In Amazon VPC, the VPC creation wizard allows one to create a single "public subnet" or have the wizard create a "public subnet" and a "private subnet". Initially, the public and private subnet option seemed good for security reasons, allowing webservers to be put in the public subnet and database servers to go in the private subnet.

But I've since learned that EC2 instances in the public subnet are not reachable from the Internet unless you associate an Amazon ElasticIP with the EC2 instance. So it seems with just a single public subnet configuration, one could just opt to not associate an ElasticIP with the database servers and end up with the same sort of security.

Can anyone explain the advantages of a public + private subnet configuration? Are the advantages of this config more to do with auto-scaling, or is it actually less secure to have a single public subnet?