Identifying a machine

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  1. The familiar DNS (Domain Name Service) form. My domain name is bruceeckel.com, so suppose I have a computer called Opus in my domain. Its domain name would be Opus.bruceeckel.com. This is exactly the kind of name that you use when you send email to people, and is often incorporated into a World-Wide-Web address.
  2. Alternatively, you can use the “dotted quad” form, which is four numbers separated by dots, such as 123.255.28.120.
In both cases, the IP address is represented internally as a 32-bit number [63] (so each of the quad numbers cannot exceed 255), and you can get a special Java object to represent this number from either of the forms above by using the static InetAddress.getByName( ) method that’s in java.net. The result is an object of type InetAddress that you can use to build a “socket” as you will see later.

As a simple example of using InetAddress.getByName( ), consider what happens if you have a dial-up Internet service provider (ISP). Each time you dial up, you are assigned a temporary IP address. But while you’re connected, your IP address has the same validity as any other IP address on the Internet. If someone connects to your machine using your IP address then they can connect to a Web server or FTP server that you have running on your machine. Of course, they need to know your IP address, and since it’s assigned each time you dial up, how can you find out what it is?

The following program uses InetAddress.getByName( ) to produce your IP address. To use it, you must know the name of your computer. It has been tested only on Windows 95, but there you can go to “Settings,” “Control Panel,” “Network,” and then select the “Identification” tab. “Computer name” is the name to put on the command line.

//: WhoAmI.java
// Finds out your network address when you're 
// connected to the Internet.
package c15;
import java.net.*;
 
public class WhoAmI {
  public static void main(String[] args) 
      throws Exception {
    if(args.length != 1) {
      System.err.println(
        "Usage: WhoAmI MachineName");
      System.exit(1);
    }
    InetAddress a = 
      InetAddress.getByName(args[0]);
    System.out.println(a);
  }
} ///:~ 

In my case, the machine is called “Colossus” (from the movie of the same name, because I keep putting bigger disks on it). So, once I’ve connected to my ISP I run the program:

java WhoAmI Colossus

I get back a message like this (of course, the address is different each time):

Colossus/199.190.87.75

If I tell my friend this address, he can log onto my personal Web server by going to the URL http://199.190.87.75 (only as long as I continue to stay connected during that session). This can sometimes be a handy way to distribute information to someone else or to test out a Web site configuration before posting it to a “real” server.

Servers and clients

The whole point of a network is to allow two machines to connect and talk to each other. Once the two machines have found each other they can have a nice, two-way conversation. But how do they find each other? It’s like getting lost in an amusement park: one machine has to stay in one place and listen while the other machine says, “Hey, where are you?”

Testing programs without a network

Port: a unique place

within the machine


[63] This means a maximum of just over four billion numbers, which is rapidly running out. The new standard for IP addresses will use a 128-bit number, which should produce enough unique IP addresses for the foreseeable future.



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