The Internet is the prime example of a large internetwork divided into different autonomous systems (such as CERFnet, SprintLink, and AlterNet). These autonomous systems are connected with backbone routers that use the Border Gateway Protocol (BGP) for communication among them. Each autonomous system is represented by a 16-bit integer assigned by Internet Network Information Center (InterNIC), which is used by BGP to avoid routing loops and implement policy-based routing on the Internet backbone. There are three types of autonomous systems:
Geoffrey Challen, Assistant Professor at the University at Buffalo, explains in a very simple manner the concept of Autonomous System in a large network like the internet.
When discussing routing, we sometimes refer to these independent networks as autonomous systems. Each autonomous system is responsible for routing packets internally. To travel across the broader internet, packets must traverse multiple autonomous systems. So it is more accurate to think of internet routing as happening across autonomous systems, rather than between individual computers.