15.2.3 Network-Layer Internetworking

The device responsible for interconnecting independent networks is the router, which operates at the Network Layer (Layer 3).

Rather than forwarding frames according to MAC addresses, routers examine logical network addresses, typically IP addresses, to determine the most appropriate path toward the destination. Each router maintains a routing table that identifies the next hop for packets destined for different networks.

When a packet arrives, the router removes the incoming Layer-2 frame, examines the destination IP address, determines the best route, and encapsulates the packet within a new Layer-2 frame appropriate to the outgoing network. In this way, the packet can traverse multiple heterogeneous networks while retaining the same network-layer addressing.

Unlike switches, routers separate broadcast domains. Broadcast traffic is therefore confined to individual LANs rather than propagating throughout an entire internetwork. This greatly improves scalability and allows very large networks to be constructed by interconnecting many smaller subnetworks.

Routers therefore form the fundamental building blocks of internetworking and provide the hierarchical packet forwarding that enables the global Internet to operate.