An Autonomous System (AS) is a large network that has a common routing policy used to serve a set of IP prefixes. An AS is assigned to a single organization and is connected to multiple ASes to route IP packets in a redundant manner. An AS is assigned a 16-bit or 32-bit number (ASN) by the IANA to Internet Service Providers (ISPs), governments, universities and enterprises.

Each AS represents a large network and has thousands of smaller subnetworks that are leased to smaller entities. All the ASes that are interconnected together throughout the world make up the Internet. Consider AS as a community that participates in the global network called the Internet.

IP data travels from one IP address (source node) to another IP address (destination node). IP address whether it is IPv4 or IPv6, a set of prefixed bits are used as a network address to determine which AS it belongs to. Each AS serves a set of IP prefixes, and this network address (IP prefix) determines how the packet should be routed to the next AS.

What is BGP

Border Gateway Protocol (BGP) is the protocol for routing IP packets to neighboring ASes. Every AS uses BGP to announce which IP prefixes they are responsible for and which other ASes they are connected to so that an IP packet can find its way to the final destination by hopping through a series of ASes. BGP routers collect routing policies from every AS in the world and create routing tables to determine the fastest path from one AS to another AS. When an IP packet arrives at BGP, it looks up the routing tables and determines the shortest path to the next AS where it should forward the packets.