NAT in plain words#
At home your phone, laptop, TV and maybe a few other devices are connected to Wi-Fi, but your internet provider usually gives your modem only one public IP address. NAT is what the modem or router does so that all these devices can reach the internet through that single address.
An analogy#
Think of an office building with one main phone number and several extensions. Outsiders see only the main number, and the receptionist knows which extension each call belongs to. In the same way, the router keeps a table of which request came from which device, so it can return the reply to the right one.
Why does it exist?#
IPv4 addresses are limited and not enough for every device in the world. With NAT, internal devices get private addresses such as 192.168.1.10, which only mean something inside the network, and one public address is shared by all of them. IPv6, with its enormous number of addresses, reduces the need for NAT.
Where you meet it#
- Port forwarding: if you want a security camera or a home game server to be reachable from outside, you must tell the router which device should receive requests on a given port.
- Online gaming: messages like "NAT Type: Strict" mean direct connections to other players are limited.
- CGNAT: some providers, especially on mobile internet, add another NAT layer of their own; in that case even port forwarding does not work.
Common mistake#
NAT blocks many unwanted incoming connections as a side effect, but it is not a firewall. Router updates and a strong password are still needed.