Send messages on Multicast Group from different subnets

[Java and Swing]

I am trying to get two machines on two different subnets to join the
same multicast group. Once they join they periodically send messages
to one another via the group, however, right now they don't seem to
know of each other. this could be a firewall issue, but I wanted to
make sure what I am doing is correct.

I try to join the multicast group at "230.0.0.1" per the Java tutorial
(http://java.sun.com/docs/books/tutorial/networking/datagrams/broadcasting.html)
.....is this address sufficient for my needs? also, what other address
could I use...i am not sure what the possibilities are?

thanks in advance.