Hi Ali,
You need two Web Server one Internal and one External. While configuring you internal web server you also add external if external particpants are allowed or not.
For internal web server you want to make sure it's on the same subnet so internal particpants can access that one. For external you need to make sure the IP configured on external one is either natted ip or public ip so that when they type the external domain name it resolves to this external server ip address.
Let me know if you have more questions.
HTH
Arun