I am creating a Co-op, puzzle game, multiplayer app. Lan help needed.

I'm creating a Co-op, puzzle game, multiplayer app and I am having trouble understanding how I can make two smartphones communicate with each other. I mean I get that one has to be the server-client and the others are going to be simple clients but my question is in a more primary level. How am I asking from my hardware to communicate so that a connection is created.