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34.1.2 Multicast Address
The destination address of Multicast message uses class D IP address with range from
224.0.0.0 to 239.255.255.255. D class address can not appear in the source IP address field
of an IP message. In the process of Unicast data transmission, the transmission path of a data
packet is from source address routing to destination address, and the transmission is
performed with hop-by-hop principle. However, in IP Multicast environment, the destination
addresses is a group instead of a single one, they form a group address. All message
receivers will join in a group, and once they do, the data flowing to the group address will be
sent to the receivers immediately and all members in the group will receive the data packets.
The members in a Multicast group are dynamic, the hosts can join and leave the Multicast
group at any time.
Multicast group can be permanent or temporary. Some of the Multicast group addresses are
assigned officially; they are called Permanent Multicast Group. Permanent Multicast Group
keeps its IP address fixed but its member structure can vary within. The member amount of
Permanent Multicast Group can be arbitrary, even zero. The IP Multicast addresses which are
not kept for use by Permanent Multicast Group can be utilized by temporary Multicast groups.
224.0.0.0 ~ 224.0.0.255 are reserved Multicast addresses (Permanent Group Address),
address 224.0.0.0 is reserved but not assigned, and other addresses are used by Routing
Protocol; 224.0.1.0 ~ 238.255.255.255 are Multicast addresses available to users (Temporary
Group Address) and are valid in the entire domain of the network; 239.0.0.0 ~
239.255.255.255 are local management Multicast addresses, which are valid only in specific
local domain. Frequently used reserved multicast address list is as follows:
Benchmark address (reserved)
224.0.0.1 Address of all hosts
224.0.0.2 Address of all Multicast Routers
224.0.0.3 Unassigned
224.0.0.4 DVMRP Router
224.0.0.5 OSPF Router
224.0.0.6 OSPF DR
224.0.0.7 ST Router
224.0.0.8 ST host
224.0.0.9 RIP-2 Router
224.0.0.10 IGRP Router
224.0.0.11 Active Agent
224.0.0.12 DHCP Server/Relay Agent
224.0.0.13 All PIM Routers
224.0.0.14 RSVP Encapsulation
224.0.0.15 All CBT Routers
224.0.0.16 Specified SBM
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