By default MongoDB listens for socket connections on port 27017. The server will fail to start if the port is not available—the most common cause of this is another instance of MongoDB that is already running.
mongod also sets up a very basic HTTP server that listens on a port 1,000 higher than the main port, in this case 28017. This means that you can get some administrative information about your database by opening a web browser and going to http://localhost:28017.
Kristina Chodorow. “MongoDB_ The Definitive Guide, 2nd Edition.epub.” iBooks.