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.