IPv6 Address Space Allocation
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After dealing for so many years with the very small IPv4 address space, the enormous number of addresses in IPv6 must have made the IETF engineers feel like kids in a candy shop. They were good kids, however, and didn't run wild grabbing all the candy they could find and gobbling it up. They very carefully considered how to divide the address space for various uses. Of course, when you have this much candy, sharing becomes pretty easy.
As was the case with IPv4, the two primary concerns in deciding how to divide the IPv6 address space were address assignment and routing. The designers of IPv6 wanted to structure the address space to make allocation of addresses to ISPs, organizations and individuals as easy as possible.
At first, this perhaps ironically, this led the creators of IPv6 back full circle to the use of specific bit sequences to identify different types of addresses, just like the old classful addressing scheme. The address type was indicated by a set of bits at the start of the address, called the format prefix (FP). The format prefix was conceptually identical to the 1 to 4 bits used in IPv4 classful addressing to denote address classes, but was variable in length, ranging from three to ten bits. Format prefixes were described in RFC 2373.
In the years following the publication of RFC 2373, the gurus who run the Internet had a change of heart regarding how address blocks should be considered. They still wanted to divide up the IPv6 address space into variably-sized blocks for different purposes. However, they realize that many people (including myself) were starting to consider the use of format prefixes to be equivalent to the old class-oriented IPv4 system.
Their main concern was that implementors might program into IPv6 hardware logic to make routing decisions based only on the first few bits of the address. This was specifically not how IPv6 is supposed to work; for one thing, the allocations are subject to change. Thus, one of the modifications made in RFC 3513 was to change the language regarding IPv6 address allocations, and specifically, to remove the term format prefix from the standard.
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