Anytime a host or a router needs to find the link-layer address of another host or router
in its network, it sends an ARP request packet. The packet includes the link-layer and IF
addresses of the sender and the IP address of the receiver. Because the sender does not
know the link-layer address of the receiver, the query is broadcast over the link using the
link-layer broadcast address, which we discuss for each protocol later (see Figure 9.7).