2-7
Multicast Addresses
To allow communication between multicast sources and multicast group members, network-layer
multicast addresses, namely, multicast IP addresses must be provided. In addition, a technique must
be available to map multicast IP addresses to link-layer multicast MAC addresses.
IP multicast addresses
1) IPv4 multicast addresses
Internet Assigned Numbers Authority (IANA) assigned the Class D address space (224.0.0.0 to
239.255.255.255) for IPv4 multicast. The specific address blocks and usages are shown in
Table 1-2.
Table 1-2 Class D IP address blocks and description
Address block Description
224.0.0.0 to 224.0.0.255
Reserved permanent group addresses. The IP address 224.0.0.0
is reserved, and other IP addresses can be used by routing
protocols and for topology searching, protocol maintenance, and
so on. Common permanent group addresses are listed in
Table
1-3. A packet destined for an address in this block will not be
forwarded beyond the local subnet regardless of the Time to Live
(TTL) value in the IP header.
224.0.1.0 to 238.255.255.255
Globally scoped group addresses. This block includes two types
of designated group addresses:
z 232.0.0.0/8: SSM group addresses, and
z 233.0.0.0/8: Glop group addresses.
239.0.0.0 to 239.255.255.255
Administratively scoped multicast addresses. These addresses
are considered to be locally rather than globally unique, and can
be reused in domains administered by different organizations
without causing conflicts. For details, refer to RFC 2365.
z The membership of a group is dynamic. Hosts can join or leave multicast groups at any time.
z “Glop” is a mechanism for assigning multicast addresses between different autonomous systems
(ASs). By filling an AS number into the middle two bytes of 233.0.0.0, you get 255 multicast
addresses for that AS. For more information, refer to RFC 2770.
Table 1-3 Some reserved multicast addresses
Address Description
224.0.0.1 All systems on this subnet, including hosts and routers
224.0.0.2 All multicast routers on this subnet
224.0.0.3 Unassigned
224.0.0.4 Distance Vector Multicast Routing Protocol (DVMRP) routers
224.0.0.5 Open Shortest Path First (OSPF) routers
224.0.0.6 OSPF designated routers/backup designated routers
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