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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.

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    You are confusing VMs with physical hosts. A physical host needs a physical connection, but virtual hosts need virtual connections. It really sounds like you need a good grounding in networking before you even attempt all the SDN stuff you have been asking about here and other places. You need to understand the network layers, how the network devices work, etc. SDN represents moving the configuration of the network devices to a controller, rather than the traditional control plane in the network device itself.
    – Ron Maupin ♦
    Jan 2, 2019 at 4:20
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    You really need to understand the network layers. A router (layer-3) really has nothing to do with ethernet (layer-2). Routers route traffic between different networks, and they can have any number of interfaces with different layer-1/2 protocols (ethernet, ATM, PPP, Wi-Fi, frame relay, HDLC, etc.). Switches (bridges) bridge frames on the same layer-2 LAN. There are some very good books that you should read.
    – Ron Maupin ♦
    Jan 2, 2019 at 4:28

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which leaves me wondering what the physical requirement is in between an ethernet chord and the server

The server (or host) needs to interface with Ethernet (or whatever is used), so at least one NIC is required.

Once an Ethernet frame or an IP packet inside the frame has been received by the NIC it is processed in software. It can be forwarded to a virtual machine, or processed in any other way and forwarded back to the physical world out of the same or another NIC, including forwarding on the L2 (switching/bridging) or L3 (routing) level.

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