Physical Address Resolution
Based on the destination IP address and the route determination process, IP determines the forwarding IP address and interface to be used to forward the packet. IP then hands the IP packet, the forwarding IP address, and the interface, to ARP.
If the forwarding IP address is the same as the destination IP address, then ARP performs a direct delivery. In a direct delivery, the MAC address corresponding to the destination IP address must be resolved.
If the forwarding IP address is not the same as the destination IP address, then ARP performs an indirect delivery. The forwarding IP address is the IP address of a router between the current IP node and the final destination. In an indirect delivery, the MAC address corresponding to the IP address of the router must be resolved.
To resolve a forwarding IP address to its MAC address, ARP uses the broadcasting facility on shared access networking technologies (such as Ethernet or Token Ring) to send out a broadcasted ARP Request frame. An ARP Reply, containing the MAC address corresponding to the requested forwarding IP address, is sent back to the sender of the ARP Request.