NetworkNick.net

Pretty much everyone knows that reserved multicast addresses are always flooded on ethernet networks. IGMP does not program filters for packets addressed to the “Reserved Multicast Address” range. There are Reserved Addresses at Layer 3 (i.e., 224.0.0.1 – 224.0.0.255) that at layer two are indistinguishable from a broader range of MAC Addresses (because the Multicast Mac Address always begins with 01005E-XXXXXX, which covers the leading byte plus one more bit of the IP Address). No filters are programmed for these, nor should they be as this will break other multicast protocols.

So the story here is, L3 reserved addresses are the 224.0.0.X, but L2 really has more reservations because of the way the IP maps to the MAC.

That being said, the following addresses are always flooded (i.e., never filtered):

224.0.0.XX 224.128.0.XX
225.0.0.XX 225.128.0.XX
226.0.0.XX 226.128.0.XX
227.0.0.XX 227.128.0.XX
228.0.0.XX 228.128.0.XX
229.0.0.XX 229.128.0.XX
230.0.0.XX 230.128.0.XX
231.0.0.XX 231.128.0.XX
232.0.0.XX 232.128.0.XX
233.0.0.XX 233.128.0.XX
234.0.0.XX 234.128.0.XX
235.0.0.XX 235.128.0.XX
236.0.0.XX 236.128.0.XX
237.0.0.XX 237.128.0.XX
238.0.0.XX 238.128.0.XX
239.0.0.XX 239.128.0.XX

Good to keep in mind when choosing that arbitrary address for your multicast application. Make sure that it doesn’t end up in the reserved L2 space!