An AS number, or Autonomous System number, is a unique identifier assigned to an Autonomous System (AS) in the context of the Internet. An AS is a collection of IP networks and routers under the control of a single organization that presents a common routing policy to the Internet. AS numbers are essential for the operation of the Border Gateway Protocol (BGP), which is used to exchange routing information between different ASes. AS numbers can be either 16-bit or 32-bit, allowing for a large number of unique identifiers. They are issued by the Internet Assigned Numbers Authority (IANA) and regional registries to organizations that need to manage their own routing policies. Each AS number helps ensure that data packets are routed efficiently across the global Internet.