Routing is the process of transferring data across an internetwork from a source host to a destination host. Routing can be understood in terms of two processes: host routing and router routing .
Host routing occurs when the sending host forwards a packet. Based on the destination network address, the sending host must decide whether to forward the packet to the destination or to a router. In Figure 1.2, the Source Host forwards the packet destined for the Destination Host to Router 1.
Router routing occurs when a router receives a packet that is to be forwarded. The packet is forwarded between routers (when the destination network is not directly attached to the router) or between a router and the destination host (when the destination network is directly attached). In Figure 1.2, Router 1 forwards the packet to Router 2. Router 2 forwards the packet to the Destination Host.
Figure 1.2 The Routing Process