By default, Internet Explorer will not automatically pass-through Windows credentials to sites that are not in the Intranet Zone.  URLs that use just the servername are included in the Intranet Zone.  Any address that uses a FQDN will, by default, always prompt for credentials regardless of how IIS is configured.  To bypass this on your LAN, add the URL to the Intranet Zone in Internet Explorer:
  1. Click the Tools menu, then select Internet Options
  2. Select the Security tab
  3. Select Local Intranet and click Sites
  4. Click Advanced
  5. Enter in the URL for the WebPortal site
  6. Click Add, then Close
  7. Click OK to save