Hi Marzo,
What are you trying to achieve here?
You can find out the IP address of the client, however if there is more than one client at that particular address then the information cannot be used to uniquely identify the client.
You can create your own unique identifiers for each client (that wont mean anything to the client - they will just mean something to you).
You can also have the clients register themselves as observers of your server, in which case you could have the server call client methods.
But these are all very different issues, so more information on what you are trying to achieve is needed

.
Regards, Andrew