How do I allow access to a website resource only from within the site?

I am hosting a public web site and I am wanting to control access to a shared directory within it.

The main site is a Moodle site (not really relevant, just noteworthy that it is PHP, SQL2008 R2) and within the learning environment I'd like to provide access to an internal site.

I have the link within Moodle working, and the internal site loads with no problems (it is a shared folder within the site), but the concern is I can access the internal site simply by entering the url in the browser.

My question therefore is, is there a way to only allow access to the internal site only via the link from the Moodle site (the request comes from Moodle), and not allow access via URL entry (or any other way for that matter!).

Preliminary research has suggested it may be possible by setting up a module within IIS 7, but before further time is spent on researching this technology, I thought I'd check with the experts. (And if this is the case any help would be appreciated!)

Thanks in advance.