Not bullet-proof but should work:
grep -oE '192\.168\.77\.[0-9]+' dhcpd.conf
Not bullet-proof as it will find also 192.168.77.999
which is not an IP.
Update:
If you are just searching for those which are free (you told that in comments) then use:
for ip in 192.168.77.{254..1} ; do
grep -oE "$ip" dhcpd.conf > /dev/null 2>&1
if [ $? != 0 ] ; then
echo "$ip is free"
fi
done