Using Existing Public IP Assigned to EC2 as Elastic IP

Question

Hi all

I have got a EC2 instance and a public IP.

I would like to use this public IP (existing one) as an "elastic" IP.

Is this possible?  I have got a few elastic IP created but I prefer
using the existing IP assigned to my EC2 instance as "elastic" IP
instead of assign a new IP to my EC2 instance.

Note: I have assigned this public IP (that was assigned to my existing EC2 instance) 
to DNS hostname e.g. abc.mycompany.com and we are providing 24x7 service at the moment.

If possible, we do not want to interrupt existing service by changing 
a new public IP, reassign it to DNS host and wait for the DNS to propagate etc.

Thanks.

Regards
Ken

Answer

Unfortunately, it is not possible - https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-instance-addressing.html#concepts-public-addresses

A public IP address is assigned to your instance from Amazon's pool of public IPv4 addresses, and is not associated with your AWS account. When a public IP address is disassociated from your instance, it is released back into the public IPv4 address pool, and you cannot reuse it.

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

Answer

Unfortunately, it is not possible to convert a existing public IP to elastic IP. The reason is that both the public and elastic IP use different IP address pool.

Hence, if an elastic IP is required, it can be associated to the instance post which the existing public IP will be released back to the pool.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-instance-addressing.html#concepts-public-addresses

Using Existing Public IP Assigned to EC2 as Elastic IP

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