Policy Routing Using Linux®

Traditional IPv4 Routing

In the simplest IPv4 routing case you would have a router and you would only need to consider how to allow two networks to talk between themselves. For this case you would only need to have an appropriate IP address assigned for each of these networks. Since this simple case does not concern any networks other than the two that are directly connected, you need only define the address and mask for each network and dictate the appropriate interface for forwarding packets to that network.

In reality, however, you often need to consider other networks beyond the scope of two connections. In these cases you must have a way to talk to the other networks. This is done by having routes that point to other routers that are responsible ...