How to use the 3rd level domain?

[eluser]@ndrea[/eluser]
Hi guys,
in these days I'm thinking about deleting my own framework and convert all the whole project to CI.
For a few days I've been reading in forums and wiki to understand CI, I can do all I need and now I have a question of absolute importance.

All 3rd level domains on my server point to root, all these paths link to the same page:
mysite.com/page.html
abc.mysite.com/page.html
xyz.mysite.com/page.html

This is because 'abc' and 'xyz' are used as account name, in the specific case 'mysite.com' links to an information page and to the real site working behind a "specific account request".
First, I wonder what the right configuration for '$config['base_url']' is.
Second, I wonder what is the best practice to create the account mapping: in the actual implementation I use mod_rewrite to take the 3rd level domain and append at querystring like "&account=my3rdLvl".
Third, how can link to the help page when I don't have the account set?

Thanks.
A.