Which technologies help identify unauthorized devices on a network?

• A. VPN
• B. 802.1X/NAC
• C. IDS
• D. SIEM

Answer: (B)

Identifying unauthorized devices on a network is best accomplished with network access control at the edge, specifically using 802.1X-based NAC. This approach requires a device to authenticate before its port on a switch or access point is allowed to carry traffic. NAC can also perform posture checks, certificate validation, or other criteria, and enforce policies accordingly. If a device isn’t recognized or isn’t compliant, it can be placed in a quarantine VLAN or blocked entirely, preventing it from accessing the rest of the network. This makes unauthorized devices identifiable and controllable right at the point of connection, rather than only after they start sending traffic.

VPN is about secure remote access into a network, not about identifying or enforcing access for devices already on the local network. An IDS looks for signs of malicious activity in traffic, which is useful for detection, but it doesn’t prevent new devices from joining the network or verify their identity. A SIEM aggregates logs and can alert on anomalies, but it doesn’t enforce access controls at the network edge to identify or isolate unauthorized devices.