Layer 2

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.

redzilla.cl — multicast

IP —

MAC —

Family —

Destination MAC

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Group type

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MAC collisions

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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.

Runs locally in your browser · no sign-up · nothing leaves your browser

How it works

The tool takes a multicast IP address and computes the destination MAC its Ethernet frames carry. For IPv4 (range 224.0.0.0/4, defined in RFC 1112) it applies the IANA prefix 01:00:5e and copies the low 23 bits of the IP, with bit 24 forced to 0; for IPv6 (range ff00::/8, RFC 2464) it uses the 33:33 prefix plus the low 32 bits of the address. It also identifies the group type: link-local, SSM, GLOP, administratively scoped (239/8), IPv6 scope or Solicited-Node.

Since an IPv4 group has 28 significant bits and only 23 reach the MAC, the top 5 bits are lost: 2^5 = 32 different IP addresses share each MAC. The tool shows a binary breakdown of which bits are kept and which are dropped, and lists the 32 colliding IPs — key to understanding why IGMP snooping can deliver traffic from a neighboring group to a host that never asked for it.

Example: the MAC of 239.255.255.250 (SSDP/UPnP)

The IP 239.255.255.250 is inside 224.0.0.0/4: IPv4 multicast, administratively scoped (239.0.0.0/8).
Take its low 23 bits: 0x7FFFFA, that is 7f:ff:fa.
Prepend the 01:00:5e prefix: the destination MAC is 01:00:5e:7f:ff:fa.
Another 31 IPs share that MAC, among them 224.127.255.250 and 238.255.255.250 (the 5 lost high bits).

Frequently asked questions

How many multicast IPs share the same MAC in IPv4?

Exactly 32. An IPv4 multicast group has 28 variable bits but the 01:00:5e MAC only carries 23, so the top 5 bits are lost and 2^5 = 32 addresses map to the same MAC. In IPv6 the 33:33 prefix carries 32 bits of the address, so the ambiguity is far smaller although it does not fully disappear.

Why do IPv4 multicast MACs start with 01:00:5e?

It is the OUI block IANA reserved for IP multicast over Ethernet, with the least significant bit of the first byte set to 1 (the group-frame flag). Only the lower half of that block is used, which is why 23 bits remain for the IP instead of 24.

What is the difference between 224.0.0.x and 239.x.x.x?

The 224.0.0.0/24 block is link-local control traffic (OSPF, VRRP, IGMP): it travels with TTL 1 and no router forwards it. The 239.0.0.0/8 block is administratively scoped, the multicast equivalent of RFC 1918 private networks: it is used freely inside an organization (IP cameras, internal IPTV, SSDP) and must not leak to the internet.

Does this conversion send my addresses to any server?

No. The whole mapping is computed locally in your browser with bit operations: no IP you type ever leaves your machine, so you can use it with the internal addressing of your network without exposing it.

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Disclaimer

We take great care to keep every tool accurate and review it thoroughly; even so, we can't guarantee it is free of errors or take responsibility for how the results are used. We recommend double-checking anything critical.

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