Packets that hold Internet Protocol data carry a 4-bit IP version number as the first field of its header.
[1][2] Currently, only IPv4 and IPv6 packets are seen on the Internet, having IP version numbers 4 and 6, respectively.
[10] A separate protocol based on reliable connections was developed and assigned version 5.
However, ST-II had reused version 5 of the original ST protocol.
The Internet Protocol that finally emerged was assigned version number 6, being the lowest free number greater than 4.