Question

I am checking whether the ipAddress is in Private Category or not. So I wrote this method below. And I am getting this as an exception-

java.net.UnknownHostException: addr is of illegal length
    at java.net.InetAddress.getByAddress(InetAddress.java:948)
    at java.net.InetAddress.getByAddress(InetAddress.java:1324)

ipAddress (172.18.36.81) is String

if(isPrivateIPAddress(ipAddress)) {

            return null;
        }


private static boolean isPrivateIPAddress(String ipAddress) {

    byte[] byteArray = null;
    InetAddress ia = null;
    try {
        byteArray = ipAddress.getBytes("UTF-16LE");
    } catch (UnsupportedEncodingException e1) {
        // TODO Auto-generated catch block
        e1.printStackTrace();
    }


    try {
        ia = InetAddress.getByAddress(byteArray);
    } catch (UnknownHostException e) {

        e.printStackTrace();
    }

    return ia.isSiteLocalAddress();
}

Answer 1

I think you’ve misunderstood how to convert an IP address from String to byte[]. The proper way to do that is to parse String to a sequence of ints, and then cast each of those to a byte. But fortunately, InetAddress already has a method to handle that for you, so you can just write:

private static boolean isPrivateIPAddress(String ipAddress)
{
    return InetAddress.getByName(ipAddress).isSiteLocalAddress();
}

(together with whatever validity-checking and error-handling you want).

Note that the above will also handle hostnames, by using DNS lookup. If you don’t want that, you’ll need to pre-check the IP-address, using something like this:

if(! Pattern.matches("(\\d{1,3}\\.){3}\\d{1,3}", ipAddress)
    throw new IllegalArgumentException();

if you’re O.K. with only supporting IPv4.