I'm having trouble finding a reference just now, but the MAC address for a modem-based link on Windows had "DESK" until it started causing real trouble. And one of those problems was that GUIDs were obtained by using "the MAC address" of the PC as an input to a DEC-Intel-Xerox-Microsoft algorithm. Around the time of this coming to a head, it came up that some manufacturers were out of MAC addresses for whatever reason, and also that some manufacturers were assuming that two random NICs on an Ethernet would not both be one of $MANUFACTURER's cards, so care was not necessary.