device on another network. Can you think of a reason why this is so?What happens in the case of subnetworks? Can a device on one subnetwork find the MAC address of a device on another subnetwork? The answer is yes, provided the source directs its question to the router. Working through a third party is called proxy ARP, and it allows the router to act as a default gateway.Sometimes a source resides on a network that has a different network number than the desired destination. If the source doesn't know the MAC address of the destination it must use the services of a router. With the router's aid, the source's data can reach its destination. A router that is used for this purpose is called a default gateway. To obtain the services of a default gateway, a source encapsulates the data so that it contains the destination MAC address of the router. A source uses the destination IP address of the host device, and not that of a router, in the IP header, because it wants the data delivered to the host device and not to a router.When a router picks up data, it strips off the data link layer information that is used in the encapsulation. It then passes the data up to the network layer where the router examines the destination IP address. It compares the destination IP address with information contained in its routing tables. If the router locates the mapped destination IP address and the MAC address, and learns that the location of the destination network is attached to one of its ports, it encapsulates the data with the new MAC address information, and forwards it to the correct destination. If the router cannot locate the mapped destination address and MAC address of the device of the final target device, it locates the MAC address of another router that can perform this function, and forwards the data to that router. This type of routing is referred to as indirect routing. 11.8Interior Gateway Protocol (IGP) and Exterior Gateway Protocol (E...
