If a user receives an "authentication required" popup when accessing manex reports (or other sections), this is typically due to using Windows authentication, which causes the "authentication required" form to pop up if someone outside the domain is trying to access it.
As long as the user is a domain member on a local network, IIS uses the credentials of the person logged in to the workstation and will not prompt.
If the workstation user is not a domain member, or if it is a public site behind a firewall/proxy, a prompt will be received.
This is a browser issue, not an IIS one. Internet Explorer (and other browsers) won't pass credentials to a site unless it is in the intranet security zone or the same domain. IP addresses aren't a domain or machine name, so the browser sees them as an internet zone and won't pass credentials. To fix this, the IP address must be added to the intranet zone in the user's browser security tab.
Or, alternately, the user can select the "edit & enable anonymous access" option under the Directory Security tab in IIS.