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Basically,

The publishers will publish their messages in the topics (a Topic can be seen as a Facebook Page)

The subscribers (Facebook Page Followers) can read the status of the publishers by reading information directly from the topics (so they connect to Facebook Pages of the publisher directly and read the status)

If the topic is shutdown, the subscribers can't read the messages (in other words if the publisher desactivate his Facebook Page, you will not be able read the messages, even if the Facebook Server which is the ROS Master is still running).

But, when you connect to the Facebook Page of the publisher, he don't know that you're reading his status. And we assume that you can not write a comment. So, There's not direct communication between you (the subscriber) and the publisher, because the communication in one-way only.

In other words, the publisher writes messages in the topic, the subscriber reads the messages from the topic. And no one cares about the location of the other or his address.

But all of this is just an abstraction: in the low level communication of the machine, the nodes need to know all the network information the nodes to communicate between them. But, the subscribers don't want to bother themselves with the IP adresses and ports, and that's why, they contact the ROS Master which will redirect the messages to the correct node.