Problem with isReachable in InetAddress class

Question

As an assignment I have to find all the alive computers on a LAN. For which I am using isReachable function of InetAddress class. But problem is

Answer 1

Here is the code which is platform independent, but requires information about any open port on the other machine (which we have most of the time).

private static boolean isReachable(String addr, int openPort, int timeOutMillis) {
    // Any Open port on other machine
    // openPort =  22 - ssh, 80 or 443 - webserver, 25 - mailserver etc.
    try {
        try (Socket soc = new Socket()) {
            soc.connect(new InetSocketAddress(addr, openPort), timeOutMillis);
        }
        return true;
    } catch (IOException ex) {
        return false;
    }
}