Which of the following layers does IPsec protect?

• A. The transport layer
• B. The application layer
• C. Both of the above
• D. Neither of the above

Answer: (D)

IPsec is a network-layer security protocol. It protects IP packets as they traverse networks, providing confidentiality, integrity, and authentication at the IP layer. In transport mode, it safeguards the payload within the IP packet (the transport-layer segment) while leaving the outer IP header exposed; in tunnel mode, it encapsulates the entire original IP packet, protecting the inner contents. Because its protection scope is the IP packet itself at the network layer, it’s not limited to either the transport layer or the application layer. So, IPsec doesn’t map cleanly to those two layers, making the direct answer that applies here: neither of the two layers.