An autonomous system number (ASN) is a unique identifier assigned to an autonomous system (AS), which is a collection of IP networks and routers under the control of a single organization. ASNs are essential for routing internet traffic, allowing different networks to communicate with each other efficiently. ASNs are used in the Border Gateway Protocol (BGP), the protocol that manages how packets are routed across the internet. Each ASN is either a 16-bit or 32-bit number, enabling organizations to manage their own IP address space and establish policies for data exchange with other networks.