virtual subdomain with htaccess

[eluser]harmstra[/eluser]
I try to create a virtual subdomain, but no workie workie.

When i put this in htaccess,

Code:

RewriteCond %{HTTP_HOST} ^subdomain[NC]
RewriteRule ^(.*)$ http://www.domain.com/test/$1 [QSA,L]

things work fine. test is a physical subdir, and the contents of /test/index.html show up when i point the browser to subdomain.domain.com.
Important: the addressbar keeps the subdomain.domain.com, while it shows www.domain.com/test/index.html

So far, so good.

Now i want to do the same thing, but redirect to a CI controller

Code:

RewriteCond %{HTTP_HOST} ^subdomain[NC]
RewriteRule ^(.*)$ http://www.domain.com/controllername/$1 [QSA,L]

Then i get a Internal Server Error, because of a redirect loop. So i add 1 line.

Code:

RewriteCond %{HTTP_HOST} ^subdomain[NC]
RewriteCond %{REQUEST_URI} !^/controllername[NC]
RewriteRule ^(.*)$ http://www.domain.com/controllername/$1 [QSA,L]

Then everything works fine, subdomain.domain.com shows the contents of www.domain.com/controllername.

But the annoying thing is, the url in the address bar changes to www.domain.com/controllername

Is there a way to avoid this?