Route Tables

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Route Tables

Route tables are used by machines/hosts on the network and by routers to determine where packets should be sent to reach their final destination. Each router builds an internal route table every time IP is loaded during system initialization. Take a closer look at a route table. Figure 5.23 illustrates an example of a route table built during the initialization of a machine configured to be a router.

Notice the five columns of information provided within the route table.

Network Address. This column represents all networks that this machine or router knows about, including entries for the default gateway, subnet and network broadcasts, the universal loopback address, and the default multicast address. In a route table, you can use names instead of IP addresses to identify networks. If you use names instead of IP addresses, the names are resolved using the networks file found in the \%system drive%\system32\drivers\etc directory. While you can configure this option, the author strongly recommends against using such names, purely from a troubleshooting perspective. If, for some reason, the networks file was deleted or corrupted, name resolution would not be your only problem. Your router would suddenly find it difficult to route packets to these networks without knowing which IP addresses represented those network IDs.
Netmask. This column simply identifies the subnet mask used for a particular network entry.
Gateway Address. This is the IP address to which packets should be sent in order to route packets to their final destination. Each network address may specify a different gateway address in which to send packets. This may be particularly true if more than one router is connected to one network segment. This column may also have self-referential entries indicating the IP address to which broadcasts should be sent, as well as the local loopback entries. You can also use names to identify these IP addresses. Any names used here will be resolved using the local hosts file on the machine. Again, while this option is supplied, the author does not recommend introducing another source of possible error by using names in route tables.
Interface. This IP address is used primarily to identify the IP address of the machine and to identify this IP address as the interface to the network. On a machine with one network card, only two entries appear. For any network address that is self-referential, the interface is 127.0.0.1, meaning that packets are not even sent onto the network. For all other communications, the IP address represents the network card interface used to communicate out onto the network. For multihomed machines, the interface IP address changes depending on which network address is configured on each network card. In this case, the interface identifies the IP address of the card connected to a particular network segment.
Metric. The metric indicates the cost or hops associated with a particular network route. The router’s job is to find the path representing the least cost or effort to get the packet to its destination. The lower the cost or hop count, the better or more efficient a particular route. On a static router, the metric for any network address will be one, indicating that the router thinks every network is only one router hop away. This is obviously not true, indicating that on static routers, this column is fairly meaningless. On dynamic routers, however, this column indicates to a router the best possible route to send packets.

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