Switches only know MACs, never IPs — so multicast frames without help flood like broadcast. L2 multicast is therefore about MAC mapping plus IGMP snooping.

MAC mapping

• IPv4 multicast → 0100.5E00.0000–0100.5E7F.FFFF (23 host bits: 3 IP bits collapse per MAC — overlapping groups are expected)
• IPv6 multicast → 33:33:xx:xx:xx:xx

Default switch behaviour: flood. Fix: IGMP snooping — see the dedicated post. Config: ip igmp snooping, verify with show ip igmp snooping groups.

Exam reflex: 0100.5E → IPv4 multicast, 33:33 → IPv6 multicast, flooding multicast → snooping disabled.