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2.5.2. Notes on Multicast Address Assignment

There are several subtle points that often deserve consideration when choosing/assigning multicast addresses. [12]
  1. Avoid 224.0.0.x
    Traffic to addresses of the form 224.0.0.x is often flooded to all switch ports. This address range is reserved for link-local uses. Many routing protocols assume that all traffic within this range will be received by all routers on the network. Hence (at least all Cisco) switches flood traffic within this range. The flooding behavior overrides the normal selective forwarding behavior of a multicast-aware switch (e.g. IGMP snooping, CGMP, etc.).
  2. Watch for 32:1 overlap
    32 non-contiguous IP multicast addresses are mapped onto each Ethernet multicast address. A receiver that joins a single IP multicast group implicitly joins 31 others due to this overlap. Of course, filtering in the operating system discards undesired multicast traffic from applications, but NIC bandwidth and CPU resources are nonetheless consumed discarding it. The overlap occurs in the 5 high-order bits, so it’s best to use the 23 low-order bits to make distinct multicast streams unique. For example, IP multicast addresses in the range to all map to unique Ethernet multicast addresses. However, IP multicast address maps to the same Ethernet multicast address as, maps to the same Ethernet multicast address as, etc.
  3. Avoid x.0.0.y and x.128.0.y
    Combining the above two considerations, it’s best to avoid using IP multicast addresses of the form x.0.0.y and x.128.0.y since they all map onto the range of Ethernet multicast addresses that are flooded to all switch ports.
  4. Watch for address assignment conflicts
    IANA administers Internet multicast addresses. Potential conflicts with Internet multicast address assignments can be avoided by using GLOP addressing (AS required) or administratively scoped addresses. Such addresses can be safely used on a network connected to the Internet without fear of conflict with multicast sources originating on the Internet. Administratively scoped addresses are roughly analogous to the unicast address space for private internets. Site-local multicast addresses are of the form 239.255.x.y, but can grow down to 239.252.x.y if needed. Organization-local multicast addresses are of the form 239.192-251.x.y, but can grow down to 239.x.y.z if needed.
For a more detailed treatment (57 pages!), see Cisco’s Guidelines for Enterprise IP Multicast Address Allocation paper.