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An Elastic IP (EIP) is an IP address that you can reserve from AWS for your account. Once you've created an Elastic IP, you can assign it to any instance of your choice. Once you reserve an Elastic IP, nobody else can use that IP address. Elastic IPs are unique because they are dynamically remappable IP addresses that make it easier to manage servers and make global changes in the cloud. Whereas static IPs are associated to a particular machine, EIPs can be reassigned to different instances when necessary as you launch and terminate servers. Typically, you will associate EIPs to your frontend servers. You can assign an EIP to a running instance or associate an EIP when an instance is launched. Be careful, you can also "steal" an EIP from one of your instances. As a best practice, you should age any new EIP before you assign it to one of your public facing servers because that IP address may still be temporarily cached and mapped to its previous instance. You do not want to accidentally inherit unintended traffic from its predecessor.
Note: By default, Amazon will allow you to reserve 5 EIPs per account. You can request more directly with Amazon. EIPs are free to use as long as they are being used (associated) by an operational instance. However, you will be charged by Amazon for unused EIPs. This is because they are publicly routable IP addresses. Public IPv4 addresses are a limited resource, so if you reserve addresses and do not use them, it is considered a wasteful practice and you will be charged by Amazon.
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