Games that use Peer-to-peer lobbies instead of dedicated servers, more popular with multiplayer co-op games.
Typically, it can be possible to join another lobby, but impossible to host (insofar as other people can't connect to it)
Typically, it can be possible to join another lobby, but impossible to host (insofar as other people can't connect to it)
Of course they can be opened manually but that assumes some technical experience, and that the ISP provided hardware gives you access to its configuration.