An autonomous system (AS) is an extensive network or group of networks with a single routing policy. Each AS is assigned a unique ASN, which is a number that identifies the AS. The Internet functions as a network of networks, with autonomous systems (ASes) being the major networks that constitute its structure. An autonomous system, defined as a large network or a conglomeration of smaller networks, operates under a unified routing policy. Every computer or device connecting to the Internet must link through an AS.
Each autonomous system governs a specific set of IP addresses, akin to how a town’s post office delivers mail to all addresses within that town. This designated range of IP addresses under the control of a particular AS is referred to as their “IP address space.” Generally, a single large organisation operates each autonomous system. These organisations can include Internet service providers (ISPs), enterprise technology companies, universities, or government organisations.