"If it's only for intranet purpose, you could use different domain names and
install a DNS server. "
In this case, every host in the local network that is set to use your DNS server would know that www.mydomain.org
is (example) ip 192.168.1.5 and that www.mydomain2.org
is 192.168.1.6. This is the "easy way" but you have to setup a DNS server.
It is well explained in the How-to I mentioned.
What I mean is that you can use 1 virtual web site for your differents sites storage. All these servers use the exact same IP but different ports (80, 81, 82..). If let say your www.mydomain.org:81
site is located on a different IP then on the home page of this site, just put something like
location = "192.168.1.6/index.html";
"Another think I'm thinking of is if you use different ports than 80 for www you could use Nat tables from Iptables to redirect by example every traffic going to port 81 is redirected to ip 192.168.1.4."
It is similar to the redirector trick. In the Firewall configuration, it is possible to redirect every thing you want by using NAT... You choose the condition on a IP/Port basis. You could by example redirect every input going to "eth1" with TCP port 81 to anywhere you wish. This is normally used when using a Proxy to redirect calls to port 80 to 8080 (the proxy)... There are good how-to on that. But it may be too complicated for your needs.
For an easy configuration of this, use Webmin.
Personally, I would go with the DNS approach.