How to get the LAN IP of a client using Java?

Question

How can i get the LAN IP-address of a computer using Java? I want the IP-address which is connected to the router and the rest of the network.

I've tried something like this:

Socket s = new Socket("www.google.com", 80);
String ip = s.getLocalAddress().getHostAddress();
s.close();

This seem to work on some cases, but sometimes it returns the loopback-address or something completely different. Also, it requires internet connection.

Does anyone got a more accurate method of doing this?

EDIT: Thought it would be better to ask here than in a comment..

What if you got many interfaces? For example, one for cable, one for wifi and one for virtual box or so. Is it impossible to actually see which one is connected to the network?

Answer 1

Try java.net.NetworkInterface

import java.net.NetworkInterface;

...

for (
    final Enumeration< NetworkInterface > interfaces =
        NetworkInterface.getNetworkInterfaces( );
    interfaces.hasMoreElements( );
)
{
    final NetworkInterface cur = interfaces.nextElement( );

    if ( cur.isLoopback( ) )
    {
        continue;
    }

    System.out.println( "interface " + cur.getName( ) );

    for ( final InterfaceAddress addr : cur.getInterfaceAddresses( ) )
    {
        final InetAddress inet_addr = addr.getAddress( );

        if ( !( inet_addr instanceof Inet4Address ) )
        {
            continue;
        }

        System.out.println(
            "  address: " + inet_addr.getHostAddress( ) +
            "/" + addr.getNetworkPrefixLength( )
        );

        System.out.println(
            "  broadcast address: " +
                addr.getBroadcast( ).getHostAddress( )
        );
    }
}

Answer 2

At first: There is no single address. Your machine has at least two adresses (127.0.0.1 on "lo" and maybe 192.168.1.1 on "eth1").

You want this: Listing network interfaces

As you may expect, you cannot automatically detect which one is connected to any of your routers, since this needs potentially complex parsing of your routing tables. But if you just want any non-local address this should be enought. To be sure, try to use this at least one time on vista or Windows 7, since they add IPv6 addresses.

import java.io.*;
import java.net.*;
import java.util.*;
import static java.lang.System.out;

public class ListNets 
{
    public static void main(String args[]) throws SocketException {
        Enumeration<NetworkInterface> nets = NetworkInterface.getNetworkInterfaces();
        for (NetworkInterface netint : Collections.list(nets))
            displayInterfaceInformation(netint);
    }

    static void displayInterfaceInformation(NetworkInterface netint) throws SocketException {
        out.printf("Display name: %s\n", netint.getDisplayName());
        out.printf("Name: %s\n", netint.getName());
        Enumeration<InetAddress> inetAddresses = netint.getInetAddresses();
        for (InetAddress inetAddress : Collections.list(inetAddresses)) {
            out.printf("InetAddress: %s\n", inetAddress);
        }
        out.printf("\n");
     }
}  

The following is sample output from the example program:

Display name: bge0
Name: bge0
InetAddress: /fe80:0:0:0:203:baff:fef2:e99d%2
InetAddress: /121.153.225.59

Display name: lo0
Name: lo0
InetAddress: /0:0:0:0:0:0:0:1%1
InetAddress: /127.0.0.1

Answer 3

This is a method I've used for a while. It includes a little hack to figure out the externally visible ip-address as well.

private List<String> getLocalHostAddresses() {

    List<String> addresses = new ArrayList<String>();

    try {
        Enumeration<NetworkInterface> e = NetworkInterface.getNetworkInterfaces();

        while (e.hasMoreElements()) {
            NetworkInterface ni = e.nextElement();
            Enumeration<InetAddress> e2 = ni.getInetAddresses();
            while (e2.hasMoreElements())
                addresses.add(e2.nextElement().getHostAddress());
        }
        URL u = new URL("http://whatismyip.org");
        BufferedReader in = new BufferedReader(new InputStreamReader(
                u.openStream()));
        addresses.add(in.readLine());
        in.close();
    } catch (Exception ignore) {
    }

    return addresses;
}

Answer 4

try {
    InetAddress addr = InetAddress.getLocalHost();

    // Get IP Address
    byte[] ipAddr = addr.getAddress();

    // Get hostname
    String hostname = addr.getHostName();
} catch (UnknownHostException e) {
}

Answer 5

As Daniel already pointed out, you cannot know which interface is the one "connected". What if, for example, the computer has multiple network interface cards which are both connected to separate physical LANs?

Let the user decide which interface to use or try them all, depending on what your use case is.