Suggest an editImprove this articleRefine the answer for “What is a multicast MAC address?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)**A multicast MAC address** is a group MAC address used to deliver a frame to a group of devices at once, subscribed to that group. **Key point:** multicast is identified by the least significant bit of the first byte of the MAC address - if it is 1, the address is multicast.Shown above the full answer for quick recall.Answer (EN)Image**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"**. --- ## 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 --- ## 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** --- ## 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) --- ## How multicast differs from other types | Type | Who it is sent to | |---|---| | Unicast | one device | | **Multicast** | a group of devices | | Broadcast | all devices | --- ## 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 --- ## Simple analogy A messenger chat: - the message is seen **only by group members** - others are not --- ## 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.For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.