If I am a station on a local network and want to communicate with another station on the same network through an IP datagram, I address a 4-byte IP address to the station in the IP datagram. In performing the transmission, I know the source IP address (my address) and the destination IP address. I can therefore complete an IP datagram. But the problem is that this IP datagram must be wrapped in a link frame, for example, in an Ethernet frame. In order to create an Ethernet frame, I need the link (6 B) addresses of both the source and the destination. I am the source and I know my link address, but I do not know the destination link address. How do I find out what this address is? The answer is by using the Address Resolution Protocol (ARP).
ARP lets you to get the link address of an opposite station when you know its IP address. The solution is simple and is shown in the following figure:
Figure 5.25: ARP
ARP sends a link broadcast to the LAN (link address FF:FF...