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.

I checked that code on my network, using broadcast, just mine IP or trying "www.google.com". Always says "not reachable". Check this answer: http://stackoverflow.com/questions/9922543/why-does-inetaddress-isreachable-return-false-when-i-can-ping-the-ip-address — Adam Michalik, Sep 10 '15 at 11:33
Possible duplicate of [How to get a list of IP connected in same network (subnet) using Java](http://stackoverflow.com/questions/3345857/how-to-get-a-list-of-ip-connected-in-same-network-subnet-using-java) — jww, Jan 28 '17 at 14:03

score 7 · Answer 1 · answered Jun 21 '17 at 09:17

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.

In my network setup, the 0 is used as the default gateway and the 255 for broadcasting, therefore no device will be connected to neither of these addresses. — The_Programmer, Jul 01 '20 at 20:51

score 3 · Answer 2 · answered Sep 10 '15 at 11:39

3

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

Advance IP scanner perhaps use an other way to discover the hosts (like a request on radmin port or a request on http).

An host can be up but not answering to ICMP ECHO REQUEST.

have you try to ping one of the host from command line?

answered Sep 10 '15 at 11:39

seneque

279
1
5

The host can be ping'ed and replies – BullyWiiPlaza Sep 10 '15 at 17:22

vgm · Answer 3 · 2015-09-10T13:07:27.743

1

Maybe try using InetAddress.getByAddress(host) instead of getByName, like this:

    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

edited Sep 10 '15 at 13:07

answered Sep 10 '15 at 13:00

vgm

145
1
2
10

Is it you, VGMoose? Cheers man ;) However, your code and application fails to find my Nintendo Wii console so your network scan is not working, sorry. File transfer works but I need to manually enter my Wii's IP address – BullyWiiPlaza Sep 17 '15 at 16:42

score 0 · Answer 4 · answered Oct 06 '18 at 21:54

0

Java 8 stream solution

    IntStream.rangeClosed(1,254).mapToObj(num -> "192.168.0."+num).parallel()
            .filter((addr) -> {
                try {
                    return InetAddress.getByName(addr).isReachable(2000);
                } catch (IOException e) {
                    return false;
                }
            }
            ).forEach(System.out::println);

answered Oct 06 '18 at 21:54

Ivo Skalický

310
1
10

That is specific to `/24` networks, but there are other network sizes, too. – Ron Maupin Oct 06 '18 at 21:59

Find all IP addresses in local network

4 Answers4

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