#!/bin/bash

for round in `seq 0 50`
do
  echo "`date`: Starting round $round"
  for offset in `seq 1 5`
  do
    let octet=offset+5*round

    [ "$octet" != "255" ] && \
       nmap -P0 -O 192.168.196.$octet > ${octet}.nmap 2>&1 &
  done
  echo "`date`: Waiting for round $round"
  wait
done