broadcast and prefix length with IPv6

[Mark Thornton]

On Vista, if you try to find the broadcast address and net prefix length
associated with an IPv4 address (InterfaceAddress.getBroadcast(),
InterfaceAddress.getNetworkPrefixLength()), you get nonsense unless IPv6
is disabled. Bug 6707289 describes the prefix length case.

Workaround: -Djava.net.preferIPv4Stack=true

Without that option, the result on my machine is
  /192.168.0.7, broadcast=/255.255.255.255, prefixLength=128

With -Djava.net.preferIPv4Stack=true
  /192.168.0.7, broadcast=/192.168.0.255, prefixLength=24

Regards,
Mark Thornton


public class TestNetworkInterfaces
{
    /**
     * Must use -Djava.net.preferIPv4Stack=true to get expected results
for broadcast address and
     * prefix length
     * @throws SocketException
     */
    @Test public void enumerateInterfaces() throws SocketException
    {
        for (Enumeration<NetworkInterface> interfaces =
NetworkInterface.getNetworkInterfaces(); interfaces.hasMoreElements();)
        {
            NetworkInterface ni = interfaces.nextElement();
            if (ni.isUp() && !ni.isVirtual())
                reportInterface(ni, "");
        }
    }

    private void reportInterface(NetworkInterface ni, String indent)
throws SocketException
    {
        if (ni.getInterfaceAddresses().isEmpty() &&
!ni.getSubInterfaces().hasMoreElements())
            return;    // has no addresses or child interfaces
        System.out.print(indent);
        System.out.print("\"");
        System.out.print(ni.getDisplayName());
        System.out.print("\"");
        if (ni.isLoopback())
        {
            System.out.print(" [loopback]");
        }
        if (ni.isPointToPoint())
        {
            System.out.print(" [ptp]");
        }
        byte[] mac = ni.getHardwareAddress();
        if (mac != null && mac.length > 0)
        {
            System.out.print(", hardware=");
            for (byte b: mac)
            {
                System.out.print(':');
                System.out.print(Integer.toHexString(b&255));
            }
        }
        System.out.println();
        indent = indent+"  ";
        for (InterfaceAddress addr: ni.getInterfaceAddresses())
        {
            System.out.print(indent);
            System.out.print(addr.getAddress());
            if (addr.getAddress().isLoopbackAddress())
                System.out.print(" [loopback]");
            System.out.print(", broadcast=");
            System.out.print(addr.getBroadcast());
            System.out.print(", prefixLength=");
            System.out.print(addr.getNetworkPrefixLength());
            //System.out.print(", searchedBroadcast=");

//System.out.print(BroadcastAddress.instance().getBroadcastAddress(addr.getAddress()));
            try
            {
                String name = addr.getAddress().getCanonicalHostName();
                if (name != null)
                {
                    System.out.print(", host=");
                    System.out.print(name);
                }
            }
            catch (Exception e)
            {

            }
            System.out.println();
        }
        for (Enumeration<NetworkInterface>
interfaces=ni.getSubInterfaces(); interfaces.hasMoreElements();)
        {
            NetworkInterface child = interfaces.nextElement();
            if (child.isUp())
            {
                reportInterface(child, indent);
            }
        }
    }
}