Exploring Autonomous System Numbers

So what are Autonomous System Numbers (ASNs), and what role do they play in the technology of the Internet? This article explores the role of ASNs as a critical element of the Internet routing architecture. We will first explore how the AS number space is structured, examine how ASNs are used in the interdomain routing environment and then look at the consumption rate of these numbers, and finally examine our options when we get to the point of likely ASN pool exhaustion. However, in order to put this into context, a brief overview of Internet routing architecture follows.

Internet routing architecture is structured as a two-level hierarchy. The environment is first partitioned into domains with each domain using an internal routing environment. These network domains use an interior routing protocol (commonly referred to as an Interior Gateway Protocol [IGP]), which maintains a complete mapping set for the current internal topology of the domain, together with the set of “best paths” between any two points within the network domain. Although this approach of having a routing protocol automatically maintaining a comprehensive view of the current topology can be made to work within even quite large routing domains, such an approach does not scale to the size of the entire Internet. Fine-grained topology information is useful only in “local” situations, and is best omitted when forming a larger view of the network. Commonly used interior routing protocols include Open Shortest Path First (OSPF), Intermediate System-to-Intermediate System (IS–IS), and Enhanced Interior Gateway Routing Protocol (EIGRP).

The second level in the routing hierarchy is the interdomain routing domain. The interdomain routing environment describes how domains interconnect, but avoids the task of maintaining transit paths within each domain. In the interdomain space, a routing path to an address is described as a sequence of domains that must be transited to reach the domain that originates that particular address prefix. Today this interdomain space is maintained using Version 4 of the Border Gateway Protocol (BGPv4).

Each routing domain is a single administrative domain, operated within a uniform set of routing policies, and is operated independently from any other domain. The domain is in effect an autonomous unit in the overall routing architecture, and is termed an Autonomous System (AS). Each of these ASs is uniquely identified using an Autonomous System Number (ASN).