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.
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.