To answer this question, you need to understand the concept of network-based intrusion detection systems (NIDSs) and their optimal placement.
Option A) On the network perimeter, to alert the network administrator of all attack attempts - This option is incorrect. Placing NIDSs on the network perimeter can provide visibility into external attacks, but it may not provide sufficient coverage for attacks originating from within the network.
Option B) On network segments with business-critical systems - This option is correct. Placing NIDSs on network segments with business-critical systems allows for targeted monitoring and detection of suspicious activities and potential attacks on these critical systems. This placement ensures that the NIDSs focus on protecting the most important assets within the network.
Option C) At the network operations center (NOC) - This option is incorrect. Placing NIDSs at the network operations center may provide visibility into network-wide activities but may not offer the same level of targeted protection for specific business-critical systems.
Option D) At an external service provider - This option is incorrect. Placing NIDSs at an external service provider may provide some level of protection, but it may not be as effective as having them directly placed on the network segments with business-critical systems.
The correct answer is B. This option is correct because placing NIDSs on network segments with business-critical systems ensures targeted monitoring and detection of potential attacks on these critical systems.