What does a routing device define?

A routing device defines the edge of the broadcast domain because routers operate at the network layer and are responsible for forwarding packets between different networks. When a broadcast packet is sent, a router will not forward that packet to other networks, effectively defining the boundary of the broadcast domain. In simple terms, a broadcast domain is a logical division of a computer network where all nodes can reach each other by broadcast at the data link layer within the same network segment.

This means that devices within the same broadcast domain can communicate with each other without the need for a router. However, when messages need to be sent to different networks (or broadcast domains), the information must go through a router, which segregates the networks and controls the flow of data, thereby ensuring that broadcast messages are contained within their respective domains.