Autonomous system (Internet)
Encyclopedia
Within the Internet
, an Autonomous System (AS) is a collection of connected Internet Protocol
(IP) routing
prefixes under the control of one or more network operators 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 adhere 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 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 Autonomous System Number (ASN).
A unique ASN is allocated to each AS for use in BGP
routing. AS numbers 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 65536 assignments. The Internet Assigned Numbers Authority
(IANA) has designated AS numbers 64512 through 65534 to be used for private purposes. The ASNs 0, 59392–64511, and 65535 are reserved by the IANA and should not be used in any routing environment. ASN 0 may be used to label non-routed networks. All other ASNs (1–54271) are subject to assignment by IANA, and, , only 49152–54271 remained unassigned. RFC 4893 introduced 32-bit AS numbers, which IANA has begun to allocate. These numbers are written either as simple integers, or in the form x.y, where x and y are 16-bit numbers. Numbers of the form 0.y are exactly the old 16-bit AS numbers, 1.y numbers and 65535.65535 are reserved, and the remainder of the space is available for allocation. The accepted textual representation of Autonomous System Numbers is defined in RFC 5396.
The number of unique autonomous networks in the routing system of the Internet exceeded 5000 in 1999, in late 2008, and in mid 2010.
(IANA) to Regional Internet Registries
(RIRs). The appropriate RIR then assigns AS numbers to entities within its designated area from the block assigned by the IANA. Entities wishing to receive an ASN
must complete the application process of their local RIR and be approved before being assigned an ASN. Current IANA ASN assignments can
be found on the IANA website.
A multihomed
Autonomous System is 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, this type of AS would not allow traffic from one AS to pass through on its way to another AS.
A stub Autonomous System refers to 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 in fact 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.
A transit Autonomous System is 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. ISPs are always transit ASs, because they provide connections from one network to another. The ISP is considered to be 'selling transit service' to the customer network, thus the term transit AS.
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, an Autonomous System (AS) is a collection of connected Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP) routing
Routing
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
prefixes under the control of one or more network operators 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
Internet service provider
An Internet service provider is a company that provides access to the Internet. Access ISPs directly connect customers to the Internet using copper wires, wireless or fiber-optic connections. Hosting ISPs lease server space for smaller businesses and host other people servers...
or a very large organization with independent connections to multiple networks, that adhere 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 BGP
Border Gateway Protocol
The Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems . It is described as a path vector 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 (ASN).
A unique ASN is allocated to each AS for use in BGP
Border Gateway Protocol
The Border Gateway Protocol is the protocol backing the core routing decisions on the Internet. It maintains a table of IP networks or 'prefixes' which designate network reachability among autonomous systems . It is described as a path vector protocol...
routing. AS numbers 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 65536 assignments. The Internet Assigned Numbers Authority
Internet Assigned Numbers Authority
The Internet Assigned Numbers Authority is the entity that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System , media types, and other Internet Protocol-related symbols and numbers...
(IANA) has designated AS numbers 64512 through 65534 to be used for private purposes. The ASNs 0, 59392–64511, and 65535 are reserved by the IANA and should not be used in any routing environment. ASN 0 may be used to label non-routed networks. All other ASNs (1–54271) are subject to assignment by IANA, and, , only 49152–54271 remained unassigned. RFC 4893 introduced 32-bit AS numbers, which IANA has begun to allocate. These numbers are written either as simple integers, or in the form x.y, where x and y are 16-bit numbers. Numbers of the form 0.y are exactly the old 16-bit AS numbers, 1.y numbers and 65535.65535 are reserved, and the remainder of the space is available for allocation. The accepted textual representation of Autonomous System Numbers is defined in RFC 5396.
The number of unique autonomous networks in the routing system of the Internet exceeded 5000 in 1999, in late 2008, and in mid 2010.
Assignment
AS numbers are assigned in blocks by the Internet Assigned Numbers AuthorityInternet Assigned Numbers Authority
The Internet Assigned Numbers Authority is the entity that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System , media types, and other Internet Protocol-related symbols and numbers...
(IANA) to Regional Internet Registries
Regional Internet registry
A regional Internet registry is an organization that manages the allocation and registration of Internet number resources within a particular region of the world...
(RIRs). The appropriate RIR then assigns AS numbers to entities within its designated area from the block assigned by the IANA. Entities wishing to receive an ASN
must complete the application process of their local RIR and be approved before being assigned an ASN. Current IANA ASN assignments can
be found on the IANA website.
Types
Autonomous Systems can be grouped into three categories, depending on their connectivity and operating policy.A multihomed
Multihoming
Multihoming is a technique used to increase the reliability of the Internet connection for an IP network. As an adjective, it is typically used to describe a customer, rather than an Internet service provider network...
Autonomous System is 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, this type of AS would not allow traffic from one AS to pass through on its way to another AS.
A stub Autonomous System refers to 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 in fact have peering
Peering
In computer networking, peering is a voluntary interconnection of administratively separate Internet networks for the purpose of exchanging traffic between the customers of each network. The pure definition of peering is settlement-free or "sender keeps all," meaning that neither party pays the...
with other Autonomous Systems that is not reflected in public route-view servers
Looking Glass Servers
Looking Glass servers are computers on the Internet running one of a variety of publicly available Looking Glass software implementations. A Looking Glass server is accessed remotely for the purpose of viewing routing info. Essentially, the server acts as a limited, read-only portal to routers of...
. Specific examples include private interconnections in the financial and transportation sectors.
A transit Autonomous System is 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. ISPs are always transit ASs, because they provide connections from one network to another. The ISP is considered to be 'selling transit service' to the customer network, thus the term transit AS.
See also
- Routing Assets DatabaseRouting Assets DatabaseRouting Assets Database , run by Merit Network, is a lookup database designed to make fundamental information about networks available. The RADb is a public registry of routing information for networks in the Internet...
(RADB) - INOC-DBAINOC-DBAThe hotline phone system is a global voice telephony network deployed and managed by Packet Clearing House that connects the network operations centers and security incident response teams of critical internet infrastructure providers such as backbone carriers, internet service providers, and...
a hotline communications system between the network operations centers of major Autonomous Systems - Administrative distanceAdministrative distanceAdministrative distance is the measure used by Cisco routers to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative distance defines the reliability of a routing protocol. Each routing protocol is prioritized...
External links
- ASN Resource Guide
- AS Number plugin for Mozilla Firefox
- CIDR and ASN assignment report update continuously
- Exploring Autonomous System Numbers
- 32-bit ASN presentation at Doha, Qatar in 11-2007
- ASN Searching and Tracing by Fixed Orbit
- ASN Lookup
- ashunt, an AS traceroute utility (part of netsniff-ng)
- Autonomous System Whois Lookup