Find all IP addresses in local network

Question

I want to find all IP addresses of devices in the local network I'm currently connected to using Java code. The useful utility Advanced IP Scanner is able to find various IP addresses in my subnet of 192.168.178/24:

According to this answer, I built my code the following way:

import java.io.IOException;
import java.net.InetAddress;

public class IPScanner
{
    public static void checkHosts(String subnet) throws IOException
    {
        int timeout = 100;
        for (int i = 1; i < 255; i++)
        {
            String host = subnet + "." + i;
            if (InetAddress.getByName(host).isReachable(timeout))
            {
                System.out.println(host + " is reachable");
            }
        }
    }

    public static void main(String[] arguments) throws IOException
    {
        checkHosts("192.168.178");
    }
}

Unfortunately, this does not print out any results, meaning that no IP addresses are reachable. Why? There are devices in my local network like seen in the Advanced IP Scanner scan.

Answer 1

Try to increase the timeout. I used about 5000ms, this helped me. In case you don't want to wait 5000ms * 254 = 21 minutes, try also this code with parallel pinging to the addresses:

public static void getNetworkIPs() {
    final byte[] ip;
    try {
        ip = InetAddress.getLocalHost().getAddress();
    } catch (Exception e) {
        return;     // exit method, otherwise "ip might not have been initialized"
    }

    for(int i=1;i<=254;i++) {
        final int j = i;  // i as non-final variable cannot be referenced from inner class
        new Thread(new Runnable() {   // new thread for parallel execution
            public void run() {
                try {
                    ip[3] = (byte)j;
                    InetAddress address = InetAddress.getByAddress(ip);
                    String output = address.toString().substring(1);
                    if (address.isReachable(5000)) {
                        System.out.println(output + " is on the network");
                    } else {
                        System.out.println("Not Reachable: "+output);
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }).start();     // dont forget to start the thread
    }
}

Worked perfectly for me.