Multicast Mac Address Mapping Considerations - Avaya 8800 Planning And Engineering, Network Design

Multicast network design
IANA has also reserved the range of 224.0.1.0 through 224.0.1.255 for well-known applications.
These addresses are also assigned by IANA to specific network applications. For example, the
Network Time Protocol (NTP) uses 224.0.1.1, and Mtrace uses 224.0.1.32. RFC 1700 contains a
complete list of these reserved addresses.
Multicast addresses in the 232.0.0.0/8 (232.0.0.0 to 232.255.255.255) range are reserved only for
source-specific multicast (SSM) applications, such as one-to-many applications. (For more
information, see draft-holbrook-ssm-00.txt). While this is the publicly reserved range for SSM
applications, private networks can use other address ranges for SSM.
Finally, addresses in the range 239.0.0.0/8 (239.0.0.0 to 239.255.255.255) are administratively
scoped addresses; they are reserved for use in private domains and must not be advertised outside
that domain. This multicast range is analogous to the 10.0.0.0/8, 172.16.0.0/20, and 192.168.0.0/16
private address ranges in the unicast IP space.
A private network should only assign multicast addresses from 224.0.2.0 through 238.255.255.255
to applications that are publicly accessible on the Internet. Multicast applications that are not publicly
accessible should be assigned addresses in the 239.0.0.0/8 range.
Although you can use any multicast address you choose on your own private network, it is generally
not good design practice to allocate public addresses to private network entities. Do not use public
addresses for unicast host or multicast group addresses on private networks. To prevent private
network addresses from escaping to a public network, you can use announce and accept policies as
described in Announce and accept policy examples

Multicast MAC address mapping considerations

Like IP, Ethernet has a range of multicast MAC addresses that natively support Layer 2 multicast
capabilities. While IP has a total of 28 addressing bits available for multicast addresses, Ethernet
has only 23 addressing bits assigned to IP multicast. The Ethernet multicast MAC address space is
much larger than 23 bits, but only a subrange of that larger space is allocated to IP multicast.
Because of this difference, 32 IP multicast addresses map to one Ethernet multicast MAC address.
IP multicast addresses map to Ethernet multicast MAC addresses by placing the low-order 23 bits of
the IP address into the low-order 23 bits of the Ethernet multicast address 01:00:5E:00:00:00. Thus,
more than one multicast address maps to the same Ethernet address (see the following figure). For
example, all 32 addresses 224.1.1.1, 224.129.1.1, 225.1.1.1, 225.129.1.1, 239.1.1.1, 239.129.1.1
map to the same 01:00:5E:01:01:01 multicast MAC address.
June 2016
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Planning and Engineering — Network Design
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