gistfile1.txt

#!/bin/bash
NETWORK=$1
if [ -z ${NETWORK} ]; then 
    echo "Usage: $0 10.20.30 (for 10.20.30.x)"
    exit 1;
fi

for h in `seq 1 254`; do 
    NAME='';
    ping -i c3 -w3 ${NETWORK}.${h} >/dev/null 2>&1 || \
      NAME=$(dig +short -x ${NETWORK}.${h}) ; 
    if [ -z "${NAME}" ]; then 
        echo ${NETWORK}.${h};
    fi; 
done