MongDB 27017 vs 27018

Setting up a MongoDB server on a separate ec2 instance and then accessing it from the application and web interface.

The default port used is 27017 by various MongoDB drivers while 28017 is used to handle HTTP requests and provides some general monitoring.
In case you are interested to start mongod on a separate port for say security reasons, take it as 6565,
then the monitor port will always be 7575.

You can also query via web browser, sudo /etc/init.d/mongod --rest. Now simple
queries can be executed like http://localhost:7575/database/collection/?filter_a=1.

I often use mongoid to access mongodb in my rails application and access the rails console using
rails c production, which links me to the mongo database present on different ec2 instance.
Use "connection strings" of the form mongodb://user:pwd@host:port/.

Refer MongoDB - HTTP Interfaces