I want to do a broadcast over different subnets, is that possible ?

A broadcast is a local function, read here: broadcast. Broadcasts can not be sent over different subnets. However, you may be able to use Multicast. Barix devices (e.g. Exstreamer M400) can also provide "rebroadcasting" functionality, which means that a device in a remote network receives a stream and sends it back out to the local network as broadcast or multicast. Using that functionality, a broadcast can be sent to a remote network.