Consider an server
ec2-50-1-2-3.compute-1.amazonaws.com, which is not publicly available and which does not have an elastic IP address. I cannot assign it an Elastic IP address as I don't have any more addresses to assign (used all 5 already on publicly-available servers).
The publicly-available servers need to access a service on
ec2-50-1-2-3.compute-1.amazonaws.com. However, if I restart that server then it may receive a different address and I'll have to update 20 websites across 5 webservers with the new address. Is there any way to refer to the
ec2-50-1-2-3.compute-1.amazonaws.com server which will persist even if I restart that server, considering that I have no more Elastic IP addresses to assign to it?
Is there any way to refer to
Key word "refer to" -- indeed, there is... a DNS
Whether your DNS is in Route 53 or elsewhere, a CNAME record refers a system asking for a particular host by name, to a different host -- also by name.
Let's say, for example, that the service this system provides is the generation of reports. In the "example.com" domain...
reports IN CNAME ec2-50-1-2-3.compute-1.amazonaws.com.
Any machine looking up "reports.example.com" from the DNS will be referred to the hostname ec2-50-1-2-3.compute-1.amazonaws.com which will of course resolve to the machine's IP address.
If the machine's IP address (and therefore, in AWS, its hostname) changes because the instance was terminated or failed or replaced, you only have to update the information in one place -- the DNS. The systems that need to access this system would be configured with "reports.example.com" instead of the other hostname, so they wouldn't have to be maintained individually.
If you are using Route 53, it's also possible to configure Route 53 to actually give out a different answer using failover routing with health checks and divert requests elsewhere when the instance isn't working properly.
Amazon will not give you any difficulty at all if you simply request more Elastic IPs. It's right here: Request to Increase Elastic IP Address Limit
It turns out the the best way to refer to other instances in AWS is to use Amazon Virtual Private Cloud (VPC). In VPC each machine gets a static internal IP address, which persists for the lifetime of the instance. In fact in VPC one can configure full networking!