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I'm very new to computer networking and I'm trying to figure out this dilemma about the Unicast, Multicast and Broadcast transmissions. I was told that MAC address are unique for each device and they don't change. So let's say my MAC address is 6C F0 49 E5 25 AA

Translating the first octet 6C = 0110 1100 The least significant bit is 0.

Does this mean that my computer allows unicast communications but not multicast? So each device depending on the least significant bit of their MAC address, can either allow unicast or multicast communications? How does it work with broadcast then? I'm sorry, but I'm really confused and I want to learn.

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    – Ron Maupin ♦
    Commented Dec 17, 2020 at 21:13

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Does this mean that my computer allows unicast communications but not multicast?

That indicates that your NIC's MAC is a unicast address - that's what it's supposed to be. The NIC also accepted broadcast and those multicasts that it is subscribed to - regardless of its hardware MAC.

Think of the hardware MAC of the NIC's default address. It only accepts frames that are destined to that unicast address, or to a broadcast/multicast address, indicated by a set bit 0 in the first octet.

A NIC can also be programmed by the driver to accept other unicast addresses, or even be reprogrammed to ignore its own hardware address.

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  • So, I understand that every device has for sure a MAC address that allows unicast transmission. But I still don't understand how the multicast comes into play. How the switch happens from 0 to 1 as less significant bit. I'm sorry if it sounds stupid. I'm completely new to this subject and I'm trying to learn the best I can.
    – Ludovica Stile
    Commented Sep 10, 2020 at 21:29
  • "I understand that every device has for sure a MAC address that allows unicast transmission." @LudovicaStile, that is not strictly true. MAC addresses are assigned to interfaces, not devices, and not all interfaces support MAC addressing. A PC could have two or more ethernet interfaces, and each of those interfaces on that single device will have its own MAC address. Interfaces that are for other types of protocols than the IEEE LAN protocols will not use MAC addresses. For example, frame relay uses DLCI, ATM uses VPI/VCI, and something like PPP uses no addressing.
    – Ron Maupin ♦
    Commented Sep 10, 2020 at 21:33
  • Ok. I think my mistake was assuming that the MAC address is like the serial number of the computer. Without considering that each transmission can have its own MAC address. I think I have a clearer view now. Thank you
    – Ludovica Stile
    Commented Sep 10, 2020 at 21:47
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I was told that MAC address are unique for each device and they don't change.

That is partially true. MAC addresses need to be unique only on the same LAN, and it is pretty easy to change the MAC address of most devices.

Does this mean that my computer allows unicast communications but not multicast? So each device depending on the least significant bit of their MAC address, can either allow unicast or multicast communications? How does it work with broadcast then?

The MAC address assigned to an interface must be a unicast MAC address because it is used as the source address for frames sent from the device, and a source MAC address must be a unicast address.

Multicast (and broadcast) MAC addresses are destination addresses. Every host must allow frames destined to the broadcast MAC address, regardless of the unicast MAC address assigned to the interface. Hosts interested in multicast traffic will subscribe to one or more multicast groups, and traffic destined to those groups will be allowed in the interface, too.

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  • So, if I understand correctly, every device has multiple MAC address? The one assigned to the interface by the OUI (unicast) and another for multicast and broadcast? I'm sorry if it sounds stupid, but this is totally new to me and I'm trying to learn. I appreciate your help very much.
    – Ludovica Stile
    Commented Sep 10, 2020 at 21:23
  • No, each interface (only for protocols that use MAC addressing, not all interfaces do; only the IEEE protocols use MAC addressing), will have an assigned (either the burned-in address, or one configured to override) unicast MAC address. All interfaces are required to recognized and allow broadcasts into the interface. An application wishing to use multicast will subscribe to a multicast group, and it will configure a hash table which the interface uses to determine if it should allow a frame with a multicast address into the interface.
    – Ron Maupin ♦
    Commented Sep 10, 2020 at 21:28
  • So for example let's say that my computer wants to send data to you and another person's computer. Your computer and the other person's computer will subscribe in a Multicast Group with its own MAC address to receive the content. The MAC address of this group in order to be multicast will have its less significant bit set on 1. And the hardware MAC address of your computer and the other person's computer won't be changed.
    – Ludovica Stile
    Commented Sep 10, 2020 at 21:38
  • IP multicast groups get translated to MAC multicast groups, so the hosts wishing to receive traffic from a multicast group will subscribe to the IP multicast group that gets translated to a MAC multicast group, which gets added to the hash table the interface uses to determine if it is interested in the multicast frames. The sender simply sets the destination MAC address to the multicast MAC address.
    – Ron Maupin ♦
    Commented Sep 10, 2020 at 21:44
  • I think it makes sense now. My explanation in the previous message was very simplified but I think I got it correctly and your explanation fills the holes. Thank you :)
    – Ludovica Stile
    Commented Sep 10, 2020 at 21:48

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