Definition
Network Address Translation (NAT) is a technique routers use to map an entire private network onto a single public IP address by rewriting the address information in packet headers. It emerged largely to conserve scarce IPv4 addresses and is now nearly universal on home routers. NAT is also the single most common reason a self-hosted service that works on your local network is unreachable from the wider internet.
How it works
Devices behind your router use private, non-routable addresses from reserved ranges (such as 192.168.x.x). When one of them connects outward, the router replaces the private source address with its own public address and records the mapping in a translation table. The modern variant, Port Address Translation (PAT, also called NAT overload or IP masquerading), adds a unique port number to each connection so that many devices can share one public address simultaneously.
Why inbound connections break
NAT only builds a translation entry when a connection originates from inside the private network. An unsolicited connection arriving from outside has no matching entry, so the router does not know which internal device should receive it and drops the packet. This is why running a reachable node, web dashboard, or VPN endpoint at home requires explicit configuration.
Carrier-grade NAT
Some ISPs add a second NAT layer of their own, called carrier-grade NAT (CGNAT), sharing one public address among many customers. Behind CGNAT you cannot open ports at all, because you do not control the outermost router, a common obstacle for home node operators.
The usual remedy for ordinary NAT is covered under Port Forwarding, while CGNAT and privacy concerns often push operators toward Tor onion services or an outbound tunnel.
In Simple Terms
Network Address Translation (NAT) is a technique routers use to map an entire private network onto a single public IP address by rewriting the address…