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An `interface identifier` that identifies a particular interface on a host
The subnet identifier plays a key role in the scalability of network layer addressing architecture. An important point to be defined in a network layer protocol is the allocation of the network layer addresses. A naive allocation scheme would be to provide an address to each host when the host is attached to the Internet on a first come first served basis. With this solution, a host in Belgium could have address ``2001:db8::1`` while another host located in Africa would use address ``2001:db8::2``. Unfortunately, this would force all routers on the Internet to maintain one route towards each host. In the network layer, scalability is often a function of the number of routes stored on the router. A network will usually work better if its routers store fewer routes and network administrators usually try to minimize the number of routes that are known by their routers. For this, they often divide their network prefix in smaller blocks. For example, consider a company with three campuses, a large one and two smaller ones. The network administrator would probably divide his block of addresses as follows :
the bottom half is used for the large campus
the top half is divided in two smaller blocks, one for each small campus
Inside each campus, the same division can be done, for example on a per building basis, starting from the buildings that host the largest number of nodes, e.g. the company datacenter. In each building, the same division can be done on a per floor basis, ... The advantage of such a hierarchical allocation of the addresses is that the routers in the large campus only need one route to reach a router in the smaller campus. The routers in the large campus would know more routes about the buildings in their campus, but they do not need to know the details of the organization of each smaller campus.
To preserve the scalability of the routing system, it is important to minimize the number of routes that are stored on each router. A router cannot store and maintain one route for each of the almost 1 billion hosts that are connected to today's Internet. Routers should only maintain routes towards blocks of addresses and not towards individual hosts. For this, hosts are grouped in `subnets` based on their location in the network. A typical subnet groups all the hosts that are part of the same enterprise. An enterprise network is usually composed of several LANs interconnected by routers. A small block of addresses from the Enterprise's block is usually assigned to each LAN.
In today's deployments, interface identifiers are always 64 bits wide. This implies that while there are :math:`2^{128}` different IPv6 addresses, they must be grouped in :math:`2^{64}` subnets. This could appear as a waste of resources, however using 64 bits for the host identifier allows IPv6 addresses to be auto-configured and also provides some benefits from a security point of view, as explained in section ICMPv6_.
In practice, there are several types of IPv6 unicast address. Most of the `IPv6 unicast addresses <http://www.iana.org/assignments/ipv6-address-space/ipv6-address-space.xhtml>`_ are allocated in blocks under the responsibility of IANA_. The current IPv6 allocations are part of the `2000::/3` address block. Regional Internet Registries (RIR) such as RIPE_ in Europe, ARIN_ in North-America or AfriNIC in Africa have each received a `block of IPv6 addresses <http://www.iana.org/assignments/ipv6-unicast-address-assignments/ipv6-unicast-address-assignments.xhtml>`_ that they sub-allocate to Internet Service Providers in their region. The ISPs then sub-allocate addresses to their customers.
When considering the allocation of IPv6 addresses, two types of address allocations are often distinguished. The RIRs allocate `provider-independent (PI)` addresses. PI addresses are usually allocated to Internet Service Providers and large companies that are connected to at least two different ISPs [CSP2009]_. Once a PI address block has been allocated to a company, this company can use its address block with the provider of its choice and change its provider at will. Internet Service Providers allocate `provider-aggregatable (PA)` address blocks from their own PI address block to their customers. A company that is connected to only one ISP should only use PA addresses. The drawback of PA addresses is that when a company using a PA address block changes its provider, it needs to change all the addresses that it uses. This can be a nightmare from an operational perspective and many companies are lobbying to obtain `PI` address blocks even if they are small and connected to a single provider. The typical size of the IPv6 address blocks are :
``/32`` for an Internet Service Provider
``/48`` for a single company
``/56`` for small user sites
``/64`` for a single user (e.g. a home user connected via ADSL)
``/128`` in the rare case when it is known that no more than one host will be attached
There is one difficulty with the utilization of these IPv6 prefixes. Consider Belnet, the Belgian research ISP that has been allocated the ``2001:6a8::/32`` prefix. Universities are connected to Belnet. UCLouvain uses prefix ``2001:6a8:3080::/48`` while the University of Liege uses ``2001:6a8:2d80::/48``. A commercial ISP uses prefix ``2a02:2788::/32``. Both Belnet and the commercial ISP are connected to the global Internet.
The Belnet network advertises prefix ``2001:6a8::/32`` that includes the prefixes from both UCLouvain and ULg. These two subnetworks can be easily reached from any internet connected host. After a few years, UCLouvain decides to increase the redundancy of its Internet connectivity and buys transit service from ISP1. A direct link between UCLouvain and the commercial ISP appears on the network and UCLouvain expects to receive packets from both Belnet and the commercial ISP.
Now, consider how a router inside ``alpha.com`` would reach a host in the ``UCLouvain`` network. This router has two routes towards ``2001:6a8:3080::1``. The first one, for prefix ``2001:6a8:3080::/48`` is via the direct link between the commercial ISP and UCLouvain. The second one, for prefix ``2001:6a8::/32`` is via the Internet and Belnet. Since :rfc:`1519` when a router knows several routes towards the same destination address, it must forward packets along the route having the longest prefix length. In the case of ``2001:6a8:3080::1``, this is the route ``2001:6a8:3080::/48`` that is used to forward the packet. This forwarding rule is called the `longest prefix match` or the `more specific match`. All IP routers implement this forwarding rule.
To understand the `longest prefix match` forwarding, consider the IPv6 routing below.
With the longest match rule, the route ``::/0`` plays a particular role. As this route has a prefix length of `0` bits, it matches all destination addresses. This route is often called the `default` route.
a packet with destination ``2a02:2788:2c4:16f::1`` received by router `R` is destined to a host on interface ``eth0`` .