Table Maintained by Router

Table Maintained by Router

Routers move information between local and remote networks. To do this, routers must use both ARP and routing tables to store information. Routing tables are not concerned with the addresses of individual hosts. Routing tables contain the addresses of networks and the best path to reach those networks. Entries can be made to the routing table in two ways: dynamically updated by information received from other routers in the network, or manually entered by a network administrator. Routers use the routing tables to determine which interface to use to forward a message to its intended destination.

If the router cannot determine where to forward a message, it will drop it. Network administrators configure a routing table with a default route to keep a packet from being dropped because the path to the destination network is not in the routing table. A default route is the interface through which the router forwards a packet containing an unknown destination IP network address. This default route usually connects to another router that can forward the packet towards its final destination network.

A router forwards a frame to one of two places: a directly connected network containing the actual destination host, or to another router on the path to reach the destination host. When a router encapsulates the frame to forward it out of an Ethernet interface, it must include a destination MAC address.

This is the MAC address of the actual destination host, if the destination host is part of a network locally connected to the router. If the router must forward the packet to another router, it will use the MAC address of the connected router. Routers obtain these MAC addresses from ARP tables.

Each router interface is part of the local network to which it is attached and maintains its own ARP table for that network. The ARP tables contain the MAC addresses and IP addresses of all of the individual hosts on that network.