js验证ip是否合法

function checkIP_zzjs()
{
obj=document.getElementById("ip").value
var exp=/^(\d{1,2}|1\d\d|2[0-4]\d|25[0-5])\.(\d{1,2}|1\d\d|2[0-4]\d|25[0-5])\.(\d{1,2}|1\d\d|2[0-4]\d|25[0-5])\.(\d{1,2}|1\d\d|2[0-4]\d|25[0-5])$/;
var reg = obj.match(exp);
if(reg==null)
{
alert("IP地址不合法！");
}
else
{
alert("IP地址合法！");
}
}
</script>