Short answer: Change in 4.2. Specify --binding=0.0.0.0 when you start the rails server.
or
rails s -b 0.0.0.0
This was a change mentioned in the release notes:
3.3 Default Host for rails server
Due to a change in Rack, rails server now listens on localhost instead of 0.0.0.0 by default. This should have minimal impact on the standard development workflow as both http://127.0.0.1:3000 and http://localhost:3000 will continue to work as before on your own machine.
However, with this change you will no longer be able to access the Rails server from a different machine, for example if your development environment is in a virtual machine and you would like to access it from the host machine. In such cases, please start the server with rails server -b 0.0.0.0 to restore the old behavior.
If you do this, be sure to configure your firewall properly such that only trusted machines on your network can access your development server.