The tale of a single MAC

Tue Jan 4 05:00:53 UTC 2011

> Date: Mon, 3 Jan 2011 08:45:54 +1030
> From: Mark Smith <nanog at 85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org>
> 
> Hi,
> 
> On Sun, 2 Jan 2011 08:50:42 -0500
> Steven Bellovin <smb at cs.columbia.edu> wrote:
> 
> > 
> > On Jan 1, 2011, at 11:33 24PM, Mark Smith wrote:
> > 
> > > On Sat, 01 Jan 2011 20:59:16 -0700
> > > Brielle Bruns <bruns at 2mbit.com> wrote:
> > > 
> > >> On 1/1/11 8:33 PM, Graham Wooden wrote:
> 
> <snip>
> 
> > >> 
> > >> Excellent example is, IIRC, the older sparc stuff, where the ethernet 
> > >> cards didn't have MAC addresses as part of the card, but were stored in 
> > >> non-volatile or battery backed memory.
> > > 
> > > This was actually the intended way to use "MAC" addresses, to used as
> > > host addresses rather than as individual interface addresses, according
> > > to the following paper -
> > > 
> > > "48-bit Absolute Internet and Ethernet Host Numbers"
> > > Yogan K. Dalal and Robert S. Printis, July 1981
> > > http://ethernethistory.typepad.com/papers/HostNumbers.pdf
> > 
> > Yup.
> > > 
> > > That paper also discusses why 48 bits were chosen as the size, despite
> > > "Ethernet systems" being limited to 1024 hosts. 
> > > 
> > > I think things evolved into MAC per NIC because when add-in NICs
> > > were invented there wasn't any appropriate non-volatile storage on the
> > > host to store the address. 
> > > 
> > On really old Sun gear, the MAC address was stored on a separate ROM chip; if the
> > motherboard was replaced, you'd just move the ROM chip to the new board.
> > 
> > I'm not sure what you mean, though, when you say "when add-in NICs were
> > invented" -- the Ethernet cards I used in 1982 plugged into Unibus slots
> > on our VAXen, so that goes back quite a ways...
> > 
> 
> More that as add-in cards supplied their own "storage" for the MAC
> address, rather than expecting it from the host (e.g. something like
> MAC addresses set by init scripts at boot or the ROM chip you
> mentioned on Suns), this has now evolved into an expected model of a
> MAC address tightly bound to an Ethernet interface and supplied by the
> Ethernet interface e.g. by an add-in board if one is added. Now that
> this model as been around for a long time, people find it a bit strange
> when MAC addresses aren't as tightly bound to a NIC/Ethernet interface.
> This is all speculation on my part though, I'd be curious if the
> reasons are different.
> 
> When I first read that paper, it was really quite surprising that "MAC"
> addresses were designed to be more general host addresses/identifiers
> that were also to be used as Ethernet addresses. One example they talk
> about is using them as unique host identifiers when sharing files via
> floppy disk.

My Ethernet experience goes back before VAXen and the DEUNA to the
original DIX Ethernet 3Com and InterLAN cards. They had the MAC in a ROM
on the card set. (Yes, they were 2 card sets with a top of the card
ribbon cable between them.) Don't confuse this with the REALLY old
Ethernet V1 3Com and Wang 1 and 10 Mbps Ethernet, which I did not
personally deal with.

I worked with early Ethernet on quite a few systems and the only one I
ever ran into that implemented the single per-system hardware MAC was
Sun, though others (notably Digital, SGI and Xerox) would re-write all
MACs with a single value derived from the network address (DECnet or
XNS) at boot time. I seem to remember that Tektronix systems also did
this before they bought the rights to the CMU TCP-IP stack and moved to
IP.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751