AWS EIP Elastic IP Addresses

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. 

Elastic IP Address Basics

The following are the basic characteristics of an Elastic IP address:

• To use an Elastic IP address, you first allocate one to your account, and then associate it with your instance or a network interface.
• When you associate an Elastic IP address with an instance or its primary network interface, the instance's public IPv4 address (if it had one) is released back into Amazon's pool of public IPv4 addresses. You cannot reuse a public IPv4 address. For more information, see Public IPv4 Addresses and External DNS Hostnames.
• You can disassociate an Elastic IP address from a resource, and reassociate it with a different resource.
• A disassociated Elastic IP address remains allocated to your account until you explicitly release it.
• To ensure efficient use of Elastic IP addresses, we impose a small hourly charge if an Elastic IP address is not associated with a running instance, or if it is associated with a stopped instance or an unattached network interface. While your instance is running, you are not charged for one Elastic IP address associated with the instance, but you are charged for any additional Elastic IP addresses associated with the instance. For more information, see Amazon EC2 Pricing.
• An Elastic IP address is for use in a specific region only. 
• When you associate an Elastic IP address with an instance that previously had a public IPv4 address, the public DNS hostname of the instance changes to match the Elastic IP address. 
• We resolve a public DNS hostname to the public IPv4 address or the Elastic IP address of the instance outside the network of the instance, and to the private IPv4 address of the instance from within the network of the instance.
  
  

  Characteristic	EC2-Classic	EC2-VPC
  Allocating an Elastic IP address	When you allocate an Elastic IP address, it's for use in EC2-Classic; however, you can migrate an Elastic IP address to the EC2-VPC platform. For more information, seeMigrating an Elastic IP Address from EC2-Classic to EC2-VPC.	When you allocate an Elastic IP address, it's for use only in a VPC.
  Associating an Elastic IP address	You associate an Elastic IP address with an instance.	An Elastic IP address is a property of a network interface. You can associate an Elastic IP address with an instance by updating the network interface attached to the instance. For more information, see Elastic Network Interfaces.
  Reassociating an Elastic IP address	If you try to associate an Elastic IP address that's already associated with another instance, the address is automatically associated with the new instance.	If your account supports EC2-VPC only, and you try to associate an Elastic IP address that's already associated with another instance, the address is automatically associated with the new instance. If you're using a VPC in an EC2-Classic account, and you try to associate an Elastic IP address that's already associated with another instance, it succeeds only if you allowed reassociation.
  Associating an Elastic IP address with a target that has an existing Elastic IP address	The existing Elastic IP address is disassociated from the instance, but remains allocated to your account.	If your account supports EC2-VPC only, the existing Elastic IP address is disassociated from the instance, but remains allocated to your account. If you're using a VPC in an EC2-Classic account, you cannot associate an Elastic IP address with a network interface or instance that has an existing Elastic IP address.
  Stopping an instance	If you stop an instance, its Elastic IP address is disassociated, and you must reassociate the Elastic IP address when you restart the instance.	If you stop an instance, its Elastic IP address remains associated.
  Assigning multiple IP addresses	Instances support only a single private IPv4 address and a corresponding Elastic IP address.	Instances support multiple IPv4 addresses, and each one can have a corresponding Elastic IP address. For more information, see Multiple IP Addresses.