How do some online games allow the player to be the host?

Question

Computer networks

For example, the Steam version of the game Left 4 Dead. In it, you can create a local server and thus become a host to which other players can connect via the Internet. At the same time, you do not need to forward ports, or even have a white IP on your device (that is, you can become a host even if you are sitting behind a NAT provider). How is this done?

Answer 1

There is an external server, it either forwards connections through itself, or helps to forward ports (like a TURN server for UDP)

Answer 2

The host is a virtual space. No one directly interacts with the player's machine except for the game servers

Answer 3

Just like TimeViewer works - the machine registers on a server known to it with a white IP. Other cars also register there. This server provides interaction, in fact, nothing is needed from the client's computer, it can even be behind nat, even in the ass, if only the tyrnet works.