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arp [ -n ] -a
arp [ -n ] -d hostname
arp [ -n ] -f filename
arp -i interface_name target_ipaddr
arp [ -n ] -s hostname hardware_addr [ temp] [ pub ]
arp -S
When hostname is the only argument, arp displays the current ARP entry for hostname. The host may be specified by name or by number, using Internet dot notation (see hosts(4tcp) and inet(7tcp)).
Options are interpreted as follows:
with argument meanings as defined under the -s option.
On interfaces with aliased IP addresses (see ifconfig(1Mtcp)), a separate ARP request is broadcast for each alias address with the sender's IP address set to the alias address.
As generally used, the sender expects the host with the target IP address to respond with an ARP reply containing its hardware address.
If the sender and target IP addresses are identical, a gratuitous ARP request is broadcast. This allows other hosts on the network to update their cache entries for the sender's IP address with the hardware address contained in the request. No reply is expected. If an interface has been configured with several alias IP addresses, a separate gratuitous ARP request must be issued for each IP address which needs to notify its corresponding hardware address to the network.
RFC 826