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An autonomous system number (ASN) is a unique number that's available globally to identify an autonomous system and which enables that system to exchange exterior routing information with other neighboring autonomous systems.
The number of autonomous system numbers is limited. For autonomous system numbers to be assigned, current guidelines need the network to be multi-homed and have a unique routing policy. Autonomous system numbers can be assigned only through a request to the local Internet registry.
Autonomous system numbers range from 1 to 64,511. When an ASN is needed, the next highest unused number is assigned. The American Registry for Internet Numbers manages IP address allocations and assignments; it is also the authority for assigning and tracking ASNs. The current ASN allocation is based on 16-bit ASNs, which could be exhausted in the near future. Other alternate approaches like 32-bit ASN approaches are currently being explored.
There are two types of autonomous system numbers: public and private. A public autonomous system number is used when the system is exchanging routing information on the public Internet with other autonomous systems. A private autonomous system number is used only if the autonomous system is communicating with a single provider via Border Gateway Protocol. In the case of a public autonomous system number, the routes will be visible on the Internet, whereas in the case of private autonomous system numbers, the routes will not be visible on the Internet.