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

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).