6
Multicast addresses
Network-layer multicast addresses (multicast IP addresses) enables communication between multicast
sources and multicast group members. In addition, a technique must be available to map multicast IP
addresses to link-layer multicast MAC addresses.
IP multicast addresses
•
IPv4 multicast addresses
Internet Assigned Numbers Authority (IANA) assigned the Class D address space (224.0.0.0 to
239.255.255.255) to IPv4 multicast.
Table 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. Other IP addresses can be used by routing protocols and
for topology searching, protocol maintenance, and so on.
lists common permanent group addresses. 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 the following
types of designated group addresses:
•
232.0.0.0/8
—SSM group addresses.
•
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 locally unique rather than globally unique, and can be
reused in domains administered by different organizations without
causing conflicts. For more information, see RFC 2365.
NOTE:
"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, see RFC 2770.
Table 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 and backup designated routers
224.0.0.7
Shared Tree (ST) routers
224.0.0.8 ST
hosts
224.0.0.9
Routing Information Protocol version 2 (RIPv2) routers
224.0.0.11
Mobile agents