ASN - Autonomous System Number

An AS is an administrative set of routers, depending on an administrative authority. There are public or assigned ASes, and private ASes. An ASes is identified by a number called ASN.

The public ASNs are any registered ASN values that are not private. This ASNs are assigned by the IANA or by Local Registries as for IP addresses. The private ASNs are ASNs that are reserved by the IANA for private use. It means that the private block must not be advertised on the global Internet. There are 1024 private ASNs that can be used for local administration. These numbers are 64512 through 65535. They are very useful to provision a set of BGP confederation.