AS (Autonomous System) is one of the essential element of BGP. BGP
is a distance vector routing protocol. AS framework provides distance
vector metric and loop detection to BGP. RFC1930 -
Guidelines for creation, selection, and registration of an
Autonomous System (AS) describes how to use AS.
AS number is tow octet digita value. So the value range is from 1 to 65535. AS numbers 64512 through 65535 are defined as private AS numbers. Private AS numbers must not to be advertised in the global Internet.