NAT (Network Address Translation)

NAT (Network Address Translation) lets a router replace the private source address of outgoing packets with its own public address, then reverse the translation for replies. The most common form, port address translation, tracks each connection by port number.

Why it matters

NAT slowed IPv4 address exhaustion by letting a whole household or company use private ranges behind one public address. A side effect is that unsolicited inbound connections have no translation entry and are dropped, which is why hosting a service behind NAT requires port forwarding. It also complicates peer-to-peer and some VPN protocols.

Example

Inside:  192.168.1.25:51000 -> 198.51.100.7:443
Router:  203.0.113.4:40012   -> 198.51.100.7:443

The server sees only 203.0.113.4. The router remembers that port 40012 maps to 192.168.1.25:51000 and returns replies there.

Common mistakes

• Treating NAT as a security feature. It is not a firewall, even though it blocks unsolicited inbound traffic by side effect.
• Chaining two NAT devices (double NAT), which breaks port forwarding.
• Expecting a server behind NAT to be reachable without a forwarding rule.

Related

• Port forwarding
• CGNAT
• IP address
• Router

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