Windows NT TCP/IP Network Administration

The Routing Table

Gateways route data between networks; but all network devices, hosts as well as gateways, must make routing decisions. For most hosts, the routing decisions are simple:

• If the destination host is on the local network, the data is delivered to the destination host.

• If the destination host is on a remote network, the data is forwarded to a local gateway.

Because routing is network-oriented, IP makes routing decisions based on the network portion of the address. The IP module determines the network part of the destination’s IP address by applying the network mask to the address. If the destination network is the local network, the mask that is applied may be the local subnet mask. If no mask is provided with the address, the address class determines the network portion of the address.

After determining the destination network, the IP module looks up the network in the local routing table.^[10]

Packets are routed toward their destination as directed by the routing table. The routing table may be built by the system administrator or by routing protocols, but the end result is the same; IP routing decisions are simple table look-ups.

You can display the routing table’s contents with the netstat -nr command or the route print command. The -r option tells netstat to display the routing table, and the -n option tells netstat to display the table in numeric form. It’s useful to display the routing table in numeric form because the destination of most routes is a network, and ...