SDN became the hottest, someone saying it would subvert  network.

So, What is SDN? Is it something we’ve never seen before?

I just started learning SDN, knowledge is not too much, my reply may be not correct. We can learn together.

SDN is a form of network "softwarization" that enhances network programmability, a network architecture redefinition, rather than a new feature or function. Compared to the original network architecture, SDN achieves a variety of functions, better, faster and in a much simpler way.