How to determine traffic of other nodes in a network from one computer?

I m doin a project on peer to peer media sharing framework to understand network topology in Java.
My query is,how to determine traffic(i.e packets that are being transferred)of other nodes in a network from one computer?

Can we use Jpcap to fin network traffic in all the computers along the path?
Any code for it?
Pls rply..:)