The most common way to use an Elastic IP (EIP) is to assign it to a server when it's launched for the first time. You can also assign an Elastic IP to a current running instance. If you select an EIP, but uncheck the "Associate IP at launch" option, the instance will be randomly assigned a Public IP address at launch time; the Elastic IP will not be assigned to the instance.
There are several different ways to configure a server so that a specific Elastic IP will be associated and used by the instance at launch time.
You can also change the configuration of a server to associate an EIP at launch time. You can only apply this setting to an inactive server that has not been launched. So, if there is a "Current" server that's already running, you'll need to modify the "Next" server.
WARNING! You can steal an Elastic IP from a running instance! Be sure to check and make sure that a running instance is not using the EIP that you want to assign to a new server.
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