Planet XGS3-24042 User Manual download pdf (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.

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