An AS number, or Autonomous System number, is a unique identifier assigned to an Autonomous System (AS) for routing purposes on the internet. Each 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 Border Gateway Protocol (BGP), which helps different networks communicate and exchange information. 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 internet registries. This system helps maintain efficient and organized routing across the global internet infrastructure.