A lot of development servers bind themselves to 127.0.0.1 by default. This is the case in Sinatra and in Rails as of 4.2. If you're like me and you're using a virtual environment like Vagrant, you'll need to serve on all interfaces so the traffic can be forwarded to the host machine.
Here's the solution for Shotgun.
shotgun -o 0.0.0.0
Here's the solution in Rails.
rails server -b 0.0.0.0