multicast

IP address range

Multicast IP addresses are in the range 224.0.0.0/4, or 224.0.0.1 through 239.255.255.254. All multicast IP addresses start with the bits 0b1110 (0x0E).

Many IP addresses in this range are reserved by the IANA. For example, 224.0.0.5 is reserved for OSPF, and 224.0.0.12 is reserved for DHCP. It is best to check with the IANA IPv4 Multicast Address Space Registry list before experimenting with some random multicast IP address. Other address are not official, but are used by convention by some popular packages. For example, the JBoss AutoDiscoveryAddress is 230.0.0.4 (even though the IANA has this listed as Unusable Locally. I'm not sure why JBoss uses this address by default. Perhaps I misunderstood the IANA documentation).

You can also limit the scope of your multicast packets by setting a short TTL in the packet. A setting of 1 or 2 is good for most local network experiments. This way you limit the range your bad packets will be seen.

MAC address range

Ethernet frames can also be specified as multicast. These frames will have the destination MAC address will have a the least significant bit set in the first octet. For example, 01-80-C2-00-00-00.