IPsec
A protocol suite that secures IP traffic with authentication and encryption, commonly used in VPNs.
IPsec is a network-layer (Layer 3) protocol suite that authenticates and encrypts IP packets, forming the foundation of most VPN implementations. It relies on two sub-protocols: Authentication Header (AH) handles integrity and origin authentication but provides no encryption, while Encapsulating Security Payload (ESP) delivers integrity plus confidentiality. Because ESP adds encryption and works through NAT where AH typically breaks, ESP alone is used in virtually all real-world deployments.
Tunnel mode wraps the entire original packet inside a new IP packet, concealing internal addressing — the standard choice for gateway-to-gateway VPNs. Transport mode encrypts only the payload while leaving the original IP header exposed, suited to direct host-to-host sessions. On the exam, watch for questions that conflate these modes or that credit AH with providing encryption, which it does not.
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