You are deploying your first EC2 instance in AWS and are using the AWS console to do this. You have chosen your AMI and your instance type and have now come to the screen where you configure your instance details. One of the things that you need to decide is whether you want to auto-assign a public IP address or not. You assume that if you do not choose this option, you will be able to assign an Elastic IP address later, which happens to be a correct assumption. Which of the below options best describes why an Elastic IP address would be preferable to a public IP address?

Answer options:

A.An Elastic IP address is free, whilst you must pay for a public IP address.
B.With an Elastic IP address, you can mask the failure of an instance or software by rapidly remapping the address to another instance in your account.
C.You can have an unlimited amount of Elastic IP addresses. However, public IP addresses are limited in number.
D.An Elastic IP address cannot be accessed from the internet like a public IP address and hence is safer from a security standpoint.

Answer – B
Option A is incorrect because public IP addresses are free.
Option B is CORRECT because you can reassign the EIP to a new instance in case of an instance failure. Thus you do not need to change any reference to the IP address in your application.
Option C is incorrect because the number of EIPs per account per region is limited (5).
Option D is incorrect because EIPs are accessible from the internet.
More information on EIPs
An Elastic IP address is a static IPv4 address designed for dynamic cloud computing. An Elastic IP address is associated with your AWS account. With an Elastic IP address, you can mask the failure of an instance or software by rapidly remapping the address to another instance in your account.
An Elastic IP address is a public IPv4 address, which is reachable from the internet. If your instance does not have a public IPv4 address, you can associate an Elastic IP address with your instance to enable communication with the internet; for example, to connect to your instance from your local computer.
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/elastic-ip-addresses-eip.html