I recently upgraded the firmware on my Draytek Vigor 2820n ADSL router to version 3.3.4 from 3.3.3. One thing that surprised me was that the change stopped me being able to access my public IP from behind the router. That is I have an internal LAN with RFC1918 addresses such as 192.168/16 and could access my public ip address which gets routed back by the router to an internal host. The 3.3.4 firmware upgrade stops that working. That makes it a bit of a pain to test connectivity to sites like my web site from inside compared to from outside and it took me a while to figure out what had happened. In the end I’ve had to implement split DNS so that internal references to the “external sites” I provide resolve to internal ip addresses internally but the public see my external ip address. A lot of work for a home network but otherwise things just don’t work properly.
So if you have a Vigor 2820n then be aware of the change in behaviour from the previous version of the firmware.