How to "listen" MAC address in ethernet network?

Question

When we send data to some server on the Internet, we specify its IP and port. If this server, for example, is written in C++, then sockets will be used on the server. But how does it work if we use Ethernet instead of the Internet? How can one computer understand that another computer sent data to it inside the ethernet network? Indeed, in this case, we do not use the IP and port to send data to the recipient, only the MAC address. How can you write a C++ program that "listens" to a MAC address and a client that can send data to the MAC address in C++? I don't fully understand Ethernet use cases in real world

Answer 1

Ethernet headers have an Ethertype field that tells ethernet to which process it should send the frame payload. You will need to establish a process and register an Ethertype number in the OS for that process in order to receive the payload of the frames with the Ethertype you are using.

You cannot just randomly choose an Ethertype number. You either reserve one with the IEEE, or you use one in an experimental range, e.g. 0x0101 to 0x01FF. There are several lists of currently reserved Ethertypes. IANA has Ethertypes, and it has information and links on how to reserve an Ethertype number and other lists.

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Also, remember that you will not be able to use IP or any of the standard protocols (transport or application) above IP that depend on IP unless you write your own. Transport protocols, such as TCP in your OS, need IP for the required pseudo-header. You will need to write custom transport-layer protocol(s), applications, and application-layer protocol(s) that you may require to use your new custom protocol using your Ethertype.

Existing transport protocols, e.g. TCP, will not use your custom Ethertype protocol, and any existing application that use existing transport-layer protocols will also not work with your custom protocol.