Programming

You can go for a client-server sort of model: The server can have two message queues that are known to all client systems.
One will be a control queue - when a client comes up it will create a new message queue and send the message queue id that is obtained to the server over the control queue. The server will store the msqids-pid(of the client) pairs - where these will be used will be seen later.
The second message queue will be the data queue - this queue will be the one where all clients send their data messages. The message type here must be something unique, the pid of the client process will be perfect.
The server can read messages from the data queue in the order that they are available - msgtyp=0 can be used. The server will then send messages to all the queues that are known to it (the msqid - pid pairs stored before will be used here), except to the pid that sent the message to it. Here you can use msgtyp=1 - you will never get the pid of any client equal to 1.

In cleanup, the server will remove the control and data message queues, the clients will delete the message queues that they created.

I know that the programming part may be a bit complicated, but with an unknown number of clients connecting, I think that this would be the best way to go.

Thanx a lot blowtorch, I'll try to figure out the coding part

one difference b/w msgqueues & shared memory.
wat is exact meaing of subnetting