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Networking Fundamentals · Intermediate

NAT Traversal (NAT-T)

Techniques that let protocols which embed addresses or lack ports, such as IPsec and VoIP, work through NAT devices.

Definition

IPsec NAT-T encapsulates ESP in UDP port 4500 so PAT devices can track it. Peer-to-peer and VoIP applications use STUN, TURN and ICE to discover public mappings and relay traffic when direct paths fail. Application Layer Gateways on firewalls rewrite embedded addresses for protocols such as SIP and FTP.

Example

An IPsec VPN client behind a home router switches from UDP 500 to UDP 4500 when NAT is detected.

Exam tip

IPsec NAT-T uses UDP 4500; IKE uses UDP 500.

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