Autonomous system (Internet)


An autonomous system is a collection of connected Internet Protocol routing prefixes under the control of one or more network operators on behalf of a single administrative entity or domain that presents a common, clearly defined routing policy to the internet.
Originally the definition required control by a single entity, typically an Internet service provider or a very large organization with independent connections to multiple networks, that adhered to a single and clearly defined routing policy, as originally defined in RFC 1771. The newer definition in RFC 1930 came into use because multiple organizations can run Border Gateway Protocol using private AS numbers to an ISP that connects all those organizations to the internet. Even though there may be multiple autonomous systems supported by the ISP, the internet only sees the routing policy of the ISP. That ISP must have an officially registered autonomous system number.
A unique ASN is allocated to each AS for use in BGP routing. ASNs are important because the ASN uniquely identifies each network on the Internet.
Until 2007, AS numbers were defined as 16-bit integers, which allowed for a maximum of 65,536 assignments. RFC 4893 introduced 32-bit AS numbers, which the Internet Assigned Numbers Authority has begun to allocate to regional Internet registry, although this proposed standard has now been replaced by RFC 6793. These numbers are written preferably as simple integers ranging from 0 to 4,294,967,295, or in the form called "asdot" which looks like x.y, where x and y are 16-bit numbers. Numbers of the form 0.y are exactly the old 16-bit AS numbers. The accepted textual representation of autonomous system numbers is defined in RFC 5396 as "asplain". The special 16-bit ASN 23456 was assigned by IANA as a placeholder for 32-bit ASN values for the case when 32-bit-ASN capable routers send BGP messages to routers with older BGP software which do not understand the new 32-bit ASNs.
The first and last ASNs of the original 16-bit integers and the last ASN of the 32-bit numbers are reserved and should not be used by operators. ASNs 64,496 to 64,511 of the original 16-bit range and 65,536 to 65,551 of the 32-bit range are reserved for use in documentation by RFC 5398. ASNs 64,512 to 65,534 of the original 16-bit AS range, and 4,200,000,000 to 4,294,967,294 of the 32-bit range are reserved for Private Use by RFC 6996, meaning they can be used internally but should not be announced to the global Internet. All other ASNs are subject to assignment by IANA.
The number of unique autonomous networks in the routing system of the Internet exceeded 5,000 in 1999, 30,000 in late 2008, 35,000 in mid-2010, 42,000 in late 2012, 54,000 in mid-2016 and 60,000 in early 2018.
The number of allocated ASNs exceeded 92,000 as of August 2019.

Assignment

AS numbers are assigned in blocks by Internet Assigned Numbers Authority to regional Internet registries. The appropriate RIR then assigns ASNs to entities within its designated area from the block assigned by IANA. Entities wishing to receive an ASN must complete the application process of their RIR, LIR or upstream service provider and be approved before being assigned an ASN. Current IANA ASN assignments to RIRs can be found on the IANA website.
There are other sources for more specific data:
A complete table of 16-bits and 32-bits ASN available:
NumberBitsDescriptionReference
016ReservedRFC1930, RFC7607
1 - 2345516Public ASNs
2345616Reserved for AS Pool TransitionRFC6793
23457 - 6449516Public ASNs
64496 - 6451116Reserved for use in documentation/sample codeRFC5398
64512 - 6553416Reserved for private useRFC1930, RFC6996
6553516ReservedRFC7300
65536 - 6555132Reserved for use in documentation and sample codeRFC4893, RFC5398
65552 - 13107132Reserved
131072 - 419999999932Public 32-bit ASNs
4200000000 - 429496729432Reserved for private useRFC6996
429496729532ReservedRFC7300

Types

Autonomous systems can be grouped into four categories, depending on their connectivity and operating policy.
  1. multihomed: An AS that maintains connections to more than one other AS. This allows the AS to remain connected to the internet in the event of a complete failure of one of their connections. However, unlike a transit AS, this type of AS would not allow traffic from one AS to pass through on its way to another AS.
  2. stub: An AS that is connected to only one other AS. This may be an apparent waste of an AS number if the network's routing policy is the same as its upstream AS's. However, the stub AS may have peering with other autonomous systems that is not reflected in public route-view servers. Specific examples include private interconnections in the financial and transportation sectors.
  3. transit: An AS that provides connections through itself to other networks. That is, network A can use network B, the transit AS, to connect to network C. If one AS is an ISP for another, then it is considered a transit AS.
  4. Internet Exchange Point : A physical infrastructure through which ISPs or content delivery networks exchange internet traffic between their networks. IXP ASNs are usually transparent.