Multicast IP ↔ MAC
Paste a multicast IP and I compute its destination MAC (IPv4 01:00:5e + 23 bits; IPv6 33:33 + 32 bits), the IPs sharing that MAC (5-bit ambiguity in IPv4) and the group range/type.
How the MAC is built
IPs sharing this MAC (32)
| # | Multicast IP | MAC |
|---|
In IPv4 the 5 high bits of the group (28 bits) don't fit in the MAC, so 32 different addresses map to the same MAC. IGMP snooping can deliver traffic from neighboring groups to a host that never asked for it.
Multicast ranges · reference
| Range | Type | Use |
|---|
How the mapping works
IPv4 MAC = 01:00:5e +
(IP & 0x7FFFFF) in the low 23 bits. Bit 24 is forced
to 0, so only 23 of the group's 28 bits reach the MAC.
Collision the 5 high bits are lost →
32 IPs (2⁵) share each MAC in IPv4.
IPv6 MAC = 33:33 + the low
32 bits of the address. More bits fit, but overlaps
across distinct blocks are still possible.
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