Why it matters

NAT is why home networks work despite IPv4 exhaustion. Understanding it clarifies connectivity issues and P2P design.

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The architecture

NAT gateway: maps outbound connections. Tracks internal IP:port ↔ external IP:port mapping in state table.

Types: full-cone, restricted-cone, port-restricted, symmetric (varying restrictiveness).

NAT translationPrivate LANmany devicesNAT gatewayIP:port mappingPublic internetfew or one IPNAT traversal: STUN discovers public mapping, TURN relays when direct fails
NAT structure.
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How it works end to end

Outbound: client → NAT → external. NAT maps and tracks. Response uses tracked mapping.

Inbound: unsolicited traffic to public IP has no mapping — dropped.

Port forwarding: manually add inbound mapping. Enables hosting services behind NAT.

NAT traversal: STUN (discover mapping), TURN (relay when direct fails), UPnP (device asks for mapping).