My task is to write a Messenger program for both internal and external staffs, I actually made it. However, I thing this is really not a good approach by using the client software keep "check-mesg" from server. So I think I am just simulating the IM program. I want to make the client app become a listening server, and let user p2p talking without a "mesg-centre" at the main server(unless offline mesg happen). The question is how do I tell the external user ( other client app ) my location while I am behind a router ? Are those other IM programs running on the client machine as a server too? and how do they get through ? Thanks in advance!