Which of the following types of firewall operates at the Network layer to examine incoming and outgoing packets?

The correct answer is C, Packet filtering. A packet filtering firewall works at the network layer, the third layer of the OSI model, where it reads the header of every incoming and outgoing packet and compares the source and destination IP address, the port number and the protocol against a list of rules, then allows the packet through or drops it. It is the oldest and fastest kind of firewall and needs little memory, but it is stateless: it judges each packet on its own and cannot look inside the data, so it misses attacks hidden in the payload. Option A is wrong because a circuit-level gateway works at the session layer and only checks that the TCP handshake is valid. Option B is wrong because an application-level gateway, or proxy firewall, works at the application layer and understands protocols such as HTTP and FTP. Option D is wrong because network filtering is not a recognised firewall type. Exam tip: packet filter at the network layer, circuit-level at the session layer, proxy at the application layer.