The AS (Autonomous System) number is one of the essential element of BGP. BGP is a distance vector routing protocol, and the AS-Path framework provides distance vector metric and loop detection to BGP. RFC1930, Guidelines for creation, selection, and registration of an Autonomous System (AS) provides some background on the concepts of an AS.
The AS number is a two octet value, ranging in value from 1 to 65535. The AS numbers 64512 through 65535 are defined as private AS numbers. Private AS numbers must not to be advertised in the global Internet.