Difference between Public IP and Elastic IP Address

Public IP and Elastic IP Address

a) Public IP address 

Instance with Public IP address will get an external DNS name (example, ec2-506-0-198-88.compute-1.amazonaws.com)  name. External DNS name will be able to resolve outside the network and to the private IPv4 address of the instance from within the network of the instance. The public IP address is mapped to the primary private IP address through network address translation (NAT).

Public IP address can't be manually associated or disassociated for your instance. AWS releases or assigns the public IP address  in following cases

1) When instance is  stopped or terminated. Stopped instance receives a new public IP

2) When you associate an Elastic IP address with your instance Public IP address for your instance will be released.

3) When you associate an Elastic IP address with the primary network interface (eth0) of your instance in a VPC.

4) When you disassociate the Elastic IP address from your instance, it receives a new public IP address.

Reboot of EC2 WON'T change the public IP address of your instance changes

If you require a persistent public IP address that can be associated to and from instances as use an Elastic IP address.

b) Elastic IP address 

An Elastic IP address is a static IPv4 address designed for dynamic cloud computing. Elastic IP address will be associated with your AWS account.

Elastic IP address doesn’t incur charges as long as the below conditions are met

1)  The instance associated with the Elastic IP address is running.

If above conditions are not met then you will be charged for the Elastic IP Address.

If you’ve stopped or terminated an EC2 instance with an associated Elastic IP address and you don’t need that Elastic IP address any more, consider disassociating or releasing the Elastic IP address.

Note: Once if the Elastic IP address is released, you can’t get the same Elastic IP address again, you have to get a different Elastic IP address.