H3C S5100-SI Operation Manual page 530

Ethernet switches

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Like having reserved the private network segment 10.0.0.0/8 for unicast, IANA has also reserved the

network segment 239.0.0.0/8 for multicast. These are administratively scoped addresses. With the

administratively scoped addresses, you can define the range of multicast domains flexibly to isolate IP

addresses between different multicast domains, so that the same multicast address can be used in

different multicast domains without causing collisions.

Ethernet multicast MAC address

When a unicast IP packet is transported in an Ethernet network, the destination MAC address is the

MAC address of the receiver. When a multicast packet is transported in an Ethernet network, a

multicast MAC address is used as the destination address because the destination is a group with an

uncertain number of members.

As stipulated by IANA, the high-order 24 bits of a multicast MAC address are 0x01005e, while the

low-order 23 bits of a MAC address are the low-order 23 bits of the multicast IP address.

mapping relationship:

Figure 1-4 Multicast address mapping

32-bit IP address

48-bit MAC address

25-bit MAC address prefix

The high-order four bits of the IP multicast address are 1110, representing the multicast ID. Only 23 bits

of the remaining 28 bits are mapped to a MAC address. Thus, five bits of the multicast IP address are

lost. As a result, 32 IP multicast addresses are mapped to the same MAC address.

describes the

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