if you are using VLC as your multicast source to stream a movie / music / whatever, make sure you are raising the TTL. by default, VLC uses a TTL of 1, which means it will not go past the local subnet.
i know older versions of VLC even if you specified the TTL in the GUI would still not update the TTL for the 'command' that it generated on the backend, so you had to manually set it. this seems to be fixed though in the latest versions for Mac and Windows.
will