Which of the following devices should be installed on a network to monitor the presence of and actively defend against attacks?

An IPS is deployed inline in the network traffic path, allowing it to detect attack signatures or anomalous behavior and immediately take action - such as dropping packets, resetting connections, or blocking source IPs - to prevent the attack from succeeding. This active, inline enforcement capability distinguishes it from passive monitoring tools.

A protocol analyzer (packet sniffer) captures and decodes network traffic for analysis but takes no defensive action against attacks.

A traffic shaper manages and prioritizes bandwidth allocation and Quality of Service but has no capability to detect or prevent attacks.

An IDS monitors traffic and generates alerts about suspicious activity but does not actively block or prevent attacks - it is passive.