in this post i will try to explain how you can install ispconfig behind a firewall using NAT. for my example, i will be using a cisco PIX 501 firewall using NAT and PAT.

so for example, i have a WAN ip address of: 205.15.20.33 and my domain name is: example.com

this is how my nameserver are setup:
example.com
ns1.nameserver.com - 205.15.20.33
ns2.nameserver.com - 205.15.20.53

as you can see from the example above, my traffic should go to the 205.15.20.33 for all example.com domain name

the server will have ip address of:
10.20.20.20

configure firewall to translate traffic coming from 205.15.20.33 to 10.20.20.20
tutorial: http://www.webune.com/forums/20101003cggg.html

so i install ispconfig 2 on server ns1.nameserver.com
during installation, ispconfig will ask for ip address:
10.20.20.20

after installation you can access ispconfig panel witht he wan ip:
https://205.15.20.33:81/index.php

once you login to admin, go to Management > Server > Settings
05p-8372-inspconfig-nat-settings.jpg

be sure to set the WAN ip address and the LAN ip address as shown on the image above
in my example i have:
Server Name: Server 1
Hostname: ns1
Domain: nameserver.com
IP Address: 205.15.20.33
Netmask: 255.255.255.0

IP List: 10.20.20.20

next, we create a domain name in the control panel. everything will be the same, except when you select the ip address select the LAN IP 10.20.20.20
05p-8372-inspconfig-isp-site.jpg

after you have created the new site, confirm the settings under DNS manager are correct, on my example this is how they are set:

DNS Entry
Domain:
05p-8372-inspconfig-dns-entry.jpg

Options:
05p-8372-inspconfig-dns-options.jpg

Record:
05p-8372-inspconfig-dns-record.jpg