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 crucial for routing internet traffic, allowing different networks to communicate with each other efficiently. They help ensure that data packets find the best path across the internet. ASNs are managed by the Internet Assigned Numbers Authority (IANA) and distributed by regional registries. Each ASN can be either a 16-bit number (ranging from 1 to 65,535) or a 32-bit number (ranging from 65,536 to 4,294,967,295), providing a vast pool of identifiers for organizations worldwide.