Why can't some websites be shown without the "www" in the URL? [closed]

I used to type the website URL without the "www" and it works fine with almost all sites that I have tried, so far.

But I realize that my university's website never show up if you don't type the "www" before.

Typing this

 http://u-psud.fr/

shows a dead page, but adding the "www" works well:

 http://www.u-psud.fr/

Why is it not working for one's own site, or why is it working for the other website :)

The "www" part is not really a standard. It's a tradition. It's up to the webmaster/sysadmin to make it work. Just because most sites have enabled both ways (with www and without www), doesn't mean that they have to. In fact, have you noticed that some websites have something like www1, www2, etc.? That's because the sysadmin can decide what he/she wants to put there (or if he/she wants to put anything there at all). As I said - it's just a tradition.

It's because the "www." is part of the DNS name that resolves to their web server's addresses. The ones that don't have "www." don't have it as part of their DNS name.