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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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6.1 Multicast addressing<br />

Multicast devices use Class D <strong>IP</strong> addresses to communicate. These addresses<br />

are contained in the range encompassing 224.0.0.0 through 239.255.255.255.<br />

For each multicast address, there exists a set of zero or more hosts that listen for<br />

packets transmitted to the address. This set of devices is called a host group. A<br />

host that sends packets to a specific group does not need to be a member of the<br />

group. The host might not even know the current members in the group. There<br />

are two types of host groups:<br />

► Permanent: Applications that are part of this type of group have an <strong>IP</strong> address<br />

permanently assigned by the IANA. Membership in this type of host group is<br />

not permanent; a host can join or leave the group as required. A permanent<br />

group continues to exist even if it has no members. The list of <strong>IP</strong> addresses<br />

assigned to permanent host groups is included in RFC 3232. These reserved<br />

addresses include:<br />

– 224.0.0.0: Reserved base address<br />

– 224.0.0.1: All systems on this subnet<br />

– 224.0.0.2: All routers on this subnet<br />

– 224.0.0.9: All R<strong>IP</strong>2 routers<br />

Other address examples include those reserved for OSPF (refer to 5.6, “Open<br />

Shortest Path First (OSPF)” on page 196). They include:<br />

– 224.0.0.5: All OSPF routers<br />

– 224.0.0.6: OSPF designated routers<br />

Additionally, IGMPv3 (defined in RFC 3376), reserves the following address:<br />

– 224.0.0.22: All IGMPv3-capable multicast routers<br />

An application can use DNS to obtain the <strong>IP</strong> address assigned to a<br />

permanent host group (refer to 12.1, “Domain Name System (DNS)” on<br />

page 426) using the domain mcast.net. It can determine the permanent group<br />

from an address by using a pointer query (refer to 12.1.6, “Mapping <strong>IP</strong><br />

addresses to domain names: Pointer queries” on page 430) in the domain<br />

224.in-addr.arpa.<br />

► Transient: Any group that is not permanent is transient. The group is available<br />

for dynamic assignment as needed. Transient groups cease to exist when the<br />

number of members drops to zero.<br />

6.1.1 Multicasting on a single physical network<br />

This process is straightforward. The sending process specifies a destination <strong>IP</strong><br />

multicast address. The device driver converts this <strong>IP</strong> address to the<br />

238 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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