Intrusion Detection Systems (IDS) are designed to monitor network and system activities for malicious activities or policy violations. A well-configured IDS can detect numerous types of malicious network traffic and computer usage that often go unnoticed by a firewall, which makes it an excellent choice for identifying unauthorized accesses or anomalies. Anti-virus software, while useful for detecting and removing malware, does not generally monitor network traffic for anomalies. Firewalls are a type of preventive control that helps to block unauthorized access based on predefined rules, but they do not detect anomalies after the traffic has passed through. Security training is essential for personnel but does not serve the specific purpose of monitoring network anomalies.