GLOSSARY · NETWORK ADDRESS TRANSLATION
NAT
NAT is a technique where a router rewrites the addresses on outgoing traffic so that many devices with private addresses can share one public IP address. The router remembers which device made each request and sends the replies back to the right one.
Why it matters
NAT is why every phone and laptop at home can browse at once using a single public address, and it is the main reason IPv4 has lasted so long. It also means outside systems cannot start a connection to a device behind it unless the router has a forwarding rule. That behaves like a basic shield, though it is not a security product.
In practice
NAT causes the common problems of hosting a game or server at home: incoming connections arrive at the router and are dropped unless you forward a port. It also affects peer-to-peer features, including WebRTC, which use helper servers to find a path through NAT. Port forwarding and the port checker are the usual way to test.
Related terms
- CGNAT: Carrier-grade NAT is NAT performed by an internet provider, not by your own router.
- Public vs. private IP address: A public IP address is reachable across the internet and is the one websites see.
- Port: A port is a number from 0 to 65535 that tells a computer which program should receive incoming network data.
- STUN: STUN is a simple protocol where a device asks a public server, "what address and port do you see me coming from?" The server replies with the answer.