In the non-authoritative group, there is no central entity and every peer (game) controls its game state. In a peer-to-peer (P2P) approach, a peer sends data to all other peers and receives data from them, assuming that information is reliable and correct (cheating-free): Non-authoritative implementation using P2P architecture.
You may not be familiar with building a multiplayer game using P2P connection, because the most popular approach involves setting up client and server as a separate entity. In this situation client is the game itself and the server is an application that contains server-side logic, connects all the players together and protects them from cheaters.
\$\begingroup\$ For an example of a true P2P game, you can look at Gunz: The Duel, which has huge issues in multiplayer and with hacking because of this. \$\endgroup\$ – fhyve Jun 5 '17 at 8:10
P2P is good if you play coop or very slow games. Even in slow RTS like in Dawn of War 2, It feels very poor. P2P on a fast paced multiplayer game is a totally unacceptable. Depending on how strong the host advantage is and the game its like playing in the future. Play a P2P COD and host is the top scoring player every game.
First of all, you want the game instances to be able to work as clients or servers. There will not be an stablished server to which everybody connects. That is, you want a peer-to-peer network architecture. Please note that while we are talking P2P, that does not mean there will not be hosts. See How can I make a peer-to-peer multiplayer game?.
P2P is more or less the default for unity. Unless you go out of your way, if you use UNet or Mirror (can't speak for newer Unity Networking), you will be using P2P by default.
This is done by sharing client IP addresses between clients in a game session together. For example, if a 16 player multiplayer game uses direct peer connectivity to update player positions, each of the 16 game clients needs to know the IP addresses of the 15 other clients to know who to send data to.
eMule is one of the most popular and reliable P2P file sharing clients around the world. It is a file sharing client based on the eDonkey2000 network. However, it goes on to offer quite a huge array of features than the standard eDonkey client.