This is an accepted version of this page An autonomous system (AS) is a collection of connected Internet Protocol (IP) routing prefixes under the control of one or more network operators on behalf of a single administrative entity or domain, that presents a common and clearly defined routing policy to the Internet.
[1] Each AS is assigned an autonomous system number (ASN), for use in Border Gateway Protocol (BGP) routing.
The IANA also maintains a registry of ASNs which are reserved for private use (and should therefore not be announced to the global Internet).
These numbers are written preferably as simple integers, in a notation referred to as "asplain",[4] ranging from 0 to 4,294,967,295 (hexadecimal 0xFFFF FFFF).
[18] There are other sources for more specific data: A complete table of available 16-bit and 32-bit ASN:[17] Autonomous systems (AS) can be grouped into four categories, depending on their connectivity and operating policy.
Instead of single AS numbers, AS-SETs can be referenced in these attributes, which simplifies management of complex routing policies.