The IP protocol features a field containing the protocol version. Since IPv4 is running out of available IP addresses and is about to be replaced, one would assume the next generation IP protocol would be named IPv5.
The only problem is, version 5 is already occupied by something else.
UNIX release conventions states that odd numbered versions are considered experimental, and not intended for public use. In the late 1970's, a protocol named The Internet Stream Protocol, or simply ST, was created to transmit audio and video.
In 2000, the protocol was updated for commercial use, and named ST2. As it outperformed IPv4 by offering connections and QoS, it was assigned Internet Protocol version 5, IPv5.