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 ( 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 the given interface (in case you had a multihomed system)

% arp -d -a

Deletes all the entries in the cache. Probably good if you want to refresh some data in it or for testing purposes.

Notice that you need to be superuser to execute the latter. The same constraint is applied if you want to add a new entry in the cache (either with -s or -S)

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