User Subdomains with only one CI core

[eluser]bretticus[/eluser]
There are so many ways to implement this that I recommend that you just google it.

Once you can reference the base path, you can make logic decisions such as which views to use, etc.

mod_rewrite is not relevant because you really don't want to redirect anything by host header as you're depending on the host header to know which site content to display.