An autonomous system (AS) is a network or a collection of networks that are all managed and supervised by a single entity or organization.
These systems are made up of many different networks but are operated under the umbrella of a single entity for easy management. Large enterprises tend to have large network infrastructures with many smaller networks, dispersed geographically but connected using a similar operating environment.
Border Gateway Protocol (BGP) is the protocol that addresses the routing of packets among different autonomous systems to connect them.
BGP uses the ASN to uniquely identify each system. This is particularly important when routing and managing routing tables for external networks or autonomous systems around their borders.
Autonomous systems numbered one to 64511 are available by IANA/ARIN (IANA/American Registry for Internet Numbers) for global use.
The 64512 to 65535 series is reserved for private and reserved purposes.