How to find the IP address of a specific device from a multicast LAN address?

Question

I am trying to program an android app dashboard(latest Java android 5.1 lollipop) that would be able to control my govee lamp which has a LAN control api which connects to a multicast address on my LAN(239.255.255.250) but I need a way to find the IP of the specific lamp so that I can send it commands after user interaction on my app.

Pinged the multicast IP and got no response(operation timed out). Looked around google for a way but couldn't find anything so wondering if someone here could help me out. TY for your time :)

Answer 1

Assuming you know the mac address you can send a reverse ARP packet (Reverse Address Resolution Protocol) to obtain the IP address. ARP itself is normally used to map an IP address to the mac address so the local router can properly forward the packet. RARP does the opposite.

Another way is to interrogate the local ARP cache from within the program on any attached device and see if that has it. So on a windows OS within the program you can initiate an arp -a in the console window to see the current arp cache. The only caveat is that the particular value may have time out and been removed.

Here is an example of the latter.

String mac = "1c-7e-81-9e-48-e8";
try {
    Runtime rt = Runtime.getRuntime();
    // send broadcast address
    rt.exec(new String[]{
            "ping","-n 1 -w 200 192.255.255.255"
    });
    // interrogate arp cache
    Process p = rt.exec(new String[]{
            "cmd.exe","/c","arp","-a"
    });

    BufferedReader in = new BufferedReader(
            new java.io.InputStreamReader(p.getInputStream()));
    String line1;
    while ((line1 = in.readLine()) != null) {
        if (line1.contains(mac)) {
            System.out.println(line1);
        }

    }

} catch (Exception e) {
    e.printStackTrace();
}

prints something like

192.168.1.107         1c-7e-81-9e-48-e8     dynamic   

The above may need to be adjusted to fulfill your exact requirements.

Of course the easiest way might be to force the issue and assign a static address of your choosing.