> Date: Mon, 3 Jan 2011 08:45:54 +1030 > From: Mark Smith <na...@85d5b20a518b8f6864949bd940457dc124746ddc.nosense.org> > > Hi, > > On Sun, 2 Jan 2011 08:50:42 -0500 > Steven Bellovin <s...@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 <br...@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: ober...@es.net Phone: +1 510 486-8634 Key fingerprint:059B 2DDF 031C 9BA3 14A4 EADA 927D EBB3 987B 3751