function checkIP2(sIPAddress)
{
    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 = sIPAddress.match(exp);
    var ErrMsg="你输入的是一个非法的IP地址段！\nIP段为：:xxx.xxx.xxx.xxx（xxx为0-255)！"    
    var Msg="你输入的是一个合法的IP地址段！"    
    if(reg==null)
    {
        alert(ErrMsg);
    }
    else
    {
        alert(reg);
    }
}

（出处：devclub）