   Comms not working on the Same Machine?
  Ok, strange problem.  I am writing a program that uses UDP Broadcast.  On my Windows XP development machine, the client can see the server's UDP Broadcast traffic, no problem (Client and Server are on the same machine.)  The machine is just plugged into a hub, no outside network connection.Now, running the identical software on my Windows 2000 machine, which is connected to the Internet through a hub and a router, the software doesn't work.  The server is still broadcasting (I can see the hub blink), but the client can't see it.  I changed the Broadcast socket to a Multicast one, no go there either.  Any clue as to why the client/server on the XP machine worked and the client/server on the W2K machine didn't work?  I am really at a loss here!   
  Broadcasts will only be sent to machines in the same subnet, if you are sending data across a router, the router is not going to route the broadcast packet.(If TCP/IP allowed this, every broadcast packet would be forwarded to every machine on the internet!)You are probably going to want to have the client initiate communication to the server, sending a directed packet to the server. The server should then keep a list of all of the connected clients. If you want to do a "broadcast" the server will have to loop through the list of "connected" clients, and send the data to each of them directly.;  A minor nit: TCP cannot broadcast, because it is by nature point-to-point. I think you meant "if IP allowed this"... It turns out, the original IP spec DID allow that (the broadcast address was called "Mars," for the planet), but it was quickly thought of as a bad idea, and most routers block broadcasts to the subnet or less.There is a socket option you have to set for broadcasts to work right: SO_BROADCAST. Also, you may wish to see whether the Windows XP machine is denying external access because of the Windows Firewall (and you might want to check the 2k machine, too). Last, you can see whether the packets make it through by looking with a sniffer like Ethereal.   