Associates an Elastic IP address with an instance or a network interface. Before you can use an Elastic IP
address, you must allocate it to your account.
An Elastic IP address is for use in either the EC2-Classic platform or in a VPC. For more information, see Elastic IP Addresses
in the Amazon Elastic Compute Cloud User Guide.
[EC2-Classic, VPC in an EC2-VPC-only account] If the Elastic IP address is already associated with a different
instance, it is disassociated from that instance and associated with the specified instance. If you associate an
Elastic IP address with an instance that has an existing Elastic IP address, the existing address is
disassociated from the instance, but remains allocated to your account.
[VPC in an EC2-Classic account] If you don't specify a private IP address, the Elastic IP address is associated
with the primary IP address. If the Elastic IP address is already associated with a different instance or a
network interface, you get an error unless you allow reassociation. You cannot associate an Elastic IP address
with an instance or network interface that has an existing Elastic IP address.
This is an idempotent operation. If you perform the operation more than once, Amazon EC2 doesn't return an error,
and you may be charged for each time the Elastic IP address is remapped to the same instance. For more
information, see the Elastic IP Addresses section of Amazon
EC2 Pricing.