TCP/IP Address Resolution For IP Multicast Addresses
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Dealing With Multiple IP Addresses Mapped To One Multicast Hardware Address
Of course, there are 28 unique bits in IP multicast addresses, so this is a bit of a problem. J What it means is that there is no unique mapping between IP multicast addresses and Ethernet multicast addresses. Since 5 of the 28 bits of the multicast group cannot be encoded in the Ethernet address, 32 (25) different IP multicast addresses map onto each possible Ethernet multicast address. In theory, this would be a problem, but in practice, it isn't. The chances of any two IP multicast addresses on a single network mapping to the same Ethernet multicast address at the same time are pretty small.
Still, it is possible that two IP multicast groups might be in use on the same physical network and might map to the same data link layer multicast address. For this reason, devices must not assume that all multicast messages they receive are for their groups; they must pass up the messages to the IP layer to check the full IP multicast address to make sure they really were supposed to get the multicast datagram they received. If they accidentally get one that was intended for a multicast group they are not a member of, they discard it. This happens infrequently so the relative lack of efficiency is not a large concern.
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