We have a lot of experience with Vigor 2860s, and have successfully setup this config, but since moving to a 3900 we are having major issues.

We have an external IP address range, lets say 50.50.50.1 - 8
We have more than one server in the office that each need to use port 80 for their services. We therefore need to setup the single WAN connection to have aliases, and then forward the traffic from the different external IP addresses to different servers. eg:
50.50.50.1 goes to Server 1
50.50.50.2 goes to Server 2
etc

We can setup the IP Aliases without an issue, the problem comes when we need to port forward. Nothing we do allows us to forward just one external IP address to one server. We can only setup rules that will forward all port 80 traffic to one particular server. We have not managed to find a way to differentiate the incoming traffic based on the external IP address. This was easy for other router models, but seems really hard for this unit. :shock:

We have tried DMZs, read guides, attempted routing rules etc. But in the end we always fail to make it work. Does anyone have a guide we could follow to achieve this please?

Have you set up all the External IP addresses in the "IP Alias" table under the WAN - Static settings?
Once this is done then under port redirect set the "Use IP Alias" to "Single Alias" and then use the dropdown box just below to select the correct Alias.
It took me a while to work it out but it does definitely work.

Neil..

Just as you were posting I finally spotted the bit that I needed.

In the port forwarding section I needed to NOT select WAN Profile All, an individual Wan interface needs to be selected, which in turn allows you to apply an alias address to the port forwarding rule. All sorted, only took several hours!

Part of the issue is that the new firmware produces a different set of menu options that is different to the guides that have been made by Draytek, oh well....we live and learn!