Autonomous System (AS)

An Autonomous System (AS) is a collection of routers whose prefixes and routing policies are under common administrative control. This could be a network service provider, a large company, a university, a division of a company, or a group of companies. The AS represents a connected group of one or more blocks of IP addresses, called IP prefixes, that have been assigned to that organization and provides a single routing policy to systems outside the AS. An IP prefix is a group of IP addresses expressed in CIDR form (i.e., address/bits, such as Autonomous Systems create a two-level hierarchy for routing in the Internet. Routing between Autonomous Systems (inter-AS routing) is external to the AS and allows one AS to send traffic to another AS. Note that most organizations do not interconect via autonomous systems but simply connect to a single ISP, which may be an autonomous system. Routers within an AS use an Interior Gateway Protocol (IGP), which handles routing between nodes inside the AS. Common interior gateway protocols include RIP, OSPF, IS-IS, EIGRP, as well as some proprietary protocols such as IGRP. Routing within an Autonomous System (intra-AS routing) is internal to that AS and invisible to those outside it. The AS administrator decides what routing algorithm should run within it. To get traffic from a host in one AS to a host in another AS, the autonomous systems need to be connected. Most ASes do not share a direct link with each other, in which case data traffic may be routed through the networks of other ASes that agree to carry the traffic. An Exterior Gateway Protocol (EGP) is a routing protocol that handles routing between Autonomous Systems (inter-AS routing). BGP version 4, the Border Gateway Protocol, is the standard EGP for inter-AS routing. At some point in the future, the Internet is expected to adopt IDRP, the OSI Inter-Domain Routing protocol.[1]

Autonomous System
source: Cloudflare

Autonomous systems were introduced to regulate organizations such as Internet service providers (ISP), educational institutions and government bodies. These systems are made up of many different networks but are operated under the umbrella of a single entity for easy management. Large enterprises tend to have large network infrastructures with many smaller networks, dispersed geographically but connected using a similar operating environment. Border Gateway Protocol (BGP) is the protocol that addresses the routing of packets among different autonomous systems to connect them. BGP uses the ASN to uniquely identify each system. This is particularly important when routing and managing routing tables for external networks or autonomous systems around their borders. Autonomous systems numbered one to 64511 are available by IANA/ARIN (IANA/American Registry for Internet Numbers) for global use. The 64512 to 65535 series is reserved for private and reserved purposes.[2]

Types of Autonomous Systems[3]
Autonomous systems (AS) can be grouped into four categories, depending on their connectivity and operating policy.

  • 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.
  • 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 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.[clarification needed]
  • Internet Exchange Point (IX or IXP): A physical infrastructure through which ISPs or content delivery networks (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.

Autonomous System Number[4]
Each autonomous system is assigned a globally unique number called an Autonomous System Number (ASN). The number serves as an identifier for the AS and is used when exchanging routing information with other autonomous systems. ASNs are available in both 16-bit and 32-bit format, although ASNs issued before 2007 were all 16-bit.

An ASN can be either public or private. An AS must have a public ASN to exchange data with other ASes on the internet. When an AS has a public ASN, all routes that originate from the AS are visible to the internet. However, an AS can obtain a private ASN if it communicates with only one provider and it uses Border Gateway Protocol (BGP) to carry out those communications. In this case, the routing policy between the AS and the provider is not visible to the internet.

The Internet Assigned Numbers Authority (IANA) manages the ASN system and coordinates the distribution of ASNs across five global regions. Each region maintains its own regional internet registry, which is responsible for issuing ASNs to individual ASes within that region. The five regional internet registries cover the following territories:

  • African Network Information Center, or AFRINIC -- Africa
  • Asia-Pacific Network Information Centre, or APNIC -- Asia/Pacific
  • American Registry for Internet Numbers, or ARIN -- Canada, USA and some Caribbean Islands
  • Latin America and Caribbean Network Information Centre, or ACNIC -- Latin America and some Caribbean Islands
  • Réseaux IP Européens Network Coordination Centre, or RIPE NCC -- Europe, the Middle East and Central Asia

Currently, IANA functions are carried out by the Public Technical Identifiers organization, an affiliate of the Internet Corporation for Assigned Names and Numbers, which is now responsible for managing IANA functions.

See Also


  1. What is Autonomous System (AS) Rutgers
  2. Explaining Autonomous Systems Techopedia
  3. What are the different Types of Autonomous Systems? Wikipedia
  4. What are autonomous system numbers? Techtarget