Hey Oko,
The EIP is yours until you release it. Be advised, if it is important, *do not release it*, as EIPs have a very short "free" lifetime at AWS and you may not get it back (though I have seen it happen if the customer opens a case very, very quickly after releasing it).
I assume you are talking about reverse dns? There's a form for reverse dns requests:
https://console.aws.amazon.com/suppo...code-ec2-email
Please note that you should have an A record *within your domain* that points to the EIP so our automation tools can verify the request easily. The A record that Amazon maintains won't play a role in the EIP's usefulness, honestly. You can have numerous A records that all point to the same IP (though typically the actual hostname is the A record and the remainder are CNAMEs...but for rDNS purposes you need to have an A within your domain for verification).
As for public/private subnets, see the following:
http://docs.aws.amazon.com/AmazonVPC...Scenario2.html
There's also a Scenario3, which covers VPN-only private subnets, which may or may not fit your requirements:
http://docs.aws.amazon.com/AmazonVPC...Scenario3.html
PM me if you want to discuss further.