Messages in this thread | | | Date | Sun, 04 Dec 2005 20:57:35 -0800 | From | "H. Peter Anvin" <> | Subject | Re: virtual interface mac adress |
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Mark Rustad wrote: > > Theoretically that is true, however there are usages that have been > approved that violate that principal. One was for TI Token Ring chips. > They were completely unable to use "global" MAC addresses - the local > bit always had to be set. Since TI could/would not fix their chips, > using the local address became allowed for a universally unique address. > > This method was later used by Apple on Ethernet for their DOS card. The > Macintosh environment would get the global address and the DOS card > would get the local one through the shared ethernet port. You might > think that you can ignore the token ring case, but you'd be wrong - > there are ethernet/token ring bridges deployed. The Apple case is also > best not ignored. I don't know how many others may be doing similar > things. > > So, I would not advise anyone to simply believe that they can use the > entire local MAC address space safely. You are also very likely to have > trouble if there is any DECnet usage in the area. Anyone else notice > that DECnet kernel patch recently? Someone must still be using it... > > This is an instance where Linus' comment a few weeks ago regarding > specs vs. reality comes into play. This is kind of an obscure area so > not a whole lot of people know about some of these things. Don't > believe everything you read in magazines regarding MAC addresses > either. I've seen some very bad advice there from time to time in this > particular area. > > I would recommend using the same MAC address with the local bit set (as > Apple did) for a single additional address. If you need more addresses > and need them to be visible on the LAN, I don't know of a reliable, > generic solution off the top of my head. >
By definition, using local addresses is probabilistic. There are moron hardware manufacturers, as you show above (which aren't even the worst of the lot, from what I've seen), but your cross-section with other local-address users will be very small (collision only likely around 2^23 nodes.)
Reducing the address space available to randomly pick from will only increase the likelihood of failure.
This is an instance where an understanding of statistics come into play.
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