What is the role of an Autonomous System Number (ASN) in Internet routing?

• A. Identify a network under a single administrative authority for BGP inter-AS routing
• B. Identify a LAN segment
• C. Assign IP addresses to hosts
• D. Indicate a router's hardware capabilities

Answer: (A)

An Autonomous System Number identifies a single administrative domain on the Internet for routing with BGP. Networks belong to an AS, a group under one admin that shares routing policies. When routers exchange routes with the wider Internet, they use BGP and label each route with the AS numbers along its path. This AS path reveals through which domains traffic will travel and enables policy decisions, like which neighbors to trust, preferred paths, and transit relationships. Inside an AS, internal protocols handle reachability, but between ASes, BGP uses the ASN to separate routing boundaries and apply policies. So, the ASN’s role is to identify the network for inter-domain routing, not to define a LAN segment, assign IP addresses, or indicate hardware capabilities.