Derek Fawcus wrote: >On Wed, Mar 30, 2005 at 11:00:08AM -0700, Michael Zappe wrote: > > >>As to question 4, nope. The OS has no good way of determining a new MAC >>address to override to (The OS doesn't have an OUI, and then how do you >>pick the serial number after that??), >> >> > >Pick a random number, set the local allocation bit (second bit on wire) >and then test the address to see if anyone responds to it. > >Didn't DECnet do something like that? > > > Yeah, but you're not guaranteed a reply to the packet. It's a distinct possibility that if you pick an address and a protocol, the other machines stack may disregard it, i.e. blackholing pings, etc. There's no good way to detect the collision. Theoretically you could use LLC to check, but I wouldn't count on that being implemented correctly, if at all... :-) If you're using locally administered addresses, you can assume that valid NICs aren't going to have this address, and the chance of a collision is low. (N in 16 million for N machines with the same OUI.) So I guess that would work! :-)
DECnet actually had a deterministic algorithm for assigning MAC addresses. You had an area number that was 6 bits, and a 10 bit node number. Then you appended this to AA:00:04:00 to get the MAC address. Multiple interfaces on the same machine all used the same MAC. >DF > > > >>and it's not considered a real >>problem because of the uniqueness of MACs. Not to mention you don't >>even want to think about how a switch could get confused by the whole >>situation... :-) >> >> Mike >> >>
