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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 addresses.
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.
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