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SubjectRe: virtual interface mac adress
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.

-hpa
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