Port forwarding

So I have been looking around the internet on multiplayer for unity but every thing I watch/read makes me even more confused on how it works! Most games you just port forward a port (usually the game has a default port) and then a player would connect with the host's ip address and port! but how could I get something like that in unity? I have tried setting masterserver.ipaddress to a text field so that the player could assign his ip address to it and a port that he has port forwarded but I get errors! So I was wondering what does the player assign Masterserver.ipaddress to? Does he go to http://www.ipchicken.com/ and use that as his ip and players use that to join his game or does he go into the cmd then type ipconfig and use his ipv4? If this still makes no sense to you, I just want to know how a player could run a server of his ip and people join the server with his ip and port! Most tutorials use the 127.0.0.1 but that means that all servers will have that ip then! So once again summed up, What does the host assign the masterserver.ipaddress to (also where would he find the ip to assign it to) and what ip would people need to join it? So yeah its sort of a lot but thanks for the help! XD