Explanation: You can place an elastic network interface on each of your web servers that connects to a mid- tier network where an application server resides. The application server can also be dual-homed to a back-end network (subnet) where the database server resides. If it is set up like this, instead of routing network packets through the dual-homed instances, each dual-homed instance receives and processes requests on the front end and initiates a connection to the back end before finally sending requests to the servers on the back-end network.
Reference:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html