An example of the good use of interoperability is that you may have had a DEC server, a Dec VAX for example, running X 11, the X Windows system. On the client side you have a Unix machine and so the X-server, because it was a standard protocol, could offer it 's services to the Unix workstation.

Another example would be a font server, where we have one or more font servers in the network. Even if the font servers ran on a Dec VAX or on Solaris or an HP Unix machine, X font clients will have access to service. So a Unix or Linux machine would be able to use the font service offered by a VAX machine.

The final reason for the advantages of interoperability I am going to mention here is "central control".

Central control is something that is familiar to those people that were working with Main Frame computers, and in a way it is ‘funny' how IT fashion is coming back on itself as now we have client server technology and this allows us to have central control of, for example, what applications to serve to and what services to serve on a network.

A classic example of client server technology is the Citrix windows client and server technology that we'll see being implemented for Windows clients in a lot of networks today.

So how is this central control implemented?

Well, I think the best way to describe this is to give some more examples.

Sometimes in client server the client and the server reside on the same machine, an example of this is X11, X Windows system, implemented in Linux. In the X Windows system, we would have an X 11 server on top of the Operating System and on top of that we would have client applications.

The client application might be X Clock or say K-Astroids or TUX-Racer. These applications are essentially our client and beneath our client we are running a server.

X is a particularly interesting one because in order to run KAstroid or TuxRacer or X-Clock, we are forced to have a server provide those services. It 's the responsibility of the server to talk to the video card, for example, to explain to the video card how to display a penguin racing down a ice slope

What makes this even more interesting is the fact that because every Unix or Linux system that you are running a GUI on you are going to have an associated X server running.

There is no reason why you couldn't display these aforementioned clients on the server. So in this case we have a machine that's offering itself as the client and we have another machine offering its X11 server as the server. Clearly we could swap those around the crossover line, for example we could run X Clock on machine B but use the services offered by machine A, X 11 server, to display the clock.

Lets have a look at a simpler example with FTP (File Transfer Protocol), in this case we might have access to Internet Solutions that has a repository of all the Linux ISO images that we want to download.

How do we do that?

Well, let us say that on our client 's side we have a client application called GFTP (Gnome File Transfer Protocol) ftp server. This GFTP server is the client and Internet Solutions ftp server, is the server.

We use our client 's to contact the server and this happens on different machines, unless you are running your own ftp server to ftp files from yourself to yourself, which really would make no sense.

This example demonstrates using http or web-based services as client servers.

On the server side we might have an Apache or IIS running as a server and on the client 's side we would have our browser, as in the Firebird, Gallion or Internet Explorer.

In that scenario, we would have an http server, usually running Apache and on the client 's side we would have a browser. Not all processing is done on a server, the server sends a web page and the browser, on receiving that web page, interprets it and displays it in a presentable form.