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Understanding Internet Registries

The move from classful address assignment to classless assignment through Regional Internet Registries (RIR) and Local Internet Registries (LIR), and why it was necessary...

Traditional Classful Assignment

The traditional classful address scheme of the public Internet is inefficient because only three different sized unicast networks can be assigned. Sub-netting allows more efficient use of a traditional class-based IP network within an organisation, by allowing it to be sub-divided. However traditional sub-netting does not propagate through the Internet routing tables, because routes are summarised back to their classful form at the administrative boundary of the network. Therefore Internet routing tables in this model must contain separate routes for each assigned class A, B or C network.

Introduction of Classless Addressing

In the mid-1990ís, the size of Internet routing tables was growing massively, to the point where performance of the backbone networks was affected. It was realised that much of the fine-grained detail in Internet routing tables was redundant, and that a more flexible hierarchy should be imposed, by allowing routes for smaller networks to be aggregated together before they were advertised into the public Internet. This also potentially allows more efficient use of available IP addresses by allocating blocks of class C addresses rather than a single class B, and by sub-netting class A and B networks into smaller allocations, rather than offering an entire classful network to an enterprise. CIDR is essentially a generalization of the sub-netting concept.

The Regional Registries

To make CIDR effective, it was necessary to impose some geographical structure on the IP address space, so that aggregation could be as effective as possible. As a result of a policy change, IP addresses are now assigned in blocks through a hierarchy of registries and service providers. In general, large blocks of IP addresses are allocated to regional registries, which will in turn assign smaller blocks of address space to Service Providers (SP), which will in turn assign yet smaller blocks to Internet Service Providers (ISP). Finally, individual users will obtain IP addresses from their respective ISP. The Regional Internet Registries and their relationship to ICANN and IANA are shown in the diagram opposite.

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