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Unformatted text preview: encing and dissemination of
mail or news to multiple recipients, for example. Without multicasting these types
of services tend to use TCP today (delivering a separate copy to each destination).
Even with multicasting, some of these applications might continue to use TCP for
2. Solicitation of servers by clients. A diskless workstation, for example, needs to
locate a bootstrap server. Today this is provided using a broadcast (as we'll see
with BOOTP in Chapter 16), but a multicast solution would impose less overhead
on the hosts that don't provide the service.
In this section we'll take a look at multicast addresses, and the next chapter looks at the
protocol used by multicasting hosts and routers (IGMP).
Multicast Group Addresses
Figure 12.2 shows the format of a class D IP address. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/broadcas.htm (8 of 12) [12/09/2001 14.47.00] Chapter 12. Broadcasting and Multicasting Figure 12.2 Format of a class D IP address.
Unlike the other three classes of IP addresses (A, B, and C), which we showed in Figure
1.5, the 28 bits allocated for the multicast group ID have no further structure.
A multicast group address is the combination of the high-order 4 bits of 1110 and the
multicast group ID. These are normally written as dotted-decimal numbers and are in the
range 184.108.40.206 through 220.127.116.11.
The set of hosts listening to a particular IP multicast address is called a host group. A host
group can span multiple networks. Membership in a host group is dynamic-hosts may join
and leave host groups at will. There is no restriction on the number of hosts in a group,
and a host does not have to belong to a group to send a message to that group.
Some multicast group addresses are assigned as well-known addresses by the IANA
(Internet Assigned Numbers Authority). "These are called permanent host groups. This is
similar to the well-known TCP and UDP port numbers. Similarly, th...
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