On May 24, 2014, at 10:15 AM, Johnny Billquist <bqt at softjar.se> wrote: ... Do I need to spell it out? :-) The hardware address is the address the card have from factory. The physical address is the address the software have programmed the card to have. Since DECnet uses specific addresses, the address is changed from the hardware address, since you do not want/need that when running DECnet. Not necessarily exactly that way. The hardware address is the default physical address. It is supposed to be globally unique (not just unique on each LAN). If you have virtual devices, like in SIMH, chances are you re responsible for this (you re in essence the manufacturer). Pedantically, if you administer MAC addresses, they should be from the locally administered address space, i.e., second bit set in the 1st byte. In practice that doesn t matter, but it avoids conflict with real hardware addresses. DECnet Phase IV uses a physical address it supplies rather than the default. Other protocols (including DECnet Phase V) don t. If your NIC type (or its driver) supports only a single physical address, the physical address changes for all protocols when you turn on DECnet Phase IV. That s why you have to turn on DECnet before LAT. However... if your NIC and driver allow per-protocol physical address, then only DECnet Phase IV uses the aa-00-04-00 address and the others continue to use the hardware address. For such systems, you have to be careful that the hardware address is unique even if DECnet is used. Most newer DEC NICs (Tulip and beyond) support multiple physical addresses, as does QNA. UNA and LANCE do not. Whether a particular OS/driver implements that is another matter. paul