Test with Java if two IPs are in the same network

Question

How can I test if two IPs are in the same network according to the subnet mask?

For example I have the IPs 1.2.3.4 and 1.2.4.3: Both are in the same network if the mask is 255.0.0.0 or 255.255.0.0 or even 255.255.248.0 but not if the mask is 255.255.255.0..

Answer 1

Try this method:

public static boolean sameNetwork(String ip1, String ip2, String mask) 
throws Exception {

    byte[] a1 = InetAddress.getByName(ip1).getAddress();
    byte[] a2 = InetAddress.getByName(ip2).getAddress();
    byte[] m = InetAddress.getByName(mask).getAddress();

    for (int i = 0; i < a1.length; i++)
        if ((a1[i] & m[i]) != (a2[i] & m[i]))
            return false;

    return true;

}

And use it like this:

sameNetwork("1.2.3.4", "1.2.4.3", "255.255.255.0")
> false

EDIT :

If you already have the IPs as InetAddress objects:

public static boolean sameNetwork(InetAddress ip1, InetAddress ip2, String mask) 
throws Exception {

    byte[] a1 = ip1.getAddress();
    byte[] a2 = ip2.getAddress();
    byte[] m = InetAddress.getByName(mask).getAddress();

    for (int i = 0; i < a1.length; i++)
        if ((a1[i] & m[i]) != (a2[i] & m[i]))
            return false;

    return true;

}

Answer 2

Simple enough: mask & ip1 == mask & ip2 - you have to interpret the IPs all as a single number for that but that should be obvious.