Re: [9fans] unique MAC address allocation?

[Michael Zappe <zapman@zappe.us>]

fgergo@gmail.com wrote:

>Not really plan9 specific, but
>1. does anybody know if and how MAC addresses are allocated to network
>card manufacturers?
>2. do manufacturers guarantee the uniqueness of MAC addresses they build?
>3. is it still true (was it ever true), that if I bought 2 NIC's,
>they'll have different MAC addresses?
>4. is it the responsibility of the OS to override the default MAC
>address if another card with the same (possibly overriden) MAC address
>was present on the LAN?
>
> thanks: gergo
>
>
Well, since my company makes network cards, I can probably help you
here! ;-)  Each vendor registers one or more 3 octet OUIs  from the
IEEE, which is used as the first 3 octets of a MAC address.  The
remaining 3 are up to the vendor to use.  Theoretically, all MAC
addresses are supposed to be unique.  For one vendor code you only have
16 million addresses available, so there is the possibility of wrap
around with MAC addresses -- it's up to the vendor to comply.  (Once you
have a practical deployed base of 90%, you can request a new OUI.) For
all practical purposes, you can assume that MAC addresses are unique.

For this part here is a good tutorial:
http://standards.ieee.org/regauth/oui/tutorials/lanman.html

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??), 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