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This module is dedicated to client server architecture. The client server concept is fundamental to the workings of the Internet, So it’s actually fairly important that you guys understand it now. Let’s start by having a look at exactly what happens when you type such a url into your web browser and press enter. The steps are as follows. Well firstly, this isn't really one of the steps involved but web server software is sitting there on that computer and it’s sitting there essentially doing nothing at all. It’s sole purpose in life is to wait for requests from browsers such as yours and as soon as those requests come in, it services them, now let’s have a look at that in a bit more detail. Your browser, which is actually known technically as a web client or a fttp client contacts that web server computer, Now the mechanism by which your browser gets in contact with the http server is not important to this particular course. We’re talking about routing and ip addresses, we don't really need to go into that in this particular course. Anyway, your browser, the web client contacts the web server and establishes a communication session. Once that session is established, your browser issues a request for the desired file, which in this case would be the new.htm file. The server software looks in the products folder on the server's machine on the hard disk and locates the file. If it’s there, it’s entirely possible that you’re requesting a file that doesn't actually exist. So it checks to see if it is there, and if it is, it then sends it back to your computer, in fact it sends it directly back to your web browser, to your web client your client, your browser then takes the file and renders it on your screen, in other words, it simply displays it on the screen for you. That's the process, what I want you to notice about this process is the web server really sits there doing nothing until it receives a request from the web client. It is the client that initiates the communication, makes a request of the server and the server provides the, well the response, what ever that response happens to be. You can think about it like this, the client is the boss and the server is the servant. Now what I really want to stress to you about in this particular module is that almost all Internet traffic is of this form. Not just web browsing, so we’re talking about email, we’re talking about news, we’re talking about FTP, we’re talking about chatting. There is a server somewhere and there is a client somewhere. Usually there's several clients for one server and the server may be dealing with many of them at once. Whenever you check your email, you fire up your email program such as Outlook express or Eudora or something like that and you connect to your mail server. Your email program is called an email client and your email server is obviously the server, so you connect to your server and say ‘have you got any new mail for me’ and the server sends it to you if there is any. At that point you close the connection again, again it's working exactly the same way. The client makes the request and the server provides the information. It’s easily summarized by saying that there is always a client server pair. The service sits around doing nothing until it’s contacted by a client and then the server fulfills the client requests and resumes doing nothing until another client asks for something and with the caveat of course that you can have several clients all making requests almost simultaneously and the server will be dealing with all of them and when your website is published, it will be published to a computer that has a server running on it. It has a http server running on it to be precise, so it will not be possible for your website to be viewed by the general public unless you have web server software running on that computer. So I hope that's a reasonable explanation of client server architecture. Something to keep in mind, whenever you’re using the Internet, try and see if you can spot the client and the server whenever you do anything that involves the Internet. We’ll now go and have a look at an example that's not http, we’ll go and have a look at FTP.