On my network, I have two Apache webservers. One running on a Windwos 10 machine with XAMPP, and the second one running on a Raspberry Pi using Raspbian. Both of them are public, and the ports are open. The XAMPP server is using port 80, and the Raspberry is port 81.
I currently have two domains, which in this case I will refer to domain A and domain B. Domain A is meant for the XAMPP server, and domain B is meant for the Raspberry server. To access the XAMPP server I simply use domainA.com, but to access the Raspberry server I have to use domainA.com:81.
So my question is, how do I use my domain B to refer to the Raspberry server with simply using http://domainB.com?
If there's a central NAT device with a single IP address which forwards the ports, then there's no way to do this the way you expect to. You want to differentiate machines by the HTTP Host header, but the NAT is operating a few levels down the OSI stack at the IP and port layer and doesn't know anything about HTTP. You will need to have an HTTP capable device (a web server) which accepts requests for both domains and reverse proxies them to the appropriate server. You could set up one of your two web server to proxy requests for the other web server, or you add a third load balancer/proxy in front of both.