By 1990 the IETF was actively interested in proposals to replace IPv4. A working group for the so-called “IP next generation”, or IPng, was created in 1993 to select the new version; RFC 155020.11.4 RTP and VoIP), as the basis for IPv6. The first IPv6 specifications, released in 1995, were RFC 1883 (now RFC 2460 , with updates) for the basic protocol, and RFC 1884 (now RFC 4291 , again with updates) for the addressing architecture.was this group’s formal solicitation of proposals. In July 1994 the IPng directors voted to accept a modified version of the “Simple Internet Protocol”, or SIP (unrelated to the Session Initiation Protocol,
SIP addresses were originally 64 bits in length, but in the month leading up to adoption as the basis for IPv6 this was increased to 128. 64 bits would probably have been enough, but the problem is less the actual number than the simplicity with which addresses can be allocated; the more bits, the easier this becomes, as sites can be given relatively large address blocks without fear of waste. A secondary consideration in the 64-to-128 leap was the potential to accommodate now-obsolete CLNP addresses (1.15 IETF and OSI), which were up to 160 bits in length, but compressible.
IPv6 has to some extent returned to the idea of a fixed division between network and host portions; for most IPv6 addresses, the first 64 bits is the network prefix (including any subnet portion) and the remaining 64 bits represents the host portion. The rule as spelled out in RFC 2460, in 1998, was that the 64/64 split would apply to all addresses except those beginning with the bits 000; those addresses were then held in reserve in the unlikely event that the 64/64 split ran into problems in the future. This was a change from 1995, when RFC 1884 envisioned 48-bit host portions and 80-bit prefixes.
While the IETF occasionally revisits the issue, at the present time the 64/64 split seems here to stay; for discussion and justification, see 8.10.1 Subnets and /64 and RFC 7421 . The 64/64 split is not automatic, however; there is no default prefix length as there was in the Class A/B/C IPv4 scheme. Thus, it is misleading to think of IPv6 as a return to something like IPv4’s classful addressing scheme. Router advertisements must always include the prefix length, and, when assigning IPv6 addresses manually, the /64 prefix length must be specified explicitly; see 8.12.3 Manual address configuration.
High-level routing, however, can, as in IPv4, be done on prefixes of any length (usually that means lengths shorter than /64). Routing can also be done on different prefix lengths at different points of the network.
IPv6 is now twenty years old, and yet usage as of 2015 remains quite modest. However, the shortage in IPv4 addresses has begun to loom ominously; IPv6 adoption rates may rise quickly if IPv4 addresses begin to climb in price.