Hi,
I've just spent 2 days trying to install DNN on my local Windows XP Pro machine and trying to access it externally. I finally solved it and, as I didn't find any answers specifically for it anywhere on the internet, would like to share it and save the time of other poor souls with the same problem.
Although I installed it numerous times with all kinds of persmission settings I was sure I had the correct settings but still got "403 forbidden access" whilst trying to access it externally. The only help I found was every man and his dog telling me to check the permissions.
I had the correct permissions set.
The problem was that a default Portal Alias is created as localhost/dotnetnuke. This allows you to access your site from the host machine.
To access the site on a local network you need to add a new Portal Alias. eg. 192.168.2.2/dotnetnuke where 192.168.2.2 is the network address of the PC hosting DNN.
Then to access it externally you need to add another Portal Alias that uses your external static ip address: #.#.#.#/dotnetnuke where #.#.#.# is your static ip address.
You need both of these for it to work as the external one passes you to your router and the internal network ip address resolves it within your network.
To add a new Portal Alias log in as the Super User (default username was "host") and in the Control Panel go to Common Tasks on the right and Site. Portal Aliases are at the bottom.
I hope this helps!