How does work shorten url services?

Hello, I know that some web sites offer a service to shorten long url. It is very useful when your site is hosted by your Internet provider for example, and that you have a long url. Or when you want to send it in an e-mail. tinyurl.com does that for example. The principle is that you enter your long url in their form and it gives you a shorten url like tinyurl.com/h4599 I was wondering how it works. I think it must use PHP. I see two possibilities. First is that the site creates an html page with a redirect header, like <META HTTP-EQUIV="REFRESH" CONTENT="0;URL=... But then it would mean that the site needs to host thousands of web pages for all its users. The second choice is to create a simple database with two fields. The first is the long url and the second is the shorten. So, when someone try tinyurl.com/h4599 it sends it to the real site. But is it possible for a web site to capture all the possible urls that it has created then compare it to its database? Because it has not created a webpage "h4599.html" for example, then the site should show a 404 page. Thanks for your help. Vous manquez d’espace pour stocker vos mails ? Yahoo! Mail vous offre GRATUITEMENT 100 Mo ! Créez votre Yahoo! Mail sur http://fr.benefits.yahoo.com/ Le nouveau Yahoo! Messenger est arrivé ! Découvrez toutes les nouveautés pour dialoguer instantanément avec vos amis. A télécharger gratuitement sur http://fr.messenger.yahoo.com