Route tables:

Linux-2.2 can pack routes into several routing tables identified by a number in the range from 1 to 255 or by name from the file /etc/iproute2/rt_tables. By default all normal routes are inserted into the main table (ID 254) and the kernel only uses this table when calculating routes.

Actually, one other table always exists, which is invisible but even more important. It is the local table (ID 255). This table consists of routes for local and broadcast addresses. The kernel maintains this table automatically and the administrator usually need not modify it or even look at it.

The multiple routing tables enter the game when policy routing is used. See sec.8, p.. In this case, the table identifier effectively becomes one more parameter, which should be added to the triplet {prefix, tos, preference} to uniquely identify the route.