Multiplayer networking only works on private network

So I'm making a simple multiplayer online applet game, and I was testing it using multicast UDP sockets instead of the typical client-server connection. This is not meant to be efficient or safe for that matter, just an experiment. Only problem is, when I try to have other people join the game from their house, it won't connect them to me, but when I use two separate computers, one that's wired in and one that's on the wifi, it works seemlessly. They can join their own game and connect to their own network, but not other peoples. Am I missing something big here? I'll post the relevant code.

InetAddress group;
DatagramPacket packet;
DatagramPacket messagePacket;
MulticastSocket socket;

socket = new MulticastSocket(4446); //random port
group = InetAddress.getByName("228.5.6.7"); //multicast address
socket.joinGroup(group);

//typical code for sending a packet
packet = new DatagramPacket(messageBuf, messageBuf.length, group, 4446);

Any ideas? I'm rather new to networking but find it a fun challenge and would like to continue learning开发者_StackOverflow社区 more about it..if you have any other tips on top of helping me to solve this problem it would be appreciated.

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You probably figured this out by now, but yes there is a huge problem you are missing. It will always work if you are local because your router doesn't mind distributing packets behind the private LAN. Anything outside the network will not want to work because the client will try to send a packet to the server, but the server is behind a NAT(Network address translation) and since the router didn't see the server send out a packet first, the router will just discard the client's packet and never connect. However if the server sends out a udp packet to try to connect, the router likes to switch ports so you don't know which port the packet will come out of. That's what packet forwarding is for on the router. So when the client sends a packet, it goes to a different port than expected and the router still discards it. There are solutions such as "hole punching". The easiest solution tho is to have a dedicated server outside of any NAT to handle the requests.