Java Tech - Chapter 13 : Networking Basics

The java.net.InetAddress class represents IP addresses. It works with either the hostname (e.g. www.kth.se) or the numerical IP address (e.g. 64.124.81.56) of an Internet host. InetAddress class offers a number of useful methods for dealing with hostnames and IP addresses.

The demo programs below illustrate some of the capabilities of the InetAddress class.

The applet LocalAddress displays the local host and IP address. Note that the SecurityManager in the browser JVM may block access to this information. You can try it in the appletviewer tool or run it as an application.

LocalAddress

import java.applet.*;
import java.awt.*;
import java.net.*;

/** Show how InetAddress can provide the local IP address.**/
public class LocalAddress extends Applet
{
  String fMessage = "";

  /**
    * Create an instance of InetAddress for the
    * local host and display the local IP address.
   **/
  public void init () {
    try {
      InetAddress local_Address =
             InetAddress.getLocalHost ();
      fMessage ="Local IP address = " +
             local_Address.toString ();
    }
    catch  (UnknownHostException e) {
      fMessage ="Unable to obtain local IP address";
    }
    System.out.println (fMessage);
  } // init

  /** Paint IP info in Applet window. **/
  public void paint (Graphics g) {
     g.drawString (fMessage,20,20);
  }

  /** Print out the local address in app mode.**/
  public static void main (String [] args) {
    LocalAddress applet =
       new LocalAddress ();
    applet.init ();
  } // main

} // Class LocalAddress

This application returns the IP address if given a host name, or returns the host name if given the IP address:

TranslateAddress

import java.net.*;

/** Translate an IP argument to a host name or
  * a host name to an IP address.
**/
class TranslateAddress {

  public static void main  (String args[])
  {

    // Look for command line argument
    if (args.length != 1) {
      System.out.println ("Error! No IP or host name address");
      System.out.println ("Usage: java TranslateAddress java.sun.com");
      System.out.println ("    or java TranslateAddress 209.249.116.143");
      System.exit (0);
    }

    try  {
      // When the argument passed is a host name  (e.g. sun.com),
      // the corresponding IP address is returned. If passed
      // an IP address, then only the IP address is returned.
      InetAddress address = InetAddress.getByName (args[0]);
      System.out.println ("Address " + args[0] + " = " + address);

      // To get the hostname when passed an IP address use
      // getHostName (), which will return the host name string.
      System.out.println ("Name of " + args[0] + " = " +
                            address.getHostName ());
    } catch  (UnknownHostException e) {
      System.out.println ("Unable to translate the address.");
    }
  } // main

} // class TranslateAddress

Example runs of this program: