linux snat gateway

[root@changxun ~]# cat nat.sh 
#!/bin/bash
# config
network=192.168.203.0/24
local_ip=192.168.203.5

function start(){
iptables -A POSTROUTING -t nat -s $network -j SNAT --to-source ${local_ip}
}

function show(){
iptables -t nat -L
}

function stop(){
iptables -D POSTROUTING -t nat -s $network -j SNAT --to-source ${local_ip}
}
echo arg:$0  $1
case $1 in
show)
  show
;;
start)
  start
;;
stop)
  stop
;;
*)
  echo usage: $0 'show|start|stop'
;;
esac