What is a multicast MAC address?

A multicast MAC address is a group MAC address used to deliver a frame to a group of devices at once, all of which are subscribed to that group.

Put simply: multicast = "one → several, but not all".

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#How multicast works at the data link layer

1. The sender specifies a multicast MAC address in the frame
2. The frame is received only by devices that are "listening" to that group
3. Other devices ignore the frame

This is useful when:

• the same message needs to be delivered to several recipients
• but a "send to everyone" (broadcast) is not needed

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#How to identify a multicast MAC address

A MAC address has the I/G (Individual / Group) bit:

• 0 → unicast
• 1 → multicast

This is the least significant bit of the first byte of the MAC address.

#Example of a multicast MAC

01:00:5E:00:00:01

• the first byte 01 → 00000001
• the least significant bit = 1 → multicast

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#Where multicast MAC addresses are used

Multicast is most often used for:

• routing protocols
• streaming video and audio
• service network protocols

Examples:

• ARP? no (this is broadcast)
• IPv4 Multicast: yes
• IPv6: yes (uses multicast instead of broadcast)

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#How multicast differs from other types

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#What switches do with multicast

By default:

• a switch may flood multicast the same way as broadcast

With additional configuration:

• the switch knows which ports are subscribed to the group
• and sends frames only there

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#Simple analogy

A messenger chat:

• the message is seen only by group members
• others are not

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#Short version for an interview

A multicast MAC address is a group MAC address used to deliver frames to a group of devices subscribed to that address.