Get all the Up ip in the local network -java

Question

For the Java project i need to scan the list of ip connected to the same local network via wlan or eth0 or anything. I need to get the list of ip address that are up in the local network.

I tried

InetAddress localHost = Inet4Address.getLocalHost();
NetworkInterface networkInterface = NetworkInterface.getByInetAddress(localHost);

for (InterfaceAddress address : networkInterface.getInterfaceAddresses()) 
{
        System.out.println(address.getNetworkPrefixLength());
}

But it gives

Exception in thread "main" java.lang.NullPointerException
at com.Server.Subnet.main(Subnet.java:17)

I think i need to follow these steps.

1. Get the subnet address of the network that i connected
2. Scan all the ip address in the subnet mask
3. List the ip address that are up

Can you give me the right implementation way

Answer 1

I tried this program to find all the up ip in the subnet of the system connected.

package com.Server;

import java.io.IOException;
import java.net.Inet4Address;
import java.net.InetAddress;
import java.net.InterfaceAddress;
import java.net.NetworkInterface;
import java.net.SocketException;
import java.net.UnknownHostException;
import java.util.Enumeration;

public class Subnet
{
    public void Subnet() throws UnknownHostException, SocketException
    {
        Enumeration e = NetworkInterface.getNetworkInterfaces();
        while(e.hasMoreElements())
        {
            NetworkInterface n = (NetworkInterface) e.nextElement();
            Enumeration ee = n.getInetAddresses();
            while (ee.hasMoreElements())
            {
                InetAddress i = (InetAddress) ee.nextElement();
                String ip = i.getHostAddress();

                String sip = ip.substring(0, ip.indexOf('.',ip.indexOf('.',ip.indexOf('.')+1) + 1) + 1);
                try {
                    for(int it=1;it<=255;it++)
                    {
                        String ipToTest = sip+it;
                        boolean online = InetAddress.getByName(itToTest).isReachable(100);
                        if (online) {
                            System.out.println(ipToTest+" is online");
                        }

                    }
                } catch (IOException e1) {
                    System.out.println(sip);
                }
            }
        }
    }
}