For a network router to know where to send packets of data it receives, it uses a routing table. The routing table contains a list of specific routing destinations, and when the router receives a packet of data, it references the routing table to know where to send that data. The routing table may also contain information on how far each destination is from the router. In essence, a routing table is a map for the router.
A routing table does not contain a list of all possible destinations. Rather, it contains a list of destinations that are next in line to the router. Each router contains this list and when it receive packets of data it directs that packet to the next link or hop in the network until it reaches its final destination. Typically the routing table contains a list of IP addresses, Gateway addresses, and other information. Below, is an example of a very basic routing table.