Learn in the textbookConnecting Devices That Make Up a Network
DefinitionSDN (Software-Defined Networking) is an approach that hands the route control of network devices over to software in one place, so the whole network's configuration can be changed centrally.
ExplanationA network device's work can be split into control, which decides where data goes, and forwarding, which actually sends it out. SDN gathers control into software called a controller (コントローラ), and each device handles forwarding as it instructs.
AnalogyInstead of adjusting the lights at each intersection one by one, a traffic control center switches every signal in the city at once.