Double NAT is a network configuration issue that can occur when you connect your Xbox to a network that has multiple routers or gateways. NAT (Network Address Translation) is a function that allows multiple devices to share one public IP address when connecting to the Internet. However, when there are multiple routers or gateways, the network is split into two different private networks, which can cause communication problems between devices.

The Xbox One may show a "Double NAT Detected" error in the network settings if it is behind multiple routers or gateways. This error can prevent the device from building a steady connection for gaming. Double NAT may cause trouble when using applications or services that require Universal Play-and-Play (UPnP) support, such as online games, VPNs, or IP cameras.

To fix the Double NAT error on Xbox One, you need to remove one NAT device from your network. If you are not sure whether your device network is a gateway or a modem, you can check the manufacturer information or model. You can also turn off UPnP on your router or open specific ports on your router.

In summary, Double NAT is a network configuration issue that can cause communication problems between devices and prevent a steady connection for gaming on Xbox One. To fix the Double NAT error, you need to remove one NAT device from your network, turn off UPnP on your router, or open specific ports on your router.