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I need help in understanding how Unicast and Multicast MAC address is differentiated as explained in wiki. According to what I understood, when the least significant bit is set as 0 in the first octet of the mac address, then it is a Unicast address and if the least significant bit is set as 1, then the address is Multicast or Broadcast.

Since the first Octet is a part of OUI (Organizationally unique identifier ) which uniquely identifies a vendor/organization and assigned by IEEE, how can the least significant bit be changed in order to differentiate a unicast or multicast address

Let us consider the following two OUI's:

00-27-C7 -> Organization A

  • Has 0 set in the least significant byte - 00 -> 0000 0000

A9-5E-4C -> Organization B

  • Has 1 set in the least significant byte - A9 -> 1010 1001

-> Does this mean that organization B can not have any unicast MAC address?

-> Is my understanding correct?

  • Remember that organizations can get more than one OUI. Cisco was originally assigned 00-00-0c (thec for Cisco), but now has many, many OUIs. Cisco uses 01-00-0c, the original OUI with the I/G bit set for multicast purposes, e.g. CDP (01-00-0c-cc-cc-cc). – Ron Maupin Jul 5 at 0:56
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OUI's are assigned. That bit is only set for assigned broadcast and multicast addresses. No organization will be given an OUI with that bit set.

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