Every device on the network maintains a routing table in memory. This table may be very simplistic, as would be the case with a low-end PC workstation, or very complex, as would be the case with a high-end router.
The table consists of pairs of IP addresses. "Where do you want to send to?" coupled with "Where do you really send to get there?". For example, a routing table may indicate that all frames destined for network 188.8.131.52 are to be sent to IP address 184.108.40.206. The immediate destination is always directly reachable; in this case we must be on network 220.127.116.11 and we trying to reach an ultimate destination on 18.104.22.168. The routing table provides a lookup for a device to tell it how to send frames.
There are two interesting exception cases. In the one case, the originating station concludes that the ultimate destination is, itself, directly reachable. In that case the frame will be forwarded directly to the destination IP address in the frame. It is not necessary to forward it to a router; the sender realizes that it is on the same subnetwork as the destination. In the other case the routing table doesn't have any matching lookup value when the masked destination address is compared against the available entries in the table. In this case a special address appears in the routing table called the Default Gateway Address. The pair of values in the table has "0.0.0.0" paired with an assigned DLC address of a router. This is the router to which frames will be sent when there is no other reference to the frames' destination in the table.
When making a routing decision the following steps are taken:
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