3Com 7757 Configuration Manual page 419

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Table of Contents

IP Multicast Protocols

Table 329 Reserved IP multicast addresses

Class D address range

224.0.0.19- 224.0.0.255

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

also reserved the network segments ranging from 239.0.0.0 to 239.255.255.255

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

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

01005e, while the low-order 23 bits of a MAC address are the low-order 23 bits of

the multicast IP address. Figure 103 describes the mapping relationship:

Figure 103 Mapping relationship between multicast IP address and multicast MAC

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.

IP multicast protocols include the multicast group management protocol and the

multicast routing protocol. Figure 104 describes the positions of the protocols

related to multicast in the network.

The specified subnetwork bandwidth management (SBM)

Virtual router redundancy protocol (VRRP)

Other protocols

Multicast Architecture

Troubleshooting

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