What is SDN? How software-defined networking changed everything

ZDNet 

The phrase software-defined networking (SDN) was coined when it was necessary to distinguish the concept from the hardware-based variety. Since that time, "SDN" has come to mean the type of dynamic configuration that takes place whenever software-based services in a data center network are made accessible through an Internet Protocol (IP) address. More to the point, SDN is networking now. In the broadest sense, any software that manages a network of dynamically assigned addresses -- addresses which represent services provided or functions performed -- is utilizing some variety of SDN. The web gave rise to the idea of addressing a function by way of a name that resolves (represents, as in a directory) to an IP address. Originally, the web was intended to be a system for addressing content in this way, but engineers soon saw how efficient an evolved form of that system could be in addressing functionality.