Work out multicast mac mapping instantly with clear inputs, formula shown and shareable results.
IPv4 multicast frames use the 01:00:5e OUI with the low 23 bits of the group address copied into the remaining bits. Because a group has 28 significant bits, five are discarded and 32 different groups collide on one MAC — so a NIC filtering only on MAC may hand unwanted groups to the kernel. IPv6 uses 33:33 followed by the low 32 bits of the group.
IPv4 group to MAC
MAC = 01:00:5e:(bit23..bit16 of group with top bit cleared):(bit15..8):(bit7..0)
IPv6 group to MAC
MAC = 33:33:(last four bytes of the IPv6 group address)
Only 23 of the 28 group bits fit in the reserved MAC space, so groups differing solely in those five high bits produce identical destination MACs.
It can. A host subscribed to one group may receive frames for an aliased group and burn CPU discarding them at the IP layer, which matters on high-rate feeds.