Find all IP addresses on the LAN

Question

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

According to this answer, I built my code as follows:

 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 any results, which means that IP addresses are not available. What for? There are devices on my local network, for example, in Advanced IP Scanner scanning.

Answer 1

InetAddress.isReachable will use ICMP ECHO REQUEST (as with ping) or a request for port 7 (echo port): http://docs.oracle.com/javase/7/docs/api/java/net/InetAddress.html#isReachable % 28int% 29

The pre-IP scanner may use a different method of locating hosts (for example, a request for the radmin port or a request for http).

The host may be turned on but not responding to an ICMP ECHO request.

Are you trying to ping one of the host from the command line?

Answer 2

Try increasing the timeout. I used about 5000 ms, it helped me. If you do not want to wait 5000 ms * 254 = 21 minutes, try also this code with parallel ping to 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 great for me.

Answer 3

Perhaps try using InetAddress.getByAddress(host) instead of getByName , for example:

  InetAddress localhost = InetAddress.getLocalHost(); byte[] ip = localhost.getAddress(); for (int i = 1; i <= 254; i++) { try { ip[3] = (byte)i; InetAddress address = InetAddress.getByAddress(ip); if (address.isReachable(100)) { output = address.toString().substring(1); System.out.print(output + " is on the network"); } } 

I took this sample for autodetection code from here