When you browse an intranet by IP address, using Internet Explorer, and the site uses integrated Windows authentication, you receive:
You are not authorized to view this page
You do not have permission to view this directory or page using the credentials you supplied.
HTTP 401.2 - Unauthorized: Logon failed due to server configuration
Internet Information Services.
The problem is caused by the way that IE determines wether the site is external (internet) or internal (intranet).
If IE finds a period in the URL, it assumes that the site is on the internet. Because integrated Windows authentication does not work accross a firwall or proxy, it does not attempt to authenticate.
To resolve the problem, add the IP address to the Exceptions, on each client, as explained in tip 2979.