Problem with some MACs

Piotrek S. komuch at
Tue Jan 4 14:55:22 UTC 2011

W dniu 11-01-04 14:13, Chuck Anderson pisze:
> 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.
They are not set via software. They are hardware adresses (the first one
is Asrock motherboard and second is motherboard too but i don't know
brand name).

> "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."
> _______________________________________________
> dhcp-users mailing list
> dhcp-users at

More information about the dhcp-users mailing list