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Map multicast IPv4 addresses to Ethernet MACs and reverse the 23-bit aliasing.
The Multicast MAC Checker converts between multicast IPv4 addresses and the Ethernet MAC addresses they map to under RFC 1112. Because only the low 23 bits of the IP survive the mapping, 32 different multicast addresses share each MAC — this tool computes the forward mapping and lists every aliased address so you can reason about cross-talk on a shared segment.
Type a multicast IP address (224.0.0.0–239.255.255.255) to compute its MAC and see the 31 other addresses that alias to it, or type a multicast MAC (starting 01:00:5E) to list all 32 candidate IPs. Either field updates the other automatically; no network lookups are required.
Input: Multicast IP or MAC — e.g. 239.5.5.5 or 01:00:5E:05:05:05.
For an IP input, the result shows the mapped MAC (01:00:5E:XX:YY:ZZ where XX is the second octet masked with 0x7F) and the 31 aliased addresses. For a MAC input, the result lists all 32 IPv4 addresses that map to it. Because the mapping preserves only 23 of the IP's 28 significant bits, hosts subscribed to one multicast group also receive frames for any of the 32 addresses sharing its MAC — they discard the unwanted frames in software, which is the aliasing the tool highlights.
→ Find the Ethernet MAC a multicast IP maps to for switch/IGMP config.
→ Identify which 32 IPs alias to a multicast MAC on a shared segment.
→ Verify a multicast address is in the valid 224.0.0.0–239.255.255.255 range.
→ Reason about multicast cross-talk when debugging group reception.
Ethernet multicast MACs encode only the low 23 bits of the 28-bit multicast IP address (the OUI 01:00:5E plus one fixed bit). The 5 lost bits create 2^5 = 32 different IP addresses that all map to the same MAC, so a host subscribed to one group receives frames for all 32 and filters the rest in software.
Take the low 23 bits of the IP address and place them into the low 23 bits of the MAC after the fixed 01:00:5E prefix and a zero bit. Equivalently, the MAC is 01:00:5E: followed by the second IP octet AND 0x7F, then the third and fourth octets unchanged.
No. IPv6 multicast uses the 33:33 prefix and maps the low 32 bits of the IPv6 multicast address into the low 32 bits of the MAC, which has no aliasing problem. This tool covers IPv4 (RFC 1112) mapping only.