In Internet routing, what is an autonomous system (AS)?

Introduction / Context:
The concept of an autonomous system is central to the design of the Internet's routing architecture. Rather than being a flat mesh of individual routers, the Internet is structured as a collection of interconnected autonomous systems. Each AS is managed by a single organization and runs its own internal routing, while using interdomain routing protocols to communicate with other ASes. Understanding what an autonomous system is helps explain how routing policies and BGP operate at scale.

Given Data / Assumptions:

The Internet consists of many networks owned by different organizations such as ISPs, enterprises and universities.
Each organization typically wants to control routing within its own network independently of others.
The term autonomous system is used in BGP and related routing standards.
We must provide a precise definition suitable for exams.

Concept / Approach:
An autonomous system is formally defined as a collection of IP networks and routers under the control of a single administrative entity that presents a common and clearly defined routing policy to the Internet. This means that within the AS, administrators can choose any Interior Gateway Protocol and internal topology. Externally, however, the AS is treated as a single unit, identified by an Autonomous System Number (ASN), when exchanging routes via BGP with other ASes. The AS acts as a building block in the hierarchical structure of global routing.

Step-by-Step Solution:
Step 1: Identify that multiple subnets, links and routers within one company or ISP are typically managed as one administrative domain. Step 2: This domain operates its own IGP and internal policies, independent of other organizations. Step 3: When communicating with the rest of the Internet, this entire domain is identified by an Autonomous System Number and expresses one coherent routing policy, for example which prefixes to advertise and which providers to prefer. Step 4: This collection of networks and routers, under one administration and policy, is the definition of an autonomous system. Step 5: Single routers, isolated computers or groups of privileged users do not fit this definition; they are either too small or do not represent a routing domain.

Verification / Alternative check:
RFC 1930 and related documents define an AS in terms very similar to option A. Practical examples include an ISP with many PoPs and routers, all using one ASN, or a large enterprise with its own AS connecting to multiple upstream providers. These cases show that an AS is more than one router and certainly more than a single host or user group.

Why Other Options Are Wrong:
A single router might belong to an AS but does not itself define the AS; the AS is the larger collection it sits in.
A standalone computer not connected to any network has no routing role and thus cannot be considered an autonomous system in routing terms.
User privilege groups have nothing to do with routing policy or administrative control over IP networks.

Common Pitfalls:
Learners sometimes think that any independent network is an AS, even if it never connects to the global Internet. While the concept can be used internally, in Internet routing standards the AS notion is tied to interdomain routing and AS numbers. Another pitfall is to ignore the policy aspect and think of AS purely as a topological grouping; in reality, shared administration and policy are key parts of the definition.

Final Answer:
An autonomous system is a collection of IP networks and routers under a single administrative control that presents a common routing policy to the Internet.