ARP control and information
ARP is the protocol used to map IP to hardware addresses. This information is kept in a cache for some time (20 m. in the original implementation).
The arp command can help you to know and control the ARP cache (This command is from a FreeBSD 7.1 box and it could differ in options or flags from your own version):
$ arp -a
www.brntech.com.tw (192.168.2.1) at 00:13:f7:96:53:02 on re0 [ethernet]
Shows all the entries in the arp cache. In my case, I know of my router IP and hardware address.
$ arp -i re0 -a
The same as the previous one but limiting the scope to ...