The Next Version of the Internet Protocol - IPv6 - Page 3
Part 3: The IPv6 Solution
The IPv6 Solution
The IPv6 global aggregation addressing architecture splits addresses into two parts. The high-order 64 bits identify the network, and the low-order 64 bits identify the node. A format prefix gives the type of IPv6 address. Next comes a top-level aggregation entity, likely to be a country or a large carrier, followed by 8 bits reserved for future growth. Then comes another aggregation entity, likely to be a large company or Internet provider, and finally a site-level aggregation entity, probably assigned by the entity above it. Such addresses are far more efficient to route across backbones.
Aggregation means any address contains its own route. The first few bits of the address might indicate, say, Europe. The packet would go to a router serving Europe, which might see Portugal in the next few bits and forward the packet to Portugal's router. From there, the packet might go on to a router in Lisbon and then on to its final destination.
Figure 2 shows that the Top-Level Aggregation ID (TLA) uses 13 bits. This gives an upper limit of no more than 8,192 (2 to the 13th power) top-level entities, which pares down the size of the routing table a backbone router would have to deal with to forward packets anywhere in an IPv6 Internet. The next 8 bits are reserved, presumably held back, just in case the TLA allocation should be bigger (or the Next-Level Aggregation ID allocation should be bigger).
Figure 2: (from RFC 2373)
NLA entities are expected to include large ISPs, among others. These entities get their address allocations from the TLAs, who also handle routing for the NLAs. Each TLA can allocate as many as 16 million or so NLA networks (2 to the 24th) The NLAs, in turn, can allocate as many as 65,536 networks each (2 to the 16th) to Site-Level Aggregation (SLA) entities. In other words, network sites. And each SLA entity still has 64 bits of address space to play around with, for as many as 18 million trillion (18,446,744,073,709,551,616) nodes per network.
While the IPv6 address is longer than we're used to, the IPv6 header is simpler than we are used to (see Figure 3). IPv6 eliminates length, identification, flag, fragment offset, header checksum, options, and padding fields that were found in IPv4 headers. Because IPv6 headers are all the same length, no length field is necessary. IPv6 prohibits fragmentation except between end nodes, so the identification, flag and fragment offset fields go away, too.
Figure 3 (from RFC 2460)
IPv6 options are handled in separate extension headers so options no longer clutter the main header. The IPv4 type-of-service field has evolved into the traffic class field, and the time-to-live field is replaced by the hop limit field. A flow label field supports IPv6 packet sequences that require the same routing treatment, such as video streams.
The simplified, standard-sized IPv6 header also makes routing easier for packets with special options. IPv4 forces routers to sense and handle all special packets, such as those using IP Security encryption and authentication. But IPv6 routers can ignore the end-to-end options and process only those relevant to the routing process.