IP History, Standards, Versions and Closely-Related Protocols
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Since the Internet Protocol is really the architectural foundation for the entire TCP/IP suite, one might have expected that it was created first, and the other protocols built upon it. That's usually how one builds a structure, after all. The history of IP, however, is a bit more complex. The functions it performs were defined at the birth of the protocol, but IP itself didn't exist for the first few years that the protocol suite was being defined.
I explore the early days of TCP/IP in the section that overviews the suite as a whole. What is notable about the development of IP is that its functions were originally part of the Transmission Control Protocol (TCP). As a formal protocol, IP was born when an early version of TCP developed in the 1970s for predecessors of the modern Internet was split into TCP at layer four and IP at layer three. The key milestone in the development of the Internet Protocol was the publishing of RFC 791, Internet Protocol, in September 1981. This standard, which was a revision of the similar RFC 760 of the previous year, defined the core functionality and characteristics of the IP that has been in widespread use for the last two decades.
The IP defined in RFC 791 was the first widely-used version of the Internet Protocol. Interestingly, however, it is not version 1 of IP but version 4! This would of course imply that there were earlier versions of the protocol at one point. Interestingly, however, there really weren't. As I mentioned above, IP was created when its functions were split out from an early version of TCP that combined both TCP and IP functions. TCP evolved through three earlier versions, and was split into TCP and IP for version 4. That version number was applied to both TCP and IP for consistency.
So, when you use IP today, you are using IP version 4, also frequently abbreviated IPv4. Unless otherwise qualified, it's safe to assume that IP means IP version 4at least for the next few years! This version number is carried in the appropriate field of all IP datagrams, as described in the topic discussing the IP datagram format.
Given that it was originally designed for an internetwork a tiny fraction of the size of our current Internet, IPv4 has proven itself remarkably capable. Various additions and changes have been made over time to how IP is used, especially with respect to addressing, but the core protocol is basically what it was in the early 1980s. There's good reason for this: changing something as fundamental as IP requires a great deal of development effort and also introduces complexities during transition.
Despite how well IPv4 has served us, it was recognized that for various reasons a new version of IP would eventually be required. Due to the difficulties associated with making such an important change, development of this new version of IP has actually been underway since the mid-1990s. This new version of IP is formally called Internet Protocol version 6 (IPv6) and also sometimes referred to as IP Next Generation or IPng. I discuss the reasons why IPv6 was developed and how it differs from IPv4 in considerable detail in the IPv6 section of this Guide.
A natural question at this point of course is: what happened to version 5 of IP? The answer is: it doesn't exist. While this may seem confusing, version 5 was in fact intentionally skipped to avoid confusion, or at least to rectify it. The problem with version 5 relates to an experimental TCP/IP protocol called the Internet Stream Protocol, Version 2, originally defined in RFC 1190. This protocol was originally seen by some as being a peer of IP at the Internet Layer in the TCP/IP architecture, and in its standard, these packets were assigned IP version 5 to differentiate them from normal IP packets (version 4). This protocol apparently never went anywhere, but to be absolutely sure that there would be no confusion, version 5 was skipped over in favor of version 6.
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