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. Each AS is assigned an autonomous system number (ASN), for use in

Border Gateway Protocol Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. BGP is classified as a path-vector routing protocol, and it make ...

(BGP) routing. Autonomous System Numbers are assigned to Local Internet Registries (LIRs) and end user organizations by their respective Regional Internet Registries (RIRs), which in turn receive blocks of ASNs for reassignment from the

Internet Assigned Numbers Authority The Internet Assigned Numbers Authority (IANA) is a standards organization that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System (DNS), media types, and other Inte ...

(IANA). The IANA also maintains a registry of ASNs which are reserved for private use (and should therefore not be announced to the global Internet). Originally, the definition required control by a single entity, typically an

Internet service provider An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise privat ...

(ISP) or a very large organization with independent connections to multiple networks, that adhered to a single and clearly defined routing policy. In March 1996, the newer definition came into use because multiple organizations can run BGP 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 ASN. Until 2007, AS numbers were defined as 16-bit integers, which allowed for a maximum of 65,536 assignments. Since then, (obsoleted by ) the IANA has begun to also assign 32-bit AS numbers to regional Internet registry (RIRs). These numbers are written preferably as simple integers, in a notation referred to as "asplain", ranging from 0 to 4,294,967,295 ( hexadecimal 0xFFFF FFFF). Or, alternatively, 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 special 16-bit ASN 23456 ("AS_TRANS") was assigned by IANA as a placeholder for 32-bit ASN values for the case when 32-bit-ASN capable routers ("new BGP speakers") send BGP messages to routers with older BGP software ("old BGP speakers") which do not understand the new 32-bit ASNs. The first and last ASNs of the original 16-bit integers (0 and 65,535) and the last ASN of the 32-bit numbers (4,294,967,295) are reserved and should not be used by operators; AS0 is used by all five RIRs to invalidate unallocated space. 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. 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. 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 100,000 as of March 2021.

Assignment

AS numbers are assigned in blocks by

(IANA) to regional Internet registries (RIRs). 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. RIRs, as part of

NRO NRO may stand for: * National Reconciliation Ordinance, a Pakistani law * National Reconnaissance Office, maintains United States reconnaissance * National Repertory Orchestra, in Colorado * ''National Review Online'', web version of the magazine ...

, can revoke AS numbers as part of their

Internet governance Internet governance consists of a system of laws, rules, policies and practices that dictate how its board members manage and oversee the affairs of any internet related-regulatory body. This article describes how the Internet was and is currentl ...

abilities. There are other sources for more specific data: *

APNIC APNIC (the Asia Pacific Network Information Centre) is the regional Internet address registry ( RIR) for the Asia-Pacific region. It is one of the world's five RIRs and is part of the Number Resource Organization ( NRO). APNIC provides numb ...

: https://ftp.apnic.net/stats/apnic/ *

RIPE NCC RIPE NCC (''Réseaux IP Européens'' Network Coordination Centre) is the regional Internet registry (RIR) for Europe, the Middle East and parts of Central Asia. Its headquarters are in Amsterdam, Netherlands, with a branch office in Dubai, UAE. ...

: https://ftp.ripe.net/ripe/stats/ *

AFRINIC AFRINIC (African Network Information Centre) is the regional Internet registry (RIR) for Africa. Its headquarters are in Ebene, Mauritius. Before AFRINIC was formed, IP addresses (IPv6 and IPv4) for Africa were distributed by the Asia-Pacific N ...

: https://ftp.afrinic.net/pub/stats/afrinic/ * ARIN: https://ftp.arin.net/pub/stats/arin/ * LACNIC: https://ftp.lacnic.net/pub/stats/lacnic/

ASN table

A complete table of 16-bits and 32-bits ASN available:

Types

Autonomous systems (AS) can be grouped into four categories, depending on their connectivity and operating policy. # ''

multihomed Multihoming is the practice of connecting a host or a computer network to more than one network. This can be done in order to increase reliability or performance. A typical host or end-user network is connected to just one network. Connecting t ...

'': 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. # '' 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. # '' transit'': An AS that acts as a router between two ASes is called a transit. Since not all ASes are directly connected with every other AS, a transit AS carries data traffic between one AS to another AS to which it has links. # ''

Internet Exchange Point Internet exchange points (IXes or IXPs) are common grounds of IP networking, allowing participant Internet service providers (ISPs) to exchange data destined for their respective networks. IXPs are generally located at places with preexisting ...

'' (IX or IXP): A physical infrastructure through which ISPs or

content delivery network A content delivery network, or content distribution network (CDN), is a geographically distributed network of proxy servers and their data centers. The goal is to provide high availability and performance by distributing the service spatially rel ...

s (CDNs) exchange Internet traffic between their networks (autonomous systems). These are often groups of local ISPs that band together to exchange data by splitting the costs of a local networking hub, avoiding the higher costs (and bandwidth charges) of a Transit AS. IXP ASNs are usually transparent. By having presence in an IXP, ASes shorten transit path to other participating ASes, thereby reducing

network latency Network delay is a design and performance characteristic of a telecommunications network. It specifies the Latency (engineering), latency for a bit of data to travel across the network from one communication endpoint to another. It is typically ...

and improving

round-trip delay In telecommunications, round-trip delay (RTD) or round-trip time (RTT) is the amount of time it takes for a signal to be sent ''plus'' the amount of time it takes for acknowledgement of that signal having been received. This time delay includes pr ...

AS-SET

Autonomous systems can be included in one or more AS-SETs, for example AS-SET of RIPE NCC "AS-12655" has AS1, AS2 and AS3 as its members, but AS1 is also included in other sets in ARIN (AS-INCAPSULA) and APNIC (AS-IMCL). Another AS-SET sources can be RADB, LEVEL3 (

tier 1 network A Tier 1 network is an Internet Protocol (IP) network that can reach every other network on the Internet solely via settlement-free interconnection (also known as settlement-free peering). Tier 1 networks can exchange traffic with other Tier 1 ne ...

now called

Lumen Technologies Lumen Technologies, Inc. (formerly CenturyLink) is an American telecommunications company headquartered in Monroe, Louisiana, that offers communications, network services, security, cloud solutions, voice, and managed services. The company is a ...

) and also ARIN has ARIN-NONAUTH source of AS-SETs. AS-SETs can be included in other AS-SETs and even form cycles.

References

External links