Public IP addresses are not persistent – i.e. if you stop/start your instance, the instance will get reassigned a new public IP address. Bummer

Elastic IPs were designed to address this.  They get allocated (initial limit of 5 EIPs per account) to your account.  You can use these throughout your AWS account – and once attached, they are persistent, through instance restarts.

Summary

Use EIPs for your load balancers and NAT instances and anything else that needs an unchanging (and publicly accessible) IP address.

If you need more than 5 EIPs, you can request AWS for a quota increase.

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