What does "autonomous system" refer to in routing?

The term "autonomous system" refers specifically to a collection of IP networks and routers under the control of a single organization that presents a common routing policy to the Internet. An autonomous system is identified by a unique Autonomous System Number (ASN) used in routing protocols, particularly in Border Gateway Protocol (BGP) which facilitates the exchange of routing information between different autonomous systems on the Internet.

In the context of routing, understanding the concept of an autonomous system is crucial because it determines how networks advertise their reachability and make routing decisions. Each autonomous system manages its routing within its network while interacting with other autonomous systems, providing a structured approach to online communication and connectivity on a larger scale.

Other options presented in the question refer to unrelated concepts. The first choice about a group of public IP addresses does not encompass the broader organizational control aspect that defines an autonomous system. The third option, a type of routing protocol, is not accurate since autonomous systems can utilize different routing protocols, and they themselves are not a protocol. Lastly, the fourth choice regarding a method of encryption is completely irrelevant to the definition of an autonomous system in networking.