i just want to know how two application interact with each other across the internet. how one send some message to other one because when IP is not fixed. please give me some coding tips by which a p2p apps connect other one across the internet when IP is not known.
please help me in detail.
Before trying to get into implementation details and coding tips, you'd better learn how P2P networks work in general, what are possible architectures, and so on (start on http://en.wikipedia.org/wiki/Peer-to-peer). Your question is too wide to "help you in detail".
Edit: To get in quicker, this can be handy: .NET P2P: Writing Peer-to-Peer Networked Apps with the Microsoft .NET Framework