JS判断IP地址格式是否正确

function checkip(ip)
{
    var pcount = 0
    var ip_length = ip.length 
    var ip_letters = "1234567890." 
    for (p=0; p < ip_length; p++)
    {
var ip_char = ip.charAt(p)
if (ip_letters.indexOf(ip_char) == -1)
{
     return false
}
    }
    for (var u = 0; u < ip_length; u++) { (ip.substr(u,1) == ".") ? pcount++ : pcount }
    if(pcount != 3) { return false }
    firstp = ip.indexOf(".")
    lastp = ip.lastIndexOf(".")
    str1 = ip.substring(0,firstp)
    ipstr_tmp = ip.substring(0,lastp)
    secondp = ipstr_tmp.lastIndexOf(".")
    str2 = ipstr_tmp.substring(firstp+1,secondp)
    str3 = ipstr_tmp.substring(secondp+1,lastp)
    str4 = ip.substring(lastp+1,ip_length)
    if (str1 == '' || str2 == '' || str3 == '' || str4 == '') { return false }
    if (str1.length > 3 || str2.length > 3 || str3.length > 3 || str4.length > 3) { return false }
    if (str1 <= 0 || str1 > 255) { return false }
    else if (str2 < 0 || str2 > 255) { return false }
    else if (str3 < 0 || str3 > 255) { return false }
    else if (str4 < 0 || str4 > 255) { return false }
    return true
}

　　只需传入相应IP值即可。