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XG2000 series User's Guide
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All Rights Reserved, Copyright (C) PFU LIMITED 2009
4.9.1 Registering Group Members
On receiving an IGMP Report message, the device registers a multicast MAC address with the MAC address table for the port
that received the IGMP Report message and the connecting multicast router port.
The following figure shows the relationship between the registered IP multicast address and the multicast MAC address.
MAC addresses that are registered with IGMP snooping are between 0100.5E00.0000 and 0100.5E7F.FFFF. An IP multicast
address is 32 bits. The first 4 bits are always 1110 followed by 28 bits that represent the IP multicast address information. Of
these 28 bits, the lower order 23 bits are mapped to a MAC address and the data in the higher order 5 bits is not used.
Therefore 32 IP multicast addresses are mapped to the same single MAC address.