By definition, multicast addresses all start with 1110 in the first half byte. Routers seeing such a destnation (never a source) address knows the msg is a multicast msg.
However, routers don’t forward any msg with destnation address 224.0.0.0 through 224.0.0.255 because these are local multicast addresses. I guess these local multicast addresses are like 192.168.* addresses.
