Routing assumes that addresses have been assigned to facilitate data delivery. In particular, routing assumes that addresses convey at least partial information about where a host is located. This permits routers to forward packets without having to rely either on broadcasting or a complete listing of all possible destinations. At the IP level, routing is used almost exclusively, primarily because the Internet was designed to construct large networks in which heavy broadcasting or huge routing tables are infeasible.
Three general prerequisites must be met to perform routing:
In the Internet environment, routing is almost always used at the IP level, and bridging almost always used at the Data Link Layer. For new network installations, the best advice is to plan for routing even if it's not used at first. This requires some advanced planning to design an addressing scheme that will work. However, the overhead is all human - hardware won't know the difference between organized and haphazard addressing schemes. Plan for the ability to put routers in strategic locations, even if those locations will initially use bridges or just signal boosters (such as Ethernet hubs and repeaters). In this manner, routers can be easily added later. Nothing is more frustrating that knowing exactly where a router should be added... and knowing that a hundred addresses must be changed before it can be.