multicast address


studying multicasting and IGMP specifically, and have a few points I don't clearly understand. First of all why do we need Ethernet multicast addresses, wouldn't IP multicasting suffice to distribure traffic among the groups of hosts? I only guess, there are protocols exploiting Ethernet-based multicasting?

And the second. From RFC1112: "IP host address is mapped to Ethernet multicast address by placing the low-order 23-bits of the IP address into the low-order 23 bits of Ethernet multicast address 01-00-5e-00-00-00".

Does this mean that in order to send a multicast packet, a host must supply a properly built MAC address in the form defined above, in its Ethernet header field (destination address)? So the OUI, purchased by equipment manufacturers would not suffice?


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If the IP packet goes over ethernet, it needs an ethernet multicast address to be multicast.


-- glen

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glen herrmannsfeldt

Layer 2 devices within the network would be unable to handle IP-only multicast addressing. Multicasting is used in a popular industrial control protocol called Ethernet/IP (the IP here stands for industrial protocol); the infrastructure devices on these control networks are almost exclusively L2 switches.

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