Amazon EC2 Instance Public IP Address

Every Amazon EC2 instance has both a private and public IP address. Both are dynamic and subject to change. The only way to have a fixed IP is with Elastic IPs.

Note that the private and public addresses will change whenever the server is relaunched. However, it will not change when the server is merely rebooted.

The public IP address is not clear to the user because it never shows up in the console. To find out what the public IP address is, do a DNS lookup on the public DNS address field.

References:
http://support.rightscale.com/06-FAQs/FAQ_0160_-_Can_the_public_or_private_IP_addresses_change_on_an_EC2_instance%3F