed to be the "god-send" over the well-used IPv4: it increased the number of bytes used in addressing from 4 bytes to 16 bytes, it introduced anycast routing, it removed the checksum from the IP layer, and lots of other improvements. One of the fields kept, of course, was the version field -- these 8 bits identify this IP header as being of version "4" when there is a 4 in there, and presumably they would use a "5" to identify this next gen version. Unfortunately, that "5" was already given to something else.
In the late 1970's, a protocol named ST -- The Internet Stream Protocol -- was created for the experimental transmission of voice, video, and distributed simulation. Two decades later, this protocol was revised to become ST2 and started to get implemented into commercial projects by groups like IBM, NeXT, Apple, and Sun. Wow did it differ a lot. ST and ST+ offered connections, instead of its connection-less IPv4 counterpart. It also guaranteed QoS. ST and ST+, were already given that magical "5".
And now as the Internet clock ticks, our PCs don't use IPv5. So we're moving onto 6.