An Architecture for IP Address Allocation with CIDR

have no defined hierarchical relationship, administrators would not be able to assign IP addresses within the domains out of some common prefix for the purpose of data abstraction. The result would be flat inter-domain routing; all routing domains would need explicit knowledge of all other routing domains that they route to. This can work well in small and medium sized internets, up to a size somewhat larger than the current Internet. However, this does not scale to very large internets. For example, we expect growth in the future to an Internet which has tens or hundreds of thousands of routing domains in North America alone. This requires a greater degree of the reachability information abstraction beyond that which can be achieved at the `routing domain' level. In the Internet, however, it should be possible to exploit the existing hierarchical routing structure interconnections, as discussed in Section 5. Thus, there is the opportunity for a group of routing domains each to be assigned an address prefix from a shorter prefix assigned to another routing domain whose function is to interconnect the group of routing domains. Each member of the group of routing domains now `owns' its (somewhat longer) prefix, from which it assigns its addresses. The most straightforward case of this occurs when there is a set of routing domains which are all attached only to a single service provider domain (e.g. regional network), and which use that provider for all external (inter-domain) traffic. A small prefix may be assigned to the provider, which then assigns slightly longer prefixes (based on the provider's prefix) to each of the routing domains that it interconnects. This allows the provider, when informing other routing domains of the addresses that it can reach, to abbreviate the reachability information for a large number of routing domains as a single prefix. This approach therefore can allow a great deal of hierarchical abbreviation of routing information, and thereby can ...