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Hosts that want to receive particular multicast data are called multicast clients. The multi-cast clients use services initiated by a client program to subscribe to the multicast group. Each multicast group is represented by a single IPv4 multicast destination address. When an IPv4 host subscribes to a multicast group, the host processes packets addressed to this mul-ticast address as well as packets addressed to its uniquely allocated unicast address. As you will see, IPv4 has set aside a special block of addresses from 224.0.0.0 to 239.255.255.255 as addresses for multicast groups. Unicast packets use the IPv4 host address of the destina-tion host as the destination address and can be routed through an internetwork. The scope of multicast traffic is often limited to the local network or routed through an internetwork.

In the multicast communication shown in Figure 6-8, the source host A, with the address

172.16.4.1, creates a single packet addressed to the multicast address 224.10.10.5. In this

example, host C and host D have an application or service running that subscribes to this

multicast group. When a copy of this packet arrives, these devices will process the packet.

Figure 6-8       Multicast Communication

Source: 172.16.4.1

Destination: 224.10.10.5

			172.16.4.253
A
172.16.4.1	B	C	D
	172.16.4.2	172.16.4.3	172.16.4.4
		224.10.10.5	224.10.10.5

The IPv4 addresses from 224.0.0.0 to 239.255.255.255 are reserved for multicast communi-cation. This multicast address range is subdivided into different types of addresses: reserved link-local addresses and globally scoped addresses. One additional type of multicast address is the administratively scoped addresses, also called limited-scope addresses.

The IPv4 multicast addresses 224.0.0.0 to 224.0.0.255 are reserved link-local addresses. These addresses are used for multicast groups on a local network. Packets to these destina-tions are always transmitted with a Time to Live (TTL) value of 1. Therefore, a router that connects the local network should never forward them outside the local network. A typical use of reserved link-local addresses is in routing protocols using multicast transmission to exchange routing information.

The globally scoped addresses are 224.0.1.0 to 238.255.255.255. They can be used to multi-cast data across the Internet. For example, 224.0.1.1 has been reserved for Network Time Protocol (NTP) to synchronize the time-of-day clocks of network devices.

Show Unicast, Broadcast, and Multicast Traffic (6.2.3.4)

In this activity, you will be able to visualize unicasts, broadcasts, and multicasts by using Packet Tracer in simulation mode. Use file e1-6234.pka on the CD-ROM that accompanies this book to perform this activity using Packet Tracer.