Originally Posted by tedeu
Oo ... first of all, super fast answer... i give you that !
That's what happens when you sit in front of your computer at work
As far as your questions goes, this is completely possible. You will need to configure your DNS server to handle the intranet website that you would like to use. For example if your site name was intranet.example.com, you would need a new A record on the DNS server that points intranet.example.com --> 192.168.12.2.
Once you have done this and your DNS is updated (Start > Run > cmd > ipconfig /flushdns, on a PC) you will be able to browse to intranet.example.com in your web browser and go to the site. But don't forget that you will need to create the site in ISPConfig before you can view the site. I believe the site will be example.com with a subdomain called intranet. If not, you may be able to make a new site called intranet.example.com. Give both a try and see which works.