autonomous system number(ASN)
A globally unique identifier given to an autonomous system (AS). An AS is a collection of Internet Protocol (IP) networks operated by one or more network operators. When an Internet user sends a message to an IP address that exists outside the user’s own AS, routers forward the message to the AS in which the address resides. In this respect, the Internet uses ASNs to route messages between autonomous systems in much the same way postal systems use postal codes to route physical mail between geographic regions.
ASNs are allocated to network operators by Regional Internet Registries.