[ephemera]

checkout arpwatch: ftp://ftp.ee.lbl.gov/arpwatch.tar.gz

or maybe this might do:

#!/bin/sh
# -ephemera

net=10.30.1             # 10.30.1/24

[ $# -lt 1 -o `id -u` -ne 0 ] && exit 1
x=1
while [ $x -le 254 ] ; do
	arp -d $net.$x >/dev/null 2>&1 
        printf "\rScanning $net.$x "
        ping -c3 $net.$x 1>/dev/null &
        x=$(($x + 1)) 
done
while [ `pgrep -P $$ ping | wc -l` -ne 0 ] ; do
        printf .
        sleep 1
done
printf " Done.\n"   
arp -na | grep $1 | sed 's/.*(\(.*\)) at .*/\1  /'

# ./script 0a:0b:0c:0d:0e:ff