In the Usenet newsgroup alt.internet.wireless, in article
<7qhlk1tmnkon0jeccv7210j6892kbmts6i@4ax.com>, Chuck wrote:

>James Knott <*email_address_deleted*m> wrote:
>>
>>Chuck wrote:
>>
>>> You absolutely must have a unique MAC address on all networked
>>> devices.

>>The MAC only has to be unique on the local network. It's irrelevant
>>elsewhere.

>What happens if you change it, and cause a conflict with another ISP
>customer?

What James means is that all Network Interface Cards that can talk
_DIRECTLY_ to each other need to be unique. On your LAN, that means
all hosts on "this" side of the router. There is nothing wrong with
having the identical MAC address on the "other" side of the router.
In a Sun workstation, _all_ network interfaces have the same MAC
address (it's determined by a parameter in the equivalent of the
BIOS on the motherboard, _not_ the network card). So, I've got a
Sun box with three network interfaces (to three different subnets)
and they all have the same MAC, though "different" IP addresses like
192.0.2.35/24 on hme0, 198.18.10.35/24 on hme1, and 198.18.24.35 on
hme2.

>Some ISPs that do dynamic addressing hash the MAC address to derive IP
>address.

Cite please. (Honest and true - no one uses a hash of the MAC,
because of the way MAC addresses are allocated, verses the size
of an IP subnet.) Just for jollies, as of the first of this month,
there are 8643 OUI assignments (the top 24 bits of the MAC address).
These are scattered in a semi-haphazard way from 00:00:00: which is
assigned to Xerox, to AC:DE:48: which is assigned to a private
entity. The assignment of the lower 24 bits of the MAC address are
totally at the pleasure of the assignee - if they want to start at
:00:00:00 and increment up or at :FF:FF:FF and work down - or both,
or something else entirely, that's fine with the IEEE. All they care
is that you don't duplicate an address (although that has happened
on a lot more than one occasion).

What you may be thinking is 'pre-assigned' addresses in DHCP, where
the poor sod who has to configure that abomination manually assigned
a MAC:IP address combination in the configuration file. Thus, when a
host with MAC address of 08:00:20:E7:54:8A comes up, the DHCP server
knows that this host is to get "that" address, and no one else.

Old guy