Short answer: your NAT type is your console’s verdict on how easily other players can connect to you. “Open” means they can, “Strict” means many can’t, and “Moderate” is in between. A strict NAT doesn’t slow your game down, but it can block party chat, hosting and matchmaking with some players. The usual causes are two routers in a row, a provider-level NAT, or a router that won’t open connections for games.

What NAT type measures

Your router uses NAT to share one public address between your devices, and by design it lets connections start from inside your home far more easily than from outside; in traditional NAT, incoming sessions are allowed only by exception. Many games connect players directly to each other, and when both players are behind NATs, their devices can’t simply reach each other’s addresses. That’s why the IETF’s NAT requirements exist: NATs are well known to cause problems for applications, and RFC 4787 sets out how a NAT should behave so that applications such as online games work consistently.

Consoles test this and give you a grade. The labels differ: Nintendo, for example, grades NAT from A, best for peer-to-peer communication, to F, unable to complete it. Others use Open, Moderate and Strict.

What’s behind the labels

Two behaviours matter most. With endpoint-independent mapping, the router keeps the same external port for your console whichever player it talks to, and RFC 4787 makes this a requirement. With endpoint-independent filtering, once your console has sent traffic out, replies from any player can reach it. Routers that are stricter about either one tend to produce stricter NAT results.

Common causes of a strict NAT

  • Double NAT: your provider’s modem is also a router, with your own router behind it. TP-Link’s fix is bridge mode on the modem router. See double NAT explained.
  • Carrier-grade NAT: your provider puts you behind its own NAT. The 100.64.0.0/10 range is set aside for that setup, so if your router’s internet address starts with 100.64 to 100.127, ask your provider about a public address.
  • UPnP off or blocked: many games rely on it to open the connections they need automatically.

Fixes, in order

NAT type isn’t the same as lag

A strict NAT stops some connections from forming; it doesn’t add delay to the ones that work. If your game connects but lags, see how to reduce ping and lag, and for whether special hardware helps, do you need a gaming router?