My suspicion would be that the DNS name of the incoming request cannot be resolved and that's why you're only getting the IP address. Is this application accessed on an intranet? What authentication method are you using? Windows Integrated or Anonymous? I suspect that anonymous authentication would cause the DNS of the request to not resolve. I might be wrong though... I'm just taking a shot in the dark.
you should check the following and how it has set as it might influence your result.
IIS's security for the application
Webconfig's impersonation and authentication
IIS's version this might be change factor as 7.5 returns differently in some cases vs iis6 ?
location of the application