Which protocol is a network-layer security protocol used for VPNs?

• A. IPsec
• B. SSH
• C. SSL/TLS
• D. PKI

Answer: (A)

IPsec protects traffic at the network layer, securing IP packets as they cross untrusted networks like the Internet. It provides confidentiality, integrity, and authentication by using two main mechanisms: ESP for encryption and optional authentication, and AH for authentication and integrity without encryption. For VPNs, IPsec typically operates in tunnel mode, encapsulating entire IP packets to connect networks or remote clients securely, with IKE handling the negotiation of security associations and keys. This combination makes IPsec the standard for network-layer VPNs, enabling secure site-to-site and remote-access connections.

SSH is for secure remote login and other application-layer tasks. SSL/TLS secures transport- or application-layer sessions (e.g., web browsers), not the raw network-layer traffic. PKI is a framework for issuing and managing digital certificates and keys, not a protocol that itself protects traffic.