I am trying to replace a 2820 router with a new 2830 but having issues using the WAN Alias.

Current setup
BT 2wirehub (81.138.252.10) with two cables one plugged into my exchange server (all my mx records point to this IP)
other cable plugged into Vigor 2820n (81.138.252.12) all my VPN traffic, web traffic

New Setup
I want to replace the above setup with One ADSL Vogor 2830 using the WAN Alias My router will have 81.138.252.12 address
and 81.138.252.10 as a WAN Alias. I have opend a port from 81.138.252.10 port 25 and point this to my exchange

Issues
I have enabled ping from the internet and can ping both IP address's but when I test exchange connection i am unable
to telnet to 81.135.252.10 port 25

If anyone could help it would be very much apriciated.

I have a feeling the Vigor can only have the first or last IP in the range.

Some background which will hopefully prove useful:

http://www.draytek.co.uk/support/kb_vigor_2ndsubnet.html

http://www.draytek.co.uk/support/kb_vigor_multinat.html

I have no firsthand experience ISP IP ranges with Vigors, so apologies if my post isn't helpful. Regards, Neal