NAT Traversal allows players to run the game server in their own internal network and be visible to players out of their own internal LAN.


When players run the game on their internal network ( by how TCP and UDP are building) players out of this network are unable to reach the machine where the game is running.


The following drawing illustrates a player out of the internal network trying to reach the server where the server is running.



Even if the Unknown Player knows the router IP Address, they are unable to reach machine 192.168.1.11 where the game server is running.


You can Enable Nat Traversal by checking the option on NetworkManager.



When NAT Traversal is enabled, the system will try to open the game port used to accept client connections on the server.


The system will map the Server Player port in the router targeting the same port on the router, this means that when some player out of the internal network tries to connect to the public IP on the port used by Server Player, the communication will be redirected to the machine where the server is running.


The following drawing illustrates how NAT Traversal works.



This tells the router that redirect communication from an unknown player to IP 192.168.1.11 and vice-versa.


In this mode, players can run the server on their internal network and accept players on the other network to play on the same server.