Raw sockets and addressing

I need to write a ftp program linking two, only two, compute开发者_如何学JAVArs, but this must be done using raw sockets on promiscuous mode. Since the socket will receive any data that come across the network device, my problem is how to address the correct program. For example: Imagine I have a server on a machine and two clients on the other. The server sends a message, how can the two clients know which of them will receive the message? Another problematic situation would be if I open a client and a server in the same machine, both may start to commune with each other.