PLANET XGS3-24042 - Manual (Page 403)

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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
224.0.0.17 All SBMS
224.0.0.18 VRRP
224.0.0.22 IGMP
When Ethernet transmits Unicast IP messages, the destination MAC address it uses is the receiver’s MAC
address. But in transmitting Multicast packets, the transmission destination is not a specific receiver any more,
but a group with uncertain members, thus Multicast MAC address is used. Multicast MAC address is
corresponding to Multicast IP address. It is prescribed in IANA (Internet Assigned Number Authority) that the
higher 25 bits in Multicast MAC address is 0x01005e, and the lower 23bits in MAC address is the lower 23bits
in Multicast IP address.
Since only 23bits out of the lower 28bits in IP Multicast address are mapped into MAC address, therefore
there are 32 IP Multicast addresses which are mapped into the same MAC address.