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Route aggregation is a technique of organizing network layer IP addresses in a hierarchical way so that addresses are "topologically significant." Route aggregation in the form of CIDR (Classless Inter-Domain Routing) helped to solve IP address depletion problem in the early 1990s. In addition, route aggregation summarizes routes so there are fewer routes to advertise across the Internet. A service provider is allocated a contiguous block of IP addresses, which it then subnets (divides into smaller allocated blocks) and leases to its downstream subscribers (which may be smaller ISPs). Because these addresses are contiguous, the ISP can advertise one route on the global Internet.

The postal ZIP code system provides an analogy. All mail with ZIP codes 9xxxx is directed to the West coast. A mail sorter on the East coast only needs to know that all mail with ZIP codes starting with 9 goes west. On the West Coast, regional and local post offices sort the mail by looking further into the ZIP code. For example, 98xxx letters are sent to Washington while 97xxx letters are sent to Oregon.

Route aggregation on the Internet is similar. CIDR does away with the old class-based IP address scheme in favor of a classless scheme that supports hierarchical addressing. As mentioned, an ISP is allocated a large block of addresses that it divides up and allocates to downstream ISPs. Allocation is handled by Internet registries, with IANA (Internet Assigned Numbers Authority) at the top. IANA allocates blocks to regional Internet registries, which include ARIN (American Registry for Internet Numbers), RIPE NCC (Réseaux IP Européens Network Coordination Centre), and APNIC (Asia Pacific Network Information Centre). These regional registries then further allocate blocks of IP addresses to local Internet registries within their geographic region. Finally, the local Internet registries assign addresses to end users. RFC 2050 (Internet Registry IP Allocation Guidelines, November 1996) discusses this hierarchy.

The important concept is that addresses are assigned in contiguous blocks on CIDR bit boundaries defined by a bit mask. For example, the class B address 180.50.0.0 (with the implied subnet mask of 255.255.0.0) is now simply referred to as 180.50.0.0/16. The /16 indicates that the first 16 bits are the network number. An ISP with this address allocation advertises 180.50.0.0/16 on the rest of the Internet. Internet routers outside the ISP's network only need to know this single address. Inside the ISPs network, routers forward incoming packets to the network of sub-ISPs and organizations to which the ISP has allocated addresses.

Note that this scheme is often referred to as "provider-based address allocation" because providers are given a block of addresses that they allocate on their own. Customers get addresses from ISPs, not IANA or other agencies. In addition, addresses are usually leased, which means that they are returned to the ISP's address block if the customer moves out of its area.

Before CIDR, the number of routers to advertise on the Internet was exceeding the capabilities of most hardware. In 1995, there were close to 65,000 routes. As CIDR aggregation has been implemented, the number of routes in the global routing table has reduced to approximately 35,000 routes.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
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