ARP protocol
A knows B's IP address, wants to learn physical address of B
A broadcasts ARP query pkt, containing B's IP address
- all machines on LAN receive ARP query
B receives ARP packet, replies to A with its (B's) physical layer address
A caches (saves) IP-to-physical address pairs until information becomes old (times out)
- soft state: information that times out (goes away) unless refreshed