multicast works just fine on a subnet. there is a need for IGMP when routing is involved, but a lot can be done on a single subnet. so, multicast UDP is quite useful.
so, does the ethernet library support UDP send with a multicast address in the destination and can a listener receive said multicast packets is a valid question. does anyone have an answer?
You appear to be ignoring a post above and making it more complicated than need be.
Beyond that, you also likely need to have a multicast router - as otherwise there really isn't a point in simply not broadcasting, which will carry only on the same segment.
If you are on the same segment, simply use UDP broadcast (address is dictated by your netmask). It is not the same as multicasting but on the same segment there is little advantage to using multicast vs broadcast. This is likely especially true since I doubt the low end ethernet chips support MAC caching/filtering, let alone multicast MAC caching/filtering. And if true, that means there is no real difference between using broadcast versus multicast on the same segment; aside from the fact things just got a heck of a lot easier.
Multicast's primary advantage is efficient broadcasting over multiple segments. If you are on the same segment, you probably don't want to use multicast but rather should just be broadcasting. The advantage here is that broadcasting is just another destination address as determined by your network address and netmask. Everyone on the segment receives it, assuming they are listening on the port.
As a footnote, some socket implementations and/or platforms require an extra flag be specified so as to allow for broadcast traffic delivery on a socket.
I've never used this before and it was the first search engine result, but here is a subnet calculator
which will allow you to figure out your broadcast address for the given network.
Edit: Added broadcast socket option note.
Edit2: Added link to calculator.