Router ARP Tables
Routers, like other network devices, maintain ARP tables to assist in transmitting packets from one network to another.
If the destination IP address of a packet is on a segment to which the router is attached, the router will forward the packet directly to the destination. To do this, however, the router must obtain the MAC address for that destination.
If the router has the MAC to IP mapping in its ARP table, the router can forward the packet directly.
If the router does not have that mapping in its ARP table, the router must issue an ARP request.