Every Ethernet port has a unique number hard-coded into it. This number is usually referred to as the port's MAC address though Apple choose to refer to it as the Ethernet ID.
Please don't confuse MAC in this context with Macintosh as it is totally unrelated and only a means of identifying your ethernet port (like a car registration number uniquely identifies a car).
To find out the MAC address of your Ethernet port:

The 12 digit hexadecimal number of the form 00:03:93:78:55:f0 is your Ethernet port's MAC address.