Multicast is a different

Multicast
Multicast is a different beast entirely. At first glance, it appears to be a hybrid of unicast and
broadcast communication, but that isn’t quite accurate. Multicast does allow point-to-multipoint
communication, which is similar to broadcasts, but it happens in a different manner. The crux of
multicast
is that it enables multiple recipients to receive messages without flooding the messages
to all hosts on a broadcast domain.
Multicast works by sending messages or data to IP
multicast group
addresses. Routers then
forward copies of the packet out every interface that has hosts
subscribed
to that group address.
This is where multicast differs from broadcast messages. With multicast communication, copies
of packets are sent only to subscribed hosts.
The difference between multicast and unicast is comparable to the difference between mailing
lists and spam. You subscribe to a mailing list when you want to receive mail from a specific
group regarding specific information—for example, a Cisco User Group mailing list. You
expect to get messages only from other members of the group regarding topics related to the
user group. In contrast, spam is unsolicited mail that arrives in your inbox. You aren’t expecting
it from the sender, nor are you likely to be interested in the content.
Multicast works in much the same way as a mailing list. You (as a user) or an application will
subscribe to a specific IP multicast group to become a member. After you become a member of the
group, IP multicast packets containing the group address in the destination field of the header
arrive at your host and are processed. If the host isn’t subscribed to the group, it will not process
packets addressed to that group. Refer to Figure 19.3 for a reference on how multicast works.
Broadcast and multicast traffic can occur at different layers of the OSI model.
Each is characterized by the fact that they are addressed to a wide group of
hosts and are not usually acknowledged. At the Data Link layer, broadcasts
manage useful concepts such as ARP. At the Network layer, they may be
responsible for routing updates or server requests. At the Application layer,
they may be misused. E-mail broadcasts are often referred to as spam.