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AWS – Private IP Addresses, Public IP addresses, and Elastic IP Addresses

A public IP address is not a static IP address, i.e. it will change if you reboot your EC2 instance

 

Elastic IP Addresses on the other hand is persistant. you can assign to one instance, or at a later date reassign it to a different instance.

Inside a vpc we can have to types of subnets:

 

  • Private subnets - This is a subnet that does not have an internet gateway attached to it. This means an instance ┬ácan't access the internet via this type of subnet. This address is persitant and will survive a reboot.
  • Public subnets - This is a subnet that does have an internet gateway attached to it.

 

In order for an instance to have internet access, it first needs to be in a vpc that contains a Public Subnet. The next thing it needs is either a public address or elastic ip address.

Attaching a Public/Elastic IP address to an instance is a bit like assigning an IP address to an instance's network card.

Elastic IP address works by associating itself to a public IP address.

 

It is possible for an instance to have a public/elastic ip address, but is a vpc that only contains a private subnet. In this situation, the instance will still not have access to the internet.

Therefore for an EC2 instance to have internet access, that instance needs to have a public/elastic IP address assigned to it, and the instance needs to also reside inside a public subnet.

 

 

 

 

 

 

 

 

 

 

http://aws.amazon.com/ec2/pricing/#Elastic_IP_Addresses

 

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/elastic-ip-addresses-eip.html