What is SDN?

Software-Defined Networking (SDN) can be best understood as a set of techniques that fundamentally change how networks are constructed, controlled, and managed. This conceptual framework separates the control plane (which dictates how data packets are routed) from the data plane (which carries the data payload), enabling more efficient and flexible network management. This abstraction allows for a more programmable and agile network environment, making it easier to adapt to changing business needs and technological advances.

Option A captures this essence by emphasizing that SDN is about controlling and managing networks rather than being limited to a specific technology or product set. This flexibility means that different SDN implementations can emerge, tailored to various environments and use cases.

The other options suggest more rigid definitions. For instance, stating that a network is defined solely by its use of a controller oversimplifies the broader intentions and methodologies of SDN. Similarly, equating SDN to a virtual appliance or a set of purchasable products limits its scope and overlooks its conceptual foundations. Understanding SDN as a comprehensive set of techniques provides a richer perspective on its role in contemporary networking.