Explain private ip.

A private IP address is a static IP address that can only communicate with other Linodes in the same datacenter. Usually they're used for communications between two of your Linodes who need to share data, such as a webserver and database. Any traffic over private IP addresses is not counted towards your monthly Network Transfer pool.

You can add a private IP address to your Linode from the Networking section of your Linode's page in the Cloud Manager:

Adding Private IP Addresses

That's about what I thought. Thanks.