Since we are all moving towards IPv6 whether we want it or not, I ask this: what happened to IPv5? Was it not cool enough for it's older brother, or did something else happen to that specification?
Answer
I believe the answer lies in the "What ever happened to IPv5?" blog post over on http://www.oreillynet.com
To quote:
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″.