This article will describe the routing process and its available algorithms. We will also discuss which routing protocols are available under Linux and how to configure them. In particular, we need to understand Internets structured around a backbone and those composed of multiple-peer networks, and consider the consequences of routing. To answer the second question, we need to consider the two basic types of route propagation algorithms and see how each supply routing information automatically. The point is that a host can route datagrams successfully even if it only has partial routing information because it can rely on a router.
Routers can also route with only partial information under certain circumstances.
To understand the criteria, imagine an Internet to be a foreign country criss-crossed with dirt roads that have directional signs posted at intersections. You leave on your journey, following the only road out of town and begin to look for directional signs. Since your destination is not listed, you continue straight. In routing jargon, you follow the default route. This imaginary voyage is similar to a datagram traversing an Internet, and the road signs are analogous to routing tables in routers along the path.