Exploring how to implement a Software Defined Network and here it explains that an SDN switch is only dealing with the data layer. But then:
Do We Need Special SDN Switches for SDN Networking?
Special SDN data switches are not required to implement software-defined networks. SDN architectures will take many different approaches as they mature and change, and each technology vendor will have a unique take on how to implement SDN. For example, VMware, with its acquisition of Nicira, will implement SDN protocols in its Switch software, which does not require specialized hardware or a controller.
So this makes it sound like you can implement the switch virtually (in software on a server), which leaves me wondering what the physical requirement is in between an ethernet chord and the server, if there is anything else in between such as a physical router or any other physical device, or if it's just ethernet straight into the server, and the server implements the network virtualization / SDN.