How to create multiplayer and networking in unity.the game should run in two r three different systems.

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C#

Unity

our problem is we are not able to create multiplayer game..we saw many links and videos it showing creation of multiplayer game in single system with two windows but we need the game to be played in two different systems.'
so please help us in creating multiplayer game in two different systems and how to connect two different systems through networking in unity which able to run as a multiplayer..please help us as soon as possible..

What I have tried:

we tried to create multiplayer games using different links and by seeing videos but its running in single system with two different systems..we used client and server codes in unity but both client and server is working in single system with two windows.