The Apache IP has to be set in the apache config files. When information comes to your real IP addres, this information is re-routed to an internal IP addres and/ or port.
192.168.1.100:80 -> Apache listens for information on this local IP address.
By default apache listens to some IP address and port 80.
You can use Apache with localhost, which means that you are using IP address
You can configure Apache to listen to any IP and any port, but the default ones are already set in your config files.
Also your web browser by default uses port 80.
Here you can read more.
Additional information(20% relevance):
A little bit about port pitfalls from the many years of experience repository. Different ports are accepted and treated by the OS and programs in different ways. For example a certain range of ports are accepted to be used by viruses and are blocked by the anti virus and/or operating system. If you use port 8080, it might be ok, but its good to test a range of ports in order to make sure that the problem does not come from that. Try 8080, 8081, then try with a big difference 9090, 9091.