The ARP protocol links OSI layers 2 and 3 by mapping IP addresses to MAC addresses. ARP packet message headers contain two operation codes: REQUEST (1) and REPLY (2).

Basically, an ARP request will broadcast “to” a particular IP address but using the “broadcast” MAC address (00:00:00:00:00:00), and the computer with that IP address will then reply in a non-broadcast fashion (since this requires that both the MAC address and IP address be filled in, such a direct reply provides the desired information by way of its very existence).

Because ARP requests are about binding a MAC address to an IP address, they are necessarily limited to the current subnet.