Question

I want to broadcast from an android device which acts as a hotspot, but using 255.255.255.255 does not work as UDP broadcast in android. But 192.168.1.255 this broadcast address works. When I check the IP address of my device its shows 192.168.1.4.

Now how to calculate address 192.168.1.255 from this address?

Answer 1

Well, if you have the broadcasting worked out and only need to change the last byte of the address to 255, you can do the following:

String getBroadcastAddress( final String address ) throws UnknownHostException {
    InetAddress addr = InetAddress.getByName( address );
    if( InetAddress instanceof Inet4Address ) {
        byte[] bytes = addr.getAddress();
        bytes[3] = 255;
        return InetAddress.getByAddress( bytes ).getHostAddress();
     } else {
         ... deal with ipv6
     }
}

Pulling the broadcast address directly from the NetworkInterface, results in this code:

public static String getBroadcast() throws SocketException {
    System.setProperty("java.net.preferIPv4Stack", "true");
    for( Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces(); interfaces.hasMoreElements(); ) {
        NetworkInterface nif = interfaces.nextElement();
        if( !nif.isLoopback() ) {
            for( InterfaceAddress addr : nif.getInterfaceAddresses() ) {
                return addr.getBroadcast().toString().substring(1);
            }
        }
    }
    return null;
}

You may have to sift through the NetworkInterface to find the one corresponding to the internal ip.