1.8 ARP

Unlike IPX, IP addresses are not the same as the hardware address of the network board, so there must be a way to discover the physical, or media access control (MAC) address. The Address Resolution Protocol (ARP) performs this task. When an IP address is mapped to a MAC address, ARP is used on broadcast networks such as Ethernet, token ring, and ARCnet. When a node uses IP to send a packet, it must determine which physical address on the network corresponds to the destination IP address. To find the physical address, the node broadcasts an ARP packet containing the destination IP address. The node with the specified destination IP address sends its physical address back to the requesting node.To speed packet transmissions and reduce the number of broadcast requests that must be examined by every node on the network, each node keeps an address resolution cache, or ARP table. Each time the node broadcasts an ARP request and receives a response, it creates an entry in its address resolution cache. The entry maps the IP address to the physical address. When the node sends an IP packet, it looks up the IP address in its cache and uses the physical address, if found. The node broadcasts an ARP request only if the IP address is not in its cache.