IP or Internet Protocol, is the primary network protocol used on the Internet, introduced by Vint Cerf and Bob Kahn in 1974.

IP version 0 to 3 was introduced and used between 1974 and 1979. After changes and refinements in initial IP protocol, version 4 was introduced in 1981, commonly known as IPv4, described in RFC 791, which become the backbone of Whole Internet in no time. (IPv4 is know as v4 because it is the fourth version of IP not because it has 4 octets, which is a common myth).

TCP/IP Protocol was designed as a part of project ARPANET, whose aim was to interconnect various universities and research institutes of USA. But with the dot com boom, TCP/IP grows like anything and its become a victim of its own success.

In Early 90's, expert realized that we will exhaust the whole IPv4 number very soon and there is a need for a new protocol, which should be enough in size to serve the internet community for at least 30-40 years. This lead to new version of Internet Protocol.

Version 5 is used for Internet Stream Protocol, an experimental streaming protocol, which never comes in public domain.

Version 6 is allocated to Simple Internet Protocol Plus, now commonly known as IPv6. This is now next most common protocol after IPv4 and is intended to replace IPv4.

Version 7, 8 and 9 is allocated for various purposes but didn't get much success and support.

If we able to see next protocol (having that much recognition as that of v4 and v6) in our lifetime, it will having a number greater than 9 for sure :).