NAT, or network address translation, is a technique used by routers to allow multiple devices on a LAN with private IP addresses to share a single public IP address. This can be useful for gaming if your router supports UPnP and you want to use it to open ports automatically for your game. However, some games (particularly those using peer-to-peer connections) may not work correctly behind a NAT router, in which case you'll need to disable NAT and use public IP addresses for all devices on your LAN.