Which statement best describes the function of a firewall in a network?

• A. To physically isolate networks.
• B. To route traffic between networks.
• C. To perform network address translation.
• D. To filter traffic between networks based on security rules, protecting against unauthorized access and threats.

Answer: (D)

Firewalls sit at the boundary between networks to enforce security rules that decide which traffic is allowed to pass. They inspect traffic against predefined policies—checking factors like where it’s coming from, where it’s going, what protocol or port is used, and whether the connection is in a valid state—and then permit or block it accordingly. This filtering capability directly protects against unauthorized access and threats by preventing risky or unwanted traffic from crossing into protected networks. Other functions like physically isolating networks, routing traffic, or performing network address translation describe different roles: isolation is about separating networks, routing moves data between networks, and NAT changes address information. While a firewall can include these capabilities, its core description here is the ability to filter traffic based on security rules to safeguard the network.