Subdomains

[eluser]CodeIgniterNewbie[/eluser]
Assume the following subdomains:

http://us.domain.com
http://ca.domain.com

When a user connects to one of the domains, I want to extract the subdomain part (i.e. "us" or "ca"), implement some logic (basically, set a session variable), then route the user to the appropriate controller (in the above case, the home controller).

If, say, the user connect to http://us.domain.com/method1/, I still need to implement the session-setting logic, and then route the user to method1 controller.

How do I do this in CI? Is this a routes issue? .htaccess issue? Some combination of both? Something else?

Thanks.