Autonomous system (Internet)

From Academic Kids

In the Internet, an autonomous system (AS) is a collection of IP networks under control of 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.

A unique AS number (or ASN) is allocated to each AS for use in BGP routing. With BGP, AS numbers are important because the ASN uniquely identifies each network on the internet.


AS numbers are assigned by the IANA, which also allocate IP addresses, to regional internet registries (RIRs) in blocks. The local RIR then assigns an AS number to an entity 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 their website: [1] (

AS numbers are currently 16-bit integers, which allow for a maximum of 65536 assignment. AS numbers are divided into 2 ranges. The first are public numbers, which may be used on the internet and range from 1 to 64511. The second range, from 64512 to 65536, are known as private numbers, and can only be used internally within an organization.


Autonomous Systems can be grouped into 3 categories, depending on their connections and operation.

A multihomed AS is an AS that maintains connections to more than one ISP. This allows the AS to remain connected to the internet in the event of a complete failure of one of their ISPs. In addition, this type of AS would not allow traffic from one ISP to pass through on its way to another ISP.

A stub AS refers to an AS that is only connected to a single ISP. This is generally regarded to be a needless use of an AS number, as the network is completely under a single ISP, and thus does not need to be uniquely identified.

A transit AS is an AS that provides connections through itself to the networks connected to it. That is, network A can use the transit AS to connect to network B. ISPs are always transit ASs, because it is their business to connect disparate networks in exchange for money. The ISP is considered to be 'selling transit service' to the end network, thus the term transit AS.

Related Topics

fr:Autonomous System fi:Autonominen järjestelmä


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