Which of the following firewall types operates at the Network layer of the OSI model and can filter data by port, interface address, source address, and destination address?

Question

Options

• ACircuit-level gateway
• BApplication gateway
• CProxy server
• DPacket Filtering

• B
  2% (1)
• C
  5% (2)
• D
  93% (38)

Explanation

Packet filtering (D) operates at the Network layer (Layer 3) of the OSI model, examining individual packets against a rule set based on source/destination IP addresses, ports, and interfaces - exactly the criteria listed in the question.

Why the distractors are wrong:

• A. Circuit-level gateway works at the Session layer (Layer 5), validating TCP handshakes without inspecting packet contents.
• B. Application gateway operates at the Application layer (Layer 7), filtering traffic based on specific application protocols (HTTP, FTP, etc.).
• C. Proxy server also functions at the Application layer, acting as an intermediary that makes requests on behalf of clients - it's a service, not a filtering mechanism tied to the Network layer.

Memory tip: Think "Packet = Post office sorting" - a packet filter just reads the address label (IP/port) on each envelope without opening it, which is exactly what Layer 3 does. If the firewall looks inside the envelope at the content, it's working at a higher layer.

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