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Re: Eth%d keeps incrementing everytime the system boots.

On Wed, Nov 21, 2007 at 10:19:30PM -0500, Douglas A. Tutty wrote:
> Silly me, but I thought that MAC addresses were actuall hard-coded into
> the port.  I thought that some site security models used known MAC
> address to decide not what IP to assign with DHCP what if to assign at
> all.  If the hardware is allowing the MAC to be reassigned, then there
> goes that idea.

The hardware is supposed to have a fixed MAC address that it uses as
default.  You can always overrride it (well on most devices you can)
with whatever you want.  The default however should stay fixed unless
you want to drive the user nuts.  For onboard network chips like the
nvidia it is usually set by the system bios, just like the motherboard
serial number, firewire MAC, etc.

> What happens if two boxes on a network segment end up with the same MAC
> address?

You get problems since switches cache which port has the device with a
specific MAC and only sends the packets that way, so the connection
would bounce between the ports for the two boxes and neither would be
very happy.  That's why every maker of ethernet devices is assigned
blocks of MAC addresses to use so that all their devices have unique
addresses and no two manufacturers can ever use the same MAC address
block, so yo uend up with unique devices.

Len Sorensen

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