Multicast addressing

[martintxo]

Hello.
I´m using VLC for multicast streaming. I have made it inside a local LAN, using 239.255.1.2 address (or someone similar). It naturally works. My question is: I have to choose the multicast group address, but when I want to stream outside this LAN, is there any problem, in theory I suppose so, but in practice, because another person had chosen the same address for another stream? If it, is there any way to avoid it, and to choose a "free" address?