When making a multiplayer game...

Should you start by making all of the game mechanics or the networking part first, for example if you are making an fps would you make the scripts for shooting and all that other stuff or start with making it possible to connect to the game

Me Personally i would get the mechanics done first. Things like Photon are designed to be easily added to the game when you need it.

best practice is first to design a game structure and separate all game to logical blocks like client-server data sending, server part, client network part, physics, interface, data storage and so on… with this structure you can start all blocks in any order by different groups of developers.