Autonomous System Numbers
ASN is an acronym for Autonomous System Number. It is a globally unique number and significant part of the internet routing architecture known as the Autonomous System. The Autonomous System Numbers are taken from a 16-bit number field. There are reserved ASNs wincluding 0, which can be used to identify non-routed networks, the 65,535 is also reserved. The ASNs starting from 64,512 up to 65,534 are reserved for private use while the ASN 23,456 is dedicated to be used in the ASN pool transition. [1] This means that the available quantity of ASN for internet routing is limited.
Autonomous System (AS)
Based on RFC 4271, Autonomous System (AS) refers to a set of routers under a single technical administration, using an interior gateway protocol (IGP) and common metrics to determine how to route packets within the AS, and using an inter-AS routing protocol to determine how to route packets to other ASes.The Network Working Group (NWG) explained that since the introduction of this definition, several IGP is commonly used for a single AS and several sets of metrics within an AS is sometimes used. The NWG also explained that the administration of an AS to other ASes still emerge with a single coherent interior routing plan and a consistent picture of the destinations that are reachable through it even when multiple IGPs and metrics are utilized.[2]