Routing Tables Overview

Overview

This lecture explains the structure and function of routing tables, which guide routers in directing network traffic based on destination addresses.

A routing table is a data set routers use to decide where to send network packets.
Early routers were just computers with two network interfaces and manually updated routing tables.
Modern operating systems still maintain routing tables to determine packet routes.
You can build a simple router using a computer with two interfaces and a manual routing table.

Routing tables typically have columns for destination network, next hop, total hops, and interface.
The destination network column identifies each network the router knows about, using network ID and netmask.
Routers check the destination IP of incoming packets to match them to known networks.
A catch-all entry handles packets that don't match explicit network listings.

The next hop column lists the IP address of the next router or indicates if the network is directly connected.
The total hops column tracks the number of steps to a destination, helping routers choose the shortest path.
Network paths may change often due to outages, congestion, or new routers, so routing tables must update regularly.
The interface column tells the router which network interface to use for each destination.

Core internet routers may have millions of routing table entries, one for each potential network destination.
Routing tables are consulted for every packet sent through the router.

Routing Table — a list routers use to determine where to send data packets based on destination IP addresses.
Destination Network — the target network, identified by network ID and netmask, for a data packet.
Next Hop — the IP address of the next router or device a packet should be forwarded to.
Total Hops — the number of steps (routers) to reach a specific network destination.
Interface — the router's network port or connection used to forward outgoing packets.