SDN (Software-Defined Networking) separates the control plane from the data plane and centralises network intelligence in a software controller. You configure network policies centrally and deploy them automatically to all switches and routers. This facilitates segmentation, micro-segmentation, and rapid response to security incidents. In an ISMS, SDN is relevant because it accelerates the implementation of network-security controls and improves consistency. At the same time, the central controller becomes a critical asset that requires strong protection.