On reboot, the IP address of an amazon instance changes. How to find the new IP address using java API?
Assuming your rebooting the instance and not launching from scratch than you can assign an elastic IP which always remains with the ec2 instance(unless you move the IP to another server). This allows you to point all your DNS to that one IP and not worry that a reboot will cause you issues.
I think thats what your asking but there are other things you could be asking. The internal IP of the server changes(if you relaunch the instance not reboot) and you can't 'reserve' it so you may need to create a script to keep you the new IP(if your pointing internal services to it).
hope that helps