IP, the Internet Protocol, is one of the pillars which supports the Internet. Almost 20 years old, first specified in a remarkably concise 45 pages in RFC 791, IP is the network-layer protocol for the Internet.
In 1991, the IETF decided that the current version of IP, called IPv4, had outlived its design. The new version of IP, called either IPng (Next Generation) or IPv6 (version 6), was the result of a long and tumultuous process which came to a head in 1994, when the IETF gave a clear direction for IPv6.
IPv6 is designed to solve the problems of IPv4. It does so by creating a new version of the protocol which serves the function of IPv4, but without the same limitations of IPv4. IPv6 is not totally different from IPv4: what you have learned in IPv4 will be valuable when you deploy IPv6. The differences between IPv6 and IPv4 are in five major areas: addressing and routing, security, network address translation, administrative workload, and support for mobile devices. IPv6 also includes an important feature: a set of possible migration and transition plans from IPv4.
Since 1994, over 30 IPv6 RFCs have been published. Changing IP means changing dozens of Internet protocols and conventions, ranging from how IP addresses are stored in DNS (domain name system) and applications, to how datagrams are sent and routed over Ethernet, PPP, Token Ring, FDDI, and every other medium, to how programmers call network functions.
The IETF, though, is not so insane as to assume that everyone is going to change everything overnight. So there are also standards and protocols and procedures for the coexistence of IPv4 and IPv6: tunneling IPv6 in IPv4, tunneling IPv4 in IPv6, running IPv4 and IPv6 on the same system (dual stack) for an extended period of time, and mixing and matching the two protocols in a variety of environments.
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