IP Multicast addresses to Ethernet Addresses


For mapping IP Multicast addresses to Ethernet Addresses the lower 23 bits of a Class D IP Address are copied to one of the IANA Designated Ethernet Addresses

Ethernet addresses reserved fotr this purpose are in the range of 01:00:53:00:00:00 through 01:00:5e:7f:ff:ff

Ethernet Addresses have a 48 bit address filed (Source and Destination 48 bits each one)

Expressed in hexadecimal numbering, the first 24 bits of an Ethernet multicast address are 01:00:5e, this indicates the frame as multicast , the next bit in the ethernet address is always 0, leaving 23 bit for the multicast address, because IP Multicast groups are 28 bits (1110XXXX XXXXXXXX XXXXXXXX XXXXXXXX) long and there are only 23 bits available the mapping cannot be one to one, so only 23 low order bits of the multicast group ID are mapped onto the ethernet address. The 5 higg order bit remain in the multicast group are ignored.

With this mapping, each Ethernet Multicast address correspings to 32 different IP Multicast Addresses (2^5)
This means that a host in one multicast group may have to filter out multicast that are intended for other groups sharing the same ehthernet address.
An example

Having the following IP Multicast Address convert it to the appropriate Ethernet MAC Representation.

224.     192.      16.       1

Multicast Ehternet

00010000.0101 1110.0100 0000.0001 0000.0000 0001
01   00   5    E    4    0    1    0    0   1



R1(config-if)#ip igmp join
Sw1#show mac-address-table multicast
Vlan    Mac Address       Type       Ports
—-    ———–       —-       —–
1    0100.5e40.1001    IGMP       Fa0/1

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