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16.1.2 Multicast Address
The multicast packets uses Class D IP address as their destination addresses, ranging
from 224.0.0.0 to 239.255.255.255. Class D addresses cannot be used in the source IP
address field of an IP packet. In unicast, the path a packet travels is from the source
address to the destination address, and the packet is transfer in the network hop-by-hop.
However, in IP multicast, the destination address of a packet is a group (group address)
instead of one single address. All information receivers are arranged in the same group.
And once a receiver joins a multicast group, data sending to the multicast address will
immediately start transferring to the receiver. All members in the group will receive the
packets. The membership for multicast group is dynamic, the hosts can join and quit a
multicast group at any time.
A multicast group can be either a perpetual one or temporary one. Part of multicast
addresses are assigned officially and referred to as the perpetual multicast group. The IP
address of a perpetual multicast group remains the same, but the membership can be
changed. A perpetual multicast group can have any number of members, even zero. The
IP multicast addresses not reserved for perpetual multicast group can be used by
temporary multicast groups.
224.0.0.0 – 224.0.0.255 are reserved multicast addresses (perpetual group address), the
address 224.0.0.0 is not used, the other addresses are available for routing protocols;
224.0.1.0 – 238.255.255.255 are multicast addresses available to users (temporary group
address), and is valid for the whole network; 239.0.0.0 – 239.255.255.255 are local
administrative multicast address and is valid for specific local ranges. The following is a
list for common reserved multicast addresses:
224.0.0.0 Base address (reserved)
224.0.0.1 All-host address
224.0.0.2 All-multicast-router address
224.0.0.3 Not for allocation
224.0.0.4 DVMRP router
224.0.0.5 OSPF router
224.0.0.6 OSPF DR
224.0.0.7 ST router
224.0.0.7 ST host
224.0.0.9 RIP-II router
224.0.0.10 IGRP router
224.0.0.11 Active proxy
224.0.0.12 DHCP Server/Relay proxy
224.0.0.13 All PIM routers
224.0.0.14 RSVP packaging
224.0.0.15 All CBT routers
224.0.0.16 Specified SBM