Routing Table

The routing table in the kernel of each host contains routes. Each entry in the routing table provides a route to a particular destination:

The routing software tries to find a route to the destination host in the route tables. When the host is not explicitly named, the routing software looks for an entry for the (sub)network where the host resides. When neither the host nor the network where the host resides is defined, the host sends the packet to a default gateway, if one has been defined. Multiple default gateways can be defined, and each is treated equally. A pointer keeps track of which default gateway has been used most recently, and the next one in the list is used for the next routing.

Routing table entries are created either of the following two ways:

• Dynamically - The routed(1M) routing daemon dynamically creates the route entries including the emetric.

• Statically - The administrator role creates static routes manually in one of two routing files. The administrator may or may not supply an emetric with the route entry.

With a small network, it is feasible to set up routes manually, and to manually make changes to the routing table when conditions change. For example, many sites have a single gateway through which all communications go to the outside world. In these cases, the single gateway can be statically defined as the default on each host on the network. Manually configuring and maintaining static routes is less feasible with large networks.