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 for those networks. In the simpler case you will have one of the connected networks that only needs to talk with the rest of the network structure and is routed with only a single default route. A default route is traditionally coded as a route to the 0.0.0.0/0 destination. However, it may be that you are within a structured network with a given finite range or that you want to implement better security, in which case your default route may only cover your network scope. The point is that a default route is really defined as the "route of last resort," which is used if no other route covers the packet destination.

These considerations illustrate the fundamental thought behind traditional IPv4 routing:

All routing is a destination-driven process.

Every packet that enters a router is inspected to determine the destination IP address. Based on that destination address the router then consults the routing table to determine where to send the packet. The only item of interest to the router is the destination address. Nothing else matters.

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