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Autonomous System
From Citizendium, the Citizens' Compendium
An autonomous system is a set of routers and addresses, under one or more administrations, which presents a common routing policy to the Internet [1] An older definition, that it was a set of routers or addresses under a common administration, is obsolete; the term routing domain is preferred for the set under single administrative control. This older definition did not cover the common case of a group of enterprise AS talking to an ISP AS, and that the ISP's policy is the only one seen by the rest of the Internet.
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Autonomous system administration
Each AS has an autonomous system number (ASN), currently 16 bit but being expanded to a 32 bit space. ASNs are assigned by address registries.
Justification for AS assignment
Policies differ among registries, but, assuming the AS requester uses IPv4 address space and routes it on the public Internet, the minimal justification is to have a /20 or larger address block, or to have a address block of /24 or larger, which is multihomed to at least two other AS.
Private AS space
In the 16-bit AS space, the ASNs 64512 through 65535 are reserved and will never be routable on the public Internet.
Introduction to AS policies
References
- ↑ Hawkinson, J. & T. Bates (July 1996), RFC 1930: Guidelines for creation, selection, and registration of an Autonomous System (AS)
