• IP Addresses:
  • Four bytes (32 bits)
  • Dotted Quads
• IP Packet Format:
  • Header (20-60 bytes) that includes from/to addresses
  • Payload (up to 65,515 bytes)

• More Addresses (16 bytes)
• Simplified Headers
• Additional Privacy and Security Features

• InetAddress :
  • public byte[] getAddress()
  • public String getHostAddress()
• Child Classes:
  • Inet4Address
  • Inet6Address

• Description:
  • A distributed database that allows users to map human readable names to IP addresses
• Interaction with the DNS:
  • Handled by a resolver that first consults a local DNS server and then, if necessary, consults foreign DNS servers
• Domain Names:
  • Are hierarchical
  • Portions are separated by '.'
  • Left portion is most specific, right portion is DNS root

• Forward Resolution (Name to Address):
  • InetAddress.getByName(host:String):InetAddress
  • InetAddress.getAllByName(host:String):InetAddress[]
• Reverse Name Resolution (Address to Name):
  • InetAddress.getByAddress(address:byte[]):InetAddress

Finding the Local Address

javaexamples/internet/MyAddress.java

import java.net.*;

/**
 * A utility for determining a machines IP address
 *
 * @version 1.0
 * @author  Prof. David Bernstein, James Madison University
 *
 */
public class MyAddress
{

    /**
     * The entry point
     *
     * @param args  The command-line arguments
     */
    public static void main(String[] args)
    {
       InetAddress ip;

       System.out.println("\n");
       try 
       {
          ip = InetAddress.getLocalHost();
          System.out.println("Address: "+ip);
       } 
       catch (UnknownHostException uhe)
       {
          System.out.println("This computer's address is unavailable!");
       }

       System.out.println("\n");
    }
}
        

Finding Any Address

javaexamples/internet/GetAddress.java

import java.net.*;

/**
 * A utility for determining the IP addresses
 * of a host given its name
 *
 * @version 1.0
 * @author  Prof. David Bernstein, James Madison University
 *
 */
public class GetAddress
{

    /**
     * The entry point
     *
     * @param args  The command-line arguments
     */
    public static void main(String[] args)
    {
       InetAddress[]      ip;
       int                i;


       System.out.println("\n");
       if (args.length == 0) 
       {
          System.out.println("You must enter a host name.");
       } 
       else
       {
          try 
          {
             ip = InetAddress.getAllByName(args[0]);

             for (i=0; i < ip.length; i++) 
             {
                System.out.println("Address: "+ip[i]);
             }
          }
          catch (UnknownHostException uhe) 
          {
             System.out.println("This computer's address is unavailable!");
          }
       }
       System.out.println("\n");
    }
}