NAT is the basis for multiple devices (with individual IPs) communicating with the internet over a single IP - usually, the router's IP.

NAT can be imagined as a table in which the router stores which device "behind him" accessed a public IP. When the response from the public IP comes, the router can route it to the device in the private network.

NAT keeps the network-internal device unknown to the outside.

NAT Traversal

NAT Travesal is used for direct connection between devices. As NAT does not allow direct connections coming from the outside.

Usual approaches for NAT Traversal:

• STUN
• UDP holepunching, see UDP Holepunching