Multicast MAC addresses
When using IPv4, the multicast MAC address is a special value that begins with 01-00-5E in hexadecimal. The remaining portion of the multicast MAC address is created by converting the lower 23 bits of the IP multicast group address into 6 hexadecimal characters.
When using IPv6, the multicast MAC address used always begins with 33:33. The remaining portion is composed of the lowest (right-most) 32 bits of the multicast IPv6 address used.
The IEEE has defined other ranges for multicast MAC addresses which include:
- 01:80:C2:XX:XX:XX - Group addresses used by standard protocols including STP, LLDP, and GVRP to name a few
- 01:1B:19:XX:XX:XX - Precision Time Protocol (PTP)
- 01:0C:CD:XX:XX:XX - IEC applications
- 01:00:0C:XX:XX:XX - Used by Cisco for many of its proprietary protocols including CDP, VTP, and UDLD