What is an autonomous system (AS)? Which roles do autonomous systems play in the Internet?

What will be an ideal response?

If we want to support very massive network like Internet, stability and policy have higher
priority to consider than route optimization because of its size and complexity. In this level of
network management, a network of networks which is called routing domain or autonomous system
is a unit for obtaining connectivity of networks.
In very large internetworks, it is necessary to divide the internetwork into separate autonomous
systems. An Autonomous System (AS) is a portion of the internetwork under the same administrative
authority. The AS may be further divided into regions, domains, or areas that define a hierarchy
within the AS. An autonomous system is a set of routers under a single technical administration. Routers in an AS can use multiple interior gateway protocols to exchange routing information inside the AS and an exterior gateway protocol to route packets outside the AS. We need to manage each of the different networks among different organizations by their managing rule or policies. That’s why we use ASs.
Note : There are several advantages of dividing autonomous system (AS) into multiple
areas. The areas of an autonomous system are groups of contiguous networks with each area
having a separate topological database. One advantage is that an area’s topology is invisible to entities outside the area, thus OSPF passes less routing traffic than if would if the AS is not partitioned. This would limit the explosion of link state updates over the entire network. Another advantage is that fewer routers participate in the exchange of routing updates. Inter- area routing is done only by the ABR, which allows for aggregation of routes and reduce unnecessary propagation of subnet information.