Why do I need ElasticIP when AWS provides Public DNS?

Elastic IP address is a static IP address provided by AWS. The EIP belong to particular user account only.

I agree that AWS provides public DNS when an instance is launched but the problem with that public DNS is it is not persistent. If the instance gets terminated or stopped / start the public DNS will be changed. If your production application is running from this server, everytime there is some disaster / stop & start the instance will have new DNS and can not be accessed from old DNS.

If this DNS is mapped with your Domain name server then on every DNS change you have to update your Domain Name mapping to new DNS. The Elastic IP here allows elasticity against any zone failure and allows the user to launch new instance, assign same elastic IP and this way the Domain Name mapping is not required to be changed.

The 5 EIPs are limited to control the IPs across internet. As AWS states “Elastic IPs are public (IPV4) internet addresses, and hence a scarce resource.We strongly encourage customers to use Elastic IPs primarily for load balancing use cases, and private IP addresses for all other internode communication”

In case the user want more than 5 EIPs for specific purpose, they can request AWS to increase the IPs for that region.

Note that, EIP when unassigned will cost the user but if it is assigned to some instance it will not cost.

The other alternative to overcome 5 EIP is start instance in some other region and use 5 EIPs available in that region. AWS also offers copying instance and copy AMI option too.

Refer http://www.newvem.com/topic/learning/guides/eip/ for more information about EIP