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> after startup the EC2 instances have new IPs

They have static ("Elastic") IP addresses:

http://aws.amazon.com/articles/1346?_encoding=UTF8&jiveR...

..don't these let you keep the same IP for your Puppet master instance?

That's correct. The problem is that the EC2 internal IPs change even though the instance would have an Elastic IP. EC2 instances use internal IPs when communicating with other EC2 instances (this is a feature of AWS DNS). As a consequence, Puppet clients cannot access the master by using the master's Elastic IP.

That sounds rather inconvenient. Are the external elastic IPs non-routable internally? I mean, if you add the IPs explicitly to hosts files then will traffic to those IPs not work?

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