Design Reference

subnets does not exist for multicast group addresses. Consequently, the usual unicast
conventions—where you reserve the all 0s subnets, all 1s subnets, all 0s host addresses, and
all 1s host addresses—do not apply.
Internet Assigned Numbers Authority (IANA) reserves addresses from 224.0.0.0 through
224.0.0.255 for link-local network applications. Multicast-capable routers do not forward
packets with an address in this range. For example, Open Shortest Path First (OSPF) uses
224.0.0.5 and 224.0.0.6, and Virtual Router Redundancy Protocol (VRRP) uses 224.0.0.18 to
communicate across local broadcast network segments.
IANA also reserves the range of 224.0.1.0 through 224.0.1.255 for well-known applications.
IANA assigns these addresses to specific network applications. For example, the Network
Time Protocol (NTP) uses 224.0.1.1, and Mtrace uses 224.0.1.32. RFC1700 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. While this
range 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. Do not advertise these
addresses outside the private 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.
In a private network, only assign multicast addresses from 224.0.2.0 through 238.255.255.255
to applications that are publicly accessible on the Internet. Assign addresses in the 239.0.0.0/8
range to multicast applications that are not publicly accessible.
Although you can use a 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.
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,
IP multicast network design
110 Network Design Reference for Avaya VSP 4000 February 2014
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