Quiz Entry - updated: 2026.07.14
What is a multicast MAC address and what are the reserved prefixes for IPv4 and IPv6 multicast?
A multicast MAC targets a group of devices. IPv4 multicast maps to the 01-00-5E prefix and IPv6 multicast to the 33-33 prefix. By default switches flood it like a broadcast and routers don't forward it.
Multicast MAC Address: Sent to a group of devices that belong to the same multicast group.
Reserved Multicast MAC Prefixes:
| Protocol | MAC Prefix | Example |
|---|---|---|
| IPv4 Multicast | 01-00-5E |
01-00-5E-xx-xx-xx |
| IPv6 Multicast | 33-33 |
33-33-xx-xx-xx-xx |
Behavior on switches:
- Flooded out all switch ports except the incoming port
- Unless the switch is configured for multicast snooping
- NOT forwarded by a router unless specifically configured
Key insight: The source address is always unicast. Only the destination can be multicast. Multicast saves bandwidth compared to sending separate unicast frames to each group member.
Examples: Routing protocol updates (OSPF, EIGRP), streaming media, online gaming.
Go deeper:
Multicast address (Wikipedia) — how IPv4 multicast maps into the 01-00-5E MAC block and IPv6 into the 33-33 block, with worked examples.