Links: https://www.zhihu.com/question/36730567/answer/83083851
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Multicast IP conversion to a multicast MAC address is a 32:1 mapping relationship.
Multicast IP Address:
The IANA assigns the class D address space to the IP multicast address. The address of the space is represented by a binary representation and the first 4 bits of the eight-bit group are represented by 1110.
Therefore the IP multicast address ranges from 224.0.0.0 to 239.255.255.255.
Multicast MAC address: The IEEE MAC address specifies that bit 0 of the first eight-bit group, Octet 0, indicates whether the destination address is a broadcast/multicast address or a unicast address.
If this bit is set, then the frame of the MAC can be defined as any host group (multicast) or as a network host (broadcast). Layer 2nd IP Multicast uses this feature to transmit IP multicast packets to the host group of the LAN segment (that is, the multicast IP to multicast MAC mapping).
Ethernet Multicast MAC address mapping:
In the case of Ethernet, IP multicast frames use the 0x0100.5exx.xxxx 24-bit prefix to start the MAC layer address. The remaining 24-bit addresses for some reason only half of these MAC addresses (that is, the last 23 bits) can be used by IP multicast. The 23 bits of the remaining MAC address space are used as mappings for layer 3rd IP multicast addresses into the 2nd tier MAC address. Because of this, the 3rd IP multicast address must be set to 32 bits of the first 4 bits to 0x1110, leaving 28 bits as meaningful IP multicast address information. These 28 bits must be mapped to only 23-bit MAC addresses that are available.
Because all 28 bits of the 3rd IP multicast address information cannot be mapped into only 23-bit available MAC address space, 5-bit address information is lost during the mapping process. Causes the 3rd IP address to be mapped to the 2nd-tier IEEE MAC address when the 2^5 or 32 IP multicast addresses are mapped to the same multicast MAC address. This means that each IEEE IP multicast MAC address can represent 32 IP multicast addresses.
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The above content excerpt from "IP Multicast network design and Development (volume 1th)" Beau Williamson
multicast IP conversion to multicast MAC address "reprint"
