Problem with some MACs

Chuck Anderson cra at WPI.EDU
Tue Jan 4 13:13:04 UTC 2011

On Tue, Jan 04, 2011 at 11:54:23AM +0100, Piotrek S. wrote:
> I noticed "little" problem with some devices in my network.
> They can't obtain IP address from DHCP (ISC 4.2.0).
> So i decided to check logs ... and can't find entries contain the
> physical adresses of these hosts.
> I thought it was a problem with network connection but at the physical
> layer everything was ok.
> I tried to change the physical address:
> d7:00:f4:ae:ef:c5 -> 00:00:f4:ae:ef:c5
> and
> 87:3c:f1:8f:13:00 -> 00:3c:f1:8f:13:00

MAC addresses that begin with an odd-numbered byte are Multicast 
Ethernet addresses.  They can't be used for normal unicast 
communication.  Are those addresses burned into some devices you have, 
or are they set via software?  Under no normal circumstances should a 
Multicast MAC be programmed into a device's hardware or software, nor 
should appear as a Source Address in a frame.

"If the least significant bit of the most significant octet of an 
address is set to 0 (zero), the frame is meant to reach only one 
receiving NIC.[5] This type of transmission is called unicast. A 
unicast frame is transmitted to all nodes within the collision domain, 
which typically ends at the nearest network switch or router. Only the 
node with the matching hardware MAC address will accept the frame; 
network frames with non-matching MAC-addresses are ignored, unless the 
device is in promiscuous mode."

"Ethernet frames with a value of 1 in the least-significant bit of the 
first octet[Note 2] of the destination address are treated as 
multicast frames and are flooded to all points on the network."

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