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3. Each user can join and leave the multicast group at any time;
4. Real time is highly demanded and certain packets drop is allowed.
IPv4 Multicast Address
1. IPv4 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. IPv4 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 IPv4 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.
92
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