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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?

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  • 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 ♦
    Commented Jul 5, 2020 at 0:56
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin ♦
    Commented Dec 17, 2020 at 18:52

1 Answer 1

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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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