Analysis of the 64-bit Boundary in IPv6 Addressing
This RFC 7421 was published in 2015
The IPv6 unicast addressing format includes a separation between the prefix used to route packets to a subnet and the interface identifier used to specify a given interface connected to that subnet.
Currently, the interface identifier is defined as 64 bits long for almost every case, leaving 64 bits for the subnet prefix.
This document describes the advantages of this fixed boundary and analyzes the issues that would be involved in treating it as a variable boundary.
RFC 7421 introduction
Rather than simply overcoming the IPv4 address shortage by doubling the address size to 64 bits, IPv6 addresses were originally chosen to be 128 bits long to provide flexibility and new possibilities.
In particular, the notion of a well-defined interface identifier was added to the IP addressing model.
The IPv6 addressing architecture [RFC4291] specifies that a unicast address is divided into n bits of subnet prefix followed by (128-n) bits of interface identifier (IID).
The bits in the IID may have significance only in the process of deriving the IID; once it is derived, the entire identifier should be treated as an opaque value [RFC7136].
Also, since IPv6 routing is entirely based on variable length prefixes (also known as variable length subnet masks), there is no basic architectural assumption that n has any particular fixed value.
All IPv6 routing protocols support prefixes of any length up to /128.
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