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For the multicast communication model, we will need two new
types of addresses, an IP multicast address and an ethernet
multicast address. An IP multicast address identifies a group of
receivers that want to receive traffic destined for the group.
Because all IP packets are encapsulated in ethernet frames, a
multicast ethernet address is also required. For the multicast model
to function correctly, hosts should be able to receive both unicast
and multicast traffic, which mandates that hosts need multiple IP
and ethernet addresses. A unicast IP and ethernet address are used
for unicast traffic and zero or more IP, and ethernet multicast
addresses are used for multicast traffic. Zero multicast addresses
are needed if the host will not be receiving multicast traffic. A
pair of multicast addresses, IP and ethernet, are required for each
multicast group that the receiver wishes to join. A major difference
between the unicast and multicast addresses is that unicast
addresses are unique on each host, while multicast addresses are
not. If five hosts wish to receive multicast traffic destined for
group A, for example, then the hosts would all listen for traffic
destined for the same multicast address, both IP and ethernet. The
amount of traffic from the unicast case would be greatly reduced, as
shown in Figure 1-7. |
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