Allows us think about a situation, you will find 2 apache server running, and something domain can be obtained. when we create a request such as this, http://domain1.com/example1.php it will request one apache server where actual domain exists. When http://domain1:8000/example1.php it will indicate a application inside a another server (other machine) within same domain group.
Now an issue is, if http://domain1:8000/example1.php is asked for, then it'll run by which server? which server will interpret it? which server will execute individuals files, either apache server in domain1 system or, a apache server that domain1:8000 (this really is other machine, that request is port submitted) points?
A server will listen on the specific port, so if you're using different ports, it'll visit whatever server is listening on that port, no matter the domain.
Since you are using port sending, it are only able to be processed by anywhere you forward the ports to. So, port 80 has been submitted for your primary server and port 8000 towards the other server. Should you did not forward, and all sorts of would the very first server, you would then have an error when the first server weren't also listening on port 8000.