Hi Ashley,
You need to understand what is happening and then work inside those rules.
a) Programs (like NetTalk, or IIS) can "listen on all IP addresses" or they can be set to listen only on a single address. (Webserver, advanced tab). (Machines can obviously be allocated multiple IP addresses).
b) Programs can listen on a port.
c) Only one program can listen to a specific IP:port combination. If you want multiple programs to be listening then either the port number or Ip address has to be unique.
The above are absolutes - each program needs a unique IP/Port combination, which no other program is using.
Your situation on a LAN bears some examination. As you said;
>> When
www.mywebsite.com hits the firewall it is directed to an internal ip address
So clearly if you want to allow external access to a server on the Lan then you need to deal with the router. There are many different routers, and some are "smarter" or "dumber".
Ideally you can get 2 different IP addresses allocated to the incoming connection- and using this the router can tell the traffic apart.
Alternatively you need to open another incoming port (to share the same IP).
Some (very few) firewall/routers allow for inspection of the header to see where the packet is going, and can route based on that, but that's unusual.
cheers
Bruce