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Routing

Routing is a method of path selection (contrast bridging).

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:

Design. A plan must exist by which addresses are assigned. Typically, addresses are broken into fields corresponding to levels in a physical hierarchy. At each level of the hierarchy, only the corresponding field in the address is used, permitting addresses to be handled in blocks. In IP, the most common designs are IP Address Classes, Subnetting, and CIDR.
Implementation. The design plan must be implemented in switching nodes, which must be able to extract path information from the addresses. Since router programming is generally not under a designer's control, designs must be limited by the features provided by manufacturers. Subnetting's great appeal lies in its great flexibility, while using a fairly simple implementation model.
Enforcement. The plan must be enforced in host addressing. A design is useless unless addresses are assigned in accordance with it. Addressing authority must be centralized, possible with subsets of the available addressing space delegated to subordinates.

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.