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6.2.3. Multicast Address
A multicast address is associated with a group of interested receivers. According to RFC
3171, addresses 224.0.0.0 to 239.255.255.255, the former Class D addresses, are
designated as multicast addresses in IPv4.
The IANA owns the OUI MAC address 01:00:5e, therefore multicast packets are delivered
by using the Ethernet MAC address range 01:00:5e:00:00:00 - 01:00:5e:7f:ff:ff. This is 23
bits of available address space.
The first octet (01) includes the broadcast/multicast bit. The lower 23 bits of the 28-bit
multicast IP address are mapped into the 23 bits of available Ethernet address space. This
means that there is ambiguity in delivering packets. If two hosts on the same subnet each
subscribe to a different multicast group whose address differs only in the first 5 bits,
Ethernet packets for both multicast groups will be delivered to both hosts, requiring the
network software in the hosts to discard the un-required packets.
Class Address Range Supports
Class A
1.0.0.1 to 126.255.255.254
Supports 16 million hosts on each of 127
networks.
Class B 128.1.0.1 to 191.255.255.254
Supports 65,000 hosts on each of 16,000
networks.
Class C 192.0.1.1 to 223.255.254.254
Supports 254 hosts on each of 2 million
networks.
Class D 224.0.0.0 to 239.255.255.255 Reserved for multicast groups.
Class E
240.0.0.0 to 254.255.255.254
Reserved for future use, or Research and
Development Purposes.