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 Multicast Address

1. Multicast IP Address:

As specified by IANA (Internet Assigned Numbers Authority), Class D IP addresses are used as

destination addresses of multicast packets. The multicast IP addresses range from

224.0.0.0~239.255.255.255. The following table displays the range and description of several

special multicast IP addresses.

Multicast IP address range Description

224.0.0.0～224.0.0.255

Reserved multicast addresses for routing protocols

and other network protocols

224.0.1.0～224.0.1.255

Addresses for video conferencing

239.0.0.0～239.255.255.255

Local management multicast addresses, which are

used in the local network only

Table 8-1 Range of the special multicast IP

2. Multicast MAC Address:

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

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

destination is not a receiver but a group with uncertain number of members, so a multicast MAC

address, a logical MAC address, is needed to be used as the destination address.

As stipulated by IANA, the high-order 24 bits of a multicast MAC address begins with 01-00-5E

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

address. The mapping relationship is described as Figure 8-2.

Figure 8-2 Mapping relationship between multicast IP address and multicast MAC address

The high-order 4 bits of the IP multicast address are 1110, identifying the multicast group. Only 23

bits of the remaining low-order 28 bits are mapped to a multicast MAC address. In that way, 5 bits

of the IP multicast address is not utilized. As a result, 32 IP multicast addresses are mapped to the

same MAC address.

 Multicast Address Table

The switch is forwarding multicast packets based on the multicast address table. As the

transmission of multicast packets can not span the VLAN, the first part of the multicast address

table is VLAN ID, based on which the received multicast packets are forwarded in the VLAN

owning the receiving port. The multicast address table is not mapped to an egress port but a group

port list. When forwarding a multicast packet, the switch looks up the multicast address table

based on the destination multicast address of the multicast packet. If the corresponding entry can

not be found in the table, the switch will broadcast the packet in the VLAN owning the receiving

port. If the corresponding entry can be found in the table, it indicates that the destination address

should be a group port list, so the switch will duplicate this multicast data and deliver each port one

copy. The general format of the multicast address table is described as Figure 8-3 below.