NAT is the practice of using private IP addresses inside of one network connected to the Internet and having a routing gateway forwarding requests to public IP addresses.

• RFC 6886 – NAT Port Mapping Protocol (NAT-PMP)
• RFC 6877 – 464xlat translation mechanism for IPv6

For Sony PlayStation networking, they differentiate between NAT Types 1 (open), 2, 3 and 4 (closed).

Microsoft says there are three types of NAT: Open, Moderate and Strict.

The detection of Nintendo Switch is considered to following behavior¹:

• Type A: Mapping: "Endpoint-Independent", Filtering: "Endpoint-Independent" or "Address-Dependent Filtering"
• Type B: Mapping: "Endpoint-Independent", Filtering: "Address and Port-Dependent"
• Type C: Mapping: the others, but the source port is predictable.
• Type D: In other cases.
• Type F: Failed to receive UDP packets.

A means it's directly connected to the internet, B means UPnP is enabled, C means some ports are blocked and D means you have Carrier-Grade NAT or a completely firewalled network.

Nintendo actually recommends opening 1-65536/udp to the Switch², which is effectively putting the Switch into an UDP DMZ. It might also use UPnP.

• nftables: Performing Network Address Translation (NAT)

see also: IPv6

• NAT66: The good, the bad, the ugly (Marco Cilloni, 2 Feb 2018) – has a section on "how to get away with NAT66"
• IPv6 ULA and NAT. Is It Better Than Global Unicast?
• What’s The Point of NAT66?
• Why Do You Need NAT66?
• We Just Might Need NAT66/NPT66