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Identifying a machine

Of course, in order to tell one machine from another and to make sure that you are connected with the machine you want, there must be some way of uniquely identifying machines on a network. Early networks were satisfied to provide unique names for machines within the local network. However, Java works within the Internet, which requires a way to uniquely identify a machine from all the others in the world . This is accomplished with the IP (Internet Protocol) address that can exist in two forms:

  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?”

The machine that “stays in one place” is called the server, and the one that seeks is called the client. This distinction is important only while the client is trying to connect to the server. Once they’ve connected, it becomes a two-way communication process and it doesn’t matter anymore that one happened to take the role of server and the other happened to take the role of the client.

So the job of the server is to listen for a connection, and that’s performed by the special server object that you create. The job of the client is to try to make a connection to a server, and this is performed by the special client object you create. Once the connection is made, you’ll see that at both server and client ends, the connection is just magically turned into an IO stream object, and from then on you can treat the connection as if you were reading from and writing to a file. Thus, after the connection is made you will just use the familiar IO commands from Chapter 10. This is one of the nice features of Java networking.

Testing programs without a network

For many reasons, you might not have a client machine, a server machine, and a network available to test your programs. You might be performing exercises in a classroom situation, or you could be writing programs that aren’t yet stable enough to put onto the network. The creators of the Internet Protocol were aware of this issue, and they created a special address called localhost to be the “local loopback” IP address for testing without a network. The generic way to produce this address in Java is:

InetAddress addr = InetAddress.getByName(null);

If you hand getByName( ) a null, it defaults to using the localhost. The InetAddress is what you use to refer to the particular machine, and you must produce this before you can go any further. You can’t manipulate the contents of an InetAddress (but you can print them out, as you’ll see in the next example). The only way you can create an InetAddress is through one of that class’s static member methods getByName( ) (which is what you’ll usually use), getAllByName( ), or getLocalHost( ).

You can also produce the local loopback address by handing it the string localhost:

InetAddress.getByName("localhost");

or by using its dotted quad form to name the reserved IP number for the loopback:

InetAddress.getByName("127.0.0.1");

All three forms produce the same result.

Port: a unique place

within the machine

An IP address isn’t enough to identify a unique server, since many servers can exist on one machine. Each IP machine also contains ports, and when you’re setting up a client or a server you must choose a port where both client and server agree to connect; if you’re meeting someone, the IP address is the neighborhood and the port is the bar.

The port is not a physical location in a machine, but a software abstraction (mainly for bookkeeping purposes). The client program knows how to connect to the machine via its IP address, but how does it connect to a desired service (potentially one of many on that machine)? That’s where the port numbers come in as second level of addressing. The idea is that if you ask for a particular port, you’re requesting the service that’s associated with the port number. The time of day is a simple example of a service. Typically, each service is associated with a unique port number on a given server machine. It’s up to the client to know ahead of time which port number the desired service is running on.

The system services reserve the use of ports 1 through 1024, so you shouldn’t use those or any other port that you know to be in use. The first choice for examples in this book will be port 8080 (in memory of the venerable old 8-bit Intel 8080 chip in my first computer, a CP/M 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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