about simple_socket_server

The least significant bit of the first address byte of an Ethernet address (network byte order) is set for Ethernet group addresses, and is zero for individual station addresses, so the first byte of your address must be even. The next least significant bit is the global/local bit. It is zero for globally unique addresses assigned through the IEEE, and 1 for addresses which are locally assigned (in which case it is up to the local network manager to ensure uniqueness within a given network).

So, for testing purposes, it is perfectly kosher to use an individual address with the local bit set. In conventional notation (network byte order), these addresses are xY:xx:xx:xx:xx:xx where x is any hex digit, and Y is one of (2, 6, A, or E). These local addresses will never conflict with a globally unique address such as those on virtually all commercial Ethernet equipment. You only have to worry about conflicts with other prototype equipment on your local Ethernet segment using a local address. Since you still have 44 bits of address that you are completely free to choose, you should be able to avoid these.

Of course, if you ever intend to sell your product commercially, you'll want to acquire a block of globally unique addresses from the IEEE and use these.