javascript非正则的ip地址验证

var ip = document.all.ip.value;          

var Letters = "1234567890."; 
            for (i=0; i < ip.length; i++)...{ 
                 var CheckChar = ip.charAt(i);
                 if(Letters.indexOf(CheckChar) == -1)...{ 
                       alert("IP地址格式不对!!，只能输入数字和“.”,格式为XXX.XXX.XXX.XXX 例如：192.168.0.1"); 
                       return false; 
                 } 
               }
               for (i = 0;i<ip.length;i++)...{ 
                (ip.charAt(i)=='.')?scount++:scount;
            } 
             if(scount!=3)...{ 
                alert ("IP地址格式不对!，只能输入数字和“.”,格式为XXX.XXX.XXX.XXX 例如：192.168.0.1"); 
                return false; 
             }
             if(scount==3)...{
                var    first = ip.indexOf("."); 
                 var    last = ip.lastIndexOf(".");
                 var    str1 = ip.substring(0,first); 
                 var    subip = ip.substring(0,last); 
                 var    sublength = subip.length; 
                 var    second = subip.lastIndexOf("."); 
                 var    str2 = subip.substring(first+1,second); 
                 var    str3 = subip.substring(second+1,sublength); 
                 var    str4 = ip.substring(last+1,ip.length); 
                  if (str1=="" || str2=="" ||str3== "" ||str4 == "")...{
                       alert("IP地址格式不对！格式为XXX.XXX.XXX.XXX 例如：192.168.0.1"); 
                     return false; 
                 }
                 if(str1<1 || str1>255)...{
                      alert("数字范围为1~255！");  
                      return false; 
                 } 
                 else if(str2< 0 || str2 >255)...{
                      alert("数字范围为0~255！"); 
                      return false; 
                 } 
                 else if(str3< 0 || str3 >255)...{
                      alert("数字范围为0~255！"); 
                      return false; 
                 } 
                 else if(str4< 0 || str4 >255)...{
                      alert("数字范围为0~255！"); 
                      return false; 
                 }
             }