¾Multicast 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
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
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.
¾Multicast 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
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