For multicast forwarding, the mac addresses in the range 01-00-5E-00-00-00 to 01-00-5E-7F-FF-FF have been reserved. The most significant 25 bits of the above 48-bit mac address are fixed while the lower 23 bits are variable. These lower 23 bits are derived from the lower 23 bits of the multicast IP address.
The IP multicast range spans from 126.96.36.199 to 188.8.131.52. The most significant 4 bits of this address range is fixed ( 1110 ). Hence when an IP multicast address maps to a mac address, out of the total of 32 bits - lower 23 are mapped to the mac address, upper 4 bits remain fixed ( as mentioned above ) and hence 32 - 23 - 4 = 5 bits do not get mapped. As a result the IP multicast addresses that vary only in these 5 bits map to the same mac address. This amounts to 32 multicast ip addresses mapping to the same mac address.
I hope you will find this useful whenever you need to convert an ip multicast address to mac address. If you like the post, do share it with your friends. You can always submit useful tools, web services and tips for us.