What is an autonomous system and what are AS Numbers (ASN)?

The term Autonomous Systems (AS) usually refers to larger IP network associations that are managed by a single routing policy. For identification purposes, a unique Autonomous System Number (ASN) is assigned to each AS. The individual Autonomous Systems together make up the Internet. The division of the World Wide Web into individual segments allows better scalability while resource consumption remains low.

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Autonomous Systems & ASNs: Definition

Autonomous Systems (AS) can be understood as a group of routers and networks that are managed as an administrative entity via a single routing policy. The Internet is composed of these same large network groups operated by individual organizations – most commonly Internet Service Providers (ISPs), technology companies, universities or government agencies. Each AS controls its own IP address space, which includes all IPs managed within the AS. The IP address space is listed in the routing policy for the AS. It also includes the other network groups connected to the AS. This information is required for the successful routing of data packets between different AS networks.

For the external communication between routers in different network groups, each AS is additionally assigned a unique number (16-bit, 32-bit), the so-called AS Number (ASN). A distinction is made between public and private ASNs, whereby public ASNs are designated for data transfer across the Internet, while private ASNs are limited to systems that are only connected to a single provider. Data is routed between different Autonomous Systems using the Border Gateway Protocol (BGP).

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Who assigns the ASN?

The Internet Assigned Numbers Authority (IANA) is responsible for the allocation of ASNs. The IANA delegates the actual allocation to the Regional Internet Registries (RIR), which are responsible for the allocation in North America (ARIN), Europe, Middle East and Central Asia (RIPE NCC), Asia-Pacific (APNIC), Latin America, the Caribbean (LACNIC) and Africa (AfriNIC).

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Autonomous Systems and AS numbers: What you need to know

Autonomous Systems (AS) are larger network groups that are managed by means of a single routing policy. Autonomous Systems are typically operated by larger organizations, network operators (ISPs), universities or government agencies. The Internet is composed of the individual AS networks. To identify the network groups, each AS has a unique ASN (AS Number). Depending on whether the AS is connected to several other Autonomous Systems or only to individual network groups, a distinction is made here between public and private ASNs. The connection between the Autonomous Systems for data routing is established using the Border Gateway Protocol (BGP).

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