how to force a default url prefix for each new blog

First of all, sorry... because the following question seems to be a FAQ, but searching on this forum didn't resolve my issue.

My need is very simple. The structure of my multisite (based on url, not subdomains) should be like the following:

http://site/casting/ (managed by admin)

The others blogs that all users will be able to create themself must have a different prefixed url (/social).

http://site/social/firstblog
http://site/social/secondblog

That's it. how to do it? Any suggestion?

If it is a really stupid question, forgive me... it's just my second post!

Thanks in advance,
e.