What is the primary function of a firewall in network security?

• A. To encrypt data traveling across networks.
• B. To filter and monitor traffic between networks according to defined rules.
• C. To route traffic based on IP addresses.
• D. To provide end-user authentication.

Answer: (B)

Firewalls filter and monitor traffic between networks based on defined rules. They act as a gatekeeper at the network boundary, inspecting data packets or connection flows and deciding whether to allow, block, or log them according to policy. This rule-driven control can rely on factors like source and destination addresses, ports, protocols, and sometimes the state of a connection, with stateful firewalls remembering ongoing conversations to enforce legitimate sessions. By doing so, they prevent unauthorized access while still permitting legitimate traffic and providing visibility through logs for monitoring and auditing. Other functions like encryption protect data contents, routing determines the path a packet takes, and authentication verifies user identity, but none of these primarily decide network access in the boundary where a firewall operates.