IPv6 Global Unicast Address Format
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Additional Notes on the Global Unicast Format
The removal of RFC 2374's fixed structure for the global routing prefix is consistent with the IPv6 development team's efforts to emphasize that bit fields and structures are used only for allocating addresses, not for routing purposes. The addresses themselves, once created, are not interpreted by hardware on an internet based on this format. To routers, the only structure is the division between the network ID and host ID is given by the prefix length that trails the IP address, and this division can occur at any bit boundary. These hardware devices just see 128 bits of an IP address and use it without any knowledge of hierarchical address divisions or levels.
Its also worth noting that the key to obtaining the allocation benefits of the aggregatable unicast address format is the abundance of bits available to us under IPv6. The ability to have these hierarchical levels while still allowing 64 bits for the interface ID is one of the main reasons why IPv6 designers went all the way from 32 bits to 128 bits for address size. By creating this structure, we maintain flexibility while avoiding the potential chaos of trying to allocate many different network sizes within our 128 bits.
Finally, note that anycast addresses are structured in the same way as unicast addresses, so they are allocated according to this same model. In contrast, multicast addresses are not; they are allocated from their own portion of the IPv6 address space.
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