This is called a reverse proxy.
If you configure a host inside the network to be the reverse proxy, then forward ports 80 (and 443 if you're doing SSL) to the reverse proxy host, and let it make proxy requests to the various internal servers based on the URL of the incoming requests.
You can do this with the IP HTTP Proxy feature, but be very careful not to allow it to proxy for anything that goes back out to the Internet, otherwise, your router will get discovered as an open proxy and hijacked by the forces of evil who will use it to either anonymize their activities or participate in DDoS activities.
When given a spoon,
you should not cling to your fork.
The soup will get cold.