Problem with isReachable in InetAddress class

Question

As an assignment I have to find all the alive computers on a LAN. For which I am using isReachable function of InetAddress class. But problem is that nothing is shown reachable to me. So I tried to have isReachable with Google's IP but still this is unreachable.

Here is the code:

import java.net.*;

public class alive{
    public static void main(String args[]){
        try{
            InetAddress ia = InetAddress.getByAddress(new byte[]{(byte)209, (byte)85, (byte)153, (byte)104});
            boolean b = ia.isReachable(10000);
            if(b){
                System.out.println("Reachable");
            }
            else{
                System.out.println("Unrachable");
            }

        }catch(Exception e){
            System.out.println("Exception: " + e.getMessage());
        }
    }
}

Output is : Unreachable

Answer 1

Here are some details on why isReachable() might not always work as expected

1. http://bordet.blogspot.com/2006/07/icmp-and-inetaddressisreachable.html
2. http://www.coderanch.com/t/206934/sockets/java/InetAdress-isReachable-Ping-Permissions

The correct way for you is to use the ICMP protocol. This is what ping uses internatlly, I believe. Here is an example that get you started.