SOURCE ROUTING
Source routing originates from the days of mainframes when one computer needed to access the mainframe, and had to know how to reach it.
Source routing is handled by the end station (a computer, workstation or mainframe). Each end station keeps track of the paths to all destinations it needs to reach. All routing intelligence is in the transmitting host. Source routing protocols generally do a better job of picking the best path, however source routing protocols usually require a lot of traffic transmission to discover these paths. Often the end station hardware needs to be a bit more sophisticated (better CPU and more RAM) to be able to process this routing information and choose optimal paths to a destination.
DEFAULT GATEWAYS
Computer's on your local network are configured to send all traffic not destined for the local network to the Default Gateway. This allows hosts to use simpler software, and need less powerful CPU's and less memory to store routing information. By relying on a default gateway, the routing process resides in special routing devices which are configured as the default gateways for each network.
Today's wide area networks (WAN's) utilize router-based routing. Router-based routing assumes the routers will handle all routing and that end stations are oblivious to all routing information except the gateway router's address for the local network. Since the routers can dynamically discover routes, and they can be built as dedicated hardware and installed at the linkage point between networks, it is often more efficient to use a router. This lowers the capability requirements of the end stations as they do not have to perform routing themselves.