You need to know what is on your network, and you need to keep your information current. How? Well, why not ping everything in range and then do arp and dns lookups for the active ones? Here is the script I used:

sysresccd IPMgt # cat ipfinder.command
function main
{
quittingtime=`date +%s -d+5days+14hours`
while [ `date +%s` -lt $quittingtime ]
do
if [ -f NonLiveIPs.current.txt ]
then
mv NonLiveIPs.current.txt NonLiveIPs.old.txt
fi
touch NonLiveIPs.current.txt
touch LiveIPs.txt
firstpart='192.168.0.'
lastpart=1
while [ $lastpart -lt 255 ]
do
(
# # %03s - three characters, padded with zeros
strlastpart=$(echo $lastpart|(awk '{printf "%03s", $1}'))
thiscommand="ping -c3 -q ${firstpart}${lastpart} 2>/dev/null|grep '100% packet loss'"
cmdresult=$(eval "$thiscommand")
echo "DEBUG: thiscommand: $thiscommand"
echo "DEBUG: cmdresult: $cmdresult"|cut -b1-80
if [ -n "$cmdresult" ]
then
echo "DEBUG: noresponse processing"
thiscommand="grep ${firstpart}{strlastpart} LiveIPs.txt"
cmdresult=$(eval "$thiscommand")
echo "DEBUG: cmdresult: $cmdresult"|cut -b1-80
if [ -z "$cmdresult" ]
then
echo "DEBUG: noresponseX2 processing"
echo "No Response: ${firstpart}${lastpart}" | tee -a NonLiveIPs.current.txt
else
echo "No Response: ${firstpart}${lastpart} - Temporarily down?"
fi
else
echo "DEBUG: responded, processing"
arpresult="$(arping2 -c1 ${firstpart}${lastpart}|grep 'index')"
echo "DEBUG: arpresult: $arpresult"
macaddr="$(echo $arpresult|awk '{print $4}')"
echo "DEBUG: macaddr: $macaddr"
if [ -z "$macaddr" ];then macaddr="00:00:00:00:00:00";fi
datestamp=$(date +%s.%d%b%Y_%H.%M.%S)
echo "DEBUG: datestamp: $datestamp"
dnsname="$(nslookup ${firstpart}${lastpart}|grep 'name'|awk -F= '{print $2}')"
echo "DEBUG: dnsname: $dnsname"
if [ -z "$dnsname" ];then dnsname=" unknown.dtfcu.com";fi
echo "${firstpart}${strlastpart} $macaddr $datestamp $dnsname" |tee -a LiveIPs.tmp
fi
) &
let lastpart++
if [ $(expr $lastpart % 5) -eq 0 ];then sleep 1;fi
done
echo "DEBUG: pausing for a minute to ensure all processing finished"
sleep 60 #Make sure the delay stays here so everything finishes first
cleanlist
echo "Pausing for five minutes"
sleep 300
date
done
}

function cleanlist
{
sort LiveIPs.tmp|uniq >LiveIPs.txt #Must follow delay
rm -f LiveIPs.tmp;touch LiveIPs.tmp
for i in `awk '{print $1"_"$2}'<LiveIPs.txt|sort|uniq`
do
echo "i: $i"
searchstr=$(echo "$i"|sed 's/_/ /g')
LastUniqContact=$(grep -F "$searchstr" LiveIPs.txt|tail -n1)
echo "$LastUniqContact"|tee -a LiveIPs.tmp
done
mv -f LiveIPs.tmp LiveIPs.txt
}
cleanlist
echo "List cleaned";sleep 1;date;
main