Have a Page Somewhere Else!

My WordPress is installed in my website’s root directory.(users can access it from http://www.sitename.com) I want to have a WordPress page in a subdomain(to be accessible from http://subdomain.sitename.com) while having other pages(and posts) on the root directory. How can I do that?
Thanks.