546 CHAPTER 41: MULTICAST OVERVIEW
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■ The membership of a group is dynamic. Hosts can join or leave multicast
groups at any time.
■ 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.
IPv6 Multicast Addresses
As defined in RFC 4291, the format of an IPv6 multicast is as follows:
Table 52 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. Commonly used permanent
group addresses are listed in Table 53. 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:
■ 232.0.0.0/8: SSM group addresses, and
■ 233.0.0.0/8: Glop group addresses; for details, see
RFC 2770.
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.
Table 53 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
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
224.0.0.12 Dynamic Host Configuration Protocol (DHCP) server/relay agent
224.0.0.13 All Protocol Independent Multicast (PIM) routers
224.0.0.14 Resource Reservation Protocol (RSVP) encapsulation
224.0.0.15 All Core-Based Tree (CBT) routers
224.0.0.16 Designated Subnetwork Bandwidth Management (SBM)
224.0.0.17 All SBMs
224.0.0.18 Virtual Router Redundancy Protocol (VRRP)
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