> Yes, its mandated by the original blue-book DIX spec.  It dates back
> to XNS, routing at the Ethernet layer, and the school that says all
> hosts should have one network address: if you have multiple MACs you
> reprogram them so that they all have the *same* MAC address.
> 
> Some protcols (phase IV decnet) still require setting all NICs on a
> host to use one MAC address, computed from the layer-3 node address.
> (which is why SunOS/Solaris acquired the feature in the first place.)

Another comment on this, from Rich Seifert, who should know (see
<seifert-ya023060042211981457470001@nntp.ix.netcom.com>):

> > The first guy was right.  MAC addresses are burned in the ROM of the NIC
> > at manufacture, and cannot be changed without replacement of the ROM.
> > 
> 
> Well, I can't remember who was the first guy and who was the second. but
> your conclusion is incorrect. While it is true that NICs generally have an
> address burned into ROM, it is not automatic that this becomes the address
> used for either destination address checking (on receive) or for source
> address generation (on transmit). Every NIC chip that I have ever seen has
> a register (sometimes more than one) for loading the NIC address(es). This
> address MAY be the same as that in the ROM, or it may be different. Some
> protocols (e.g., DECnet) require that the MAC address be changed to map to
> the network layer address--this avoids the need for an ARP-like protocol in
> DECnet.

Steinar Haug, Nethelp consulting, sthaug@nethelp.no