What happened to IPv5?
Version 5 of the IP family was an experimental protocol developed in the 1980s. IPv5 (also called the Internet Stream Protocol) was never widely deployed. Since the number 5 was already allocated, this number was not considered for the successor to IPv4. Several proposals were suggested as the IPv4 successor, and each was assigned a number. In the end, it happened that the one with version number 6 was selected.

Today is the day IPv6 finally goes live. For as long as there has been an Internet IPv4 has been synonymous with IP and nobody really stopped to think about which version of the protocol it was. But IPv4 has outlived its usefulness.