When a critical legacy server is segmented into a private network, the security control being used is compensating. Compensating controls are alternative measures put in place to satisfy a security requirement when the primary control is not feasible or practical. In this case, segmenting the legacy server into a private network serves as a compensating control to protect it from potential vulnerabilities that cannot be mitigated directly.
Compensating: Provides an alternative method to achieve the desired security outcome when the primary control is not possible.
Deterrent: Aims to discourage potential attackers but does not directly address segmentation.
Corrective: Used to correct or mitigate the impact of an incident after it has occurred.
Preventive: Aims to prevent security incidents but is not specific to the context of segmentation.