Routing

Routing moves IP packets across network boundaries. A switch forwards frames within a single broadcast domain; a router forwards packets between different networks by reading destination IP addresses and choosing the best next hop.

Every device connected to a network makes routing decisions. Hosts decide whether to send directly (same subnet) or to the default gateway. Routers decide which of several possible paths leads closest to the destination.

This course covers how routing works at the protocol-independent level: the routing table, forwarding decisions, static routes, dynamic routing protocol categories, route selection, and the design choices that make networks converge, scale, and remain stable.

The routing table

A routing table is a data structure that maps destination network prefixes to next-hop addresses and outgoing interfaces. Every router and every host has one.

A routing table entry contains:

• Destination prefix. A network address and prefix length (e.g., 10.1.0.0/16).
• Next hop. The IP address of the next router toward the destination, or "directly connected" for local networks.
• Outgoing interface. The local interface to send the packet through.
• Metric. A cost value used to compare routes learned from the same protocol.
• Administrative distance. A preference value used to compare routes learned from different sources.

Routes enter the table through three mechanisms: directly connected networks (learned automatically from interface configuration), static routes (configured manually by an administrator), and dynamic routing protocols (learned from neighboring routers).

The forwarding decision

When a packet arrives, the router: