The Design

Let's look at how we will achieve this. Previously, we showed how to configure routing to allow data to flow between the two subnets by using a gateway (we defined the two interfaces “indium” and “tin” to accomplish this). Figure 13.1 shows the network at the stage where we had successfully connected all the systems together and had implemented any required routes and gateways.

We will now use “xenon” to perform a similar function, but this time to the external network. Once this has been set up, all the other machines will be able to access the external network using this route. This is normally implemented by using a default router, which is a piece of hardware such as a specialist device or another Sun workstation (as we used before). When a packet is sent from a system to a route where the destination address is not defined in the routing table, then the data will be passed to the system defined as the default router, which will then forward it to the correct machine, or perhaps to another router that has access to the destination system. We can see from this that each machine doesn't necessarily need to know where every system is—it only needs to know the whereabouts of the next router. In other words, the default router is a catchall for any addresses we do not know about—we expect it to know what to do with them.

Another obvious advantage of doing this is that we don't have to add a route to every machine we want to communicate with, which would be practically impossible for an Internet connection.