Usually, packets cannot be transferred from a server to client(in a client server architecture) unless the server's router is configured to forward ports or the client and server may be is a same network.
So how does in P2P architecture, each person who is downloading the file is capable of sending the parts of the file to some-one located in somewhere(seeders). There are various problems like Routers are initially not configured to act as a server. So how does a peer sends a part of file to another peer without configuring the router and modifying the firewall permissions?
Sorry for my bad English! I hope you understand my question