Autonomous System Numbers

What are Autonomous System Numbers?

An Autonomous System (AS) is a group of one or more IP prefixes (lists of IP addresses accessible on a network) run by one or more network operators that maintain a single, clearly-defined routing policy. Network operators need Autonomous System Numbers (ASNs) to control routing within their networks and to exchange routing information with other Internet Service Providers (ISPs).

There are two different formats to represent ASNs: 2-byte and 4-byte.

A 2-byte ASN is a 16-bit number. This format provides for 65,536 ASNs (0 to 65535). From these ASNs, the Internet Assigned Numbers Authority (IANA) reserved 1,023 of them (64512 to 65534) for private use.

A 4-byte ASN is a 32-bit number. This format provides for 232 or 4,294,967,296 ASNs (0 to 4294967295). IANA reserved a block of 94,967,295 ASNs (4200000000 to 4294967294) for private use.

Up until the Internet Engineering Task Force (IETF) proposed a gradual transition to 4-byte ASNs in 2007, all ASNs were 2-byte. There is no longer a distinction between a 2-byte and 4-byte ASN, and all ASNs should be considered 4-byte.

For more information about specifications for ASNs, see the following Requests for Comments (RFCs):

  • RFC 1930 – Guidelines for creation, selection, and registration of an Autonomous System (AS)
  • RFC 5396 – Textual Representation of Autonomous System (AS) Numbers
  • RFC 6793 – BGP Support for Four-Octet AS Number Space
  • RFC 6996 - Autonomous System (AS) Reservation for Private Use

To begin your ASN request, visit the Requesting IPs or ASNs page.

What are the different types of Autonomous Systems?

There are three different kinds of Autonomous Systems:

  1. A multi-homed AS connects to two or more ASes so it can maintain its Internet connection should one AS connection fail.
  2. A stub AS connects to only one other AS, though it may have its own private connections not visible to the rest of the Internet.
  3. A transit AS acts as a link between two or more other ASes, allowing for data to pass through it, even data from unassociated networks. ISPs, for example, offer their customers and their customers' networks access to other networks and the Internet via transit AS.

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