1-7Multicast AddressesTo allow communication between multicast sources and multicast group members, network-layermulticast addresses, namely, multicast IP addresses must be provided. In addition, a technique must beavailable to map multicast IP addresses to link-layer multicast MAC addresses.IP multicast addressesInternet Assigned Numbers Authority (IANA) assigned the Class D address space (224.0.0.0 to239.255.255.255) for IPv4 multicast. The specific address blocks and usages are shown in Table 1-2.Table 1-2 Class D IP address blocks and descriptionAddress block Description224.0.0.0 to 224.0.0.255Reserved permanent group addresses. The IP address224.0.0.0 is reserved, and other IP addresses can be usedby routing protocols and for topology searching, protocolmaintenance, and so on. Common permanent groupaddresses are listed in Table 1-3. A packet destined for anaddress in this block will not be forwarded beyond the localsubnet regardless of the Time to Live (TTL) value in the IPheader.224.0.1.0 to 238.255.255.255Globally scoped group addresses. This block includes twotypes of designated group addresses:z 232.0.0.0/8: SSM group addresses, andz 233.0.0.0/8: Glop group addresses.239.0.0.0 to 239.255.255.255Administratively scoped multicast addresses. Theseaddresses are considered to be locally rather than globallyunique, and can be reused in domains administered bydifferent 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 multicastaddresses for that AS. For more information, refer to RFC 2770.Table 1-3 Some reserved multicast addressesAddress Description224.0.0.1 All systems on this subnet, including hosts and routers224.0.0.2 All multicast routers on this subnet224.0.0.3 Unassigned224.0.0.4 Distance Vector Multicast Routing Protocol (DVMRP) routers224.0.0.5 Open Shortest Path First (OSPF) routers224.0.0.6 OSPF designated routers/backup designated routers
