Associates an Elastic IP address with an instance or a network
interface.
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.
[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.
This is an idempotent operation. If you perform the operation more
than once, Amazon EC2 doesn't return an error.