Network attached devices are generally not directly addressable from outside their local network. There's a thing called "network address translation" on router

Network attached devices are generally not directly addressable from outside their local network. There's a thing called "network address translation" on routers which hides the addresses used internally. The router substitutes it's own address for the internal ones on internet facing connections, acting on their behalf, and just remembers which connections belong to which local device. For new incoming connections, it won't know which device it's receiving the connection request on behalf of. That's unless that device announces to the router it's waiting for the connection using a thing called uPnP, or you manually configure the router to forward to a specific device for specific types of connections. When uPnP or manual forwarding aren't there, intermediaries on the internet allow both A and B to make outgoing connection requests to set up the link, which is no trouble for any router to do.