I have 3 web servers (ec2 instances) in a vpc public subnet, and they all have EIPs.
I have the usual Internet Gateway with the route to ensure those instances will be able to call external services, and a load balancer (ELB) rotating incoming traffic between those instances.
The main issue is that the application on the servers will connect to a lot of external services/APIs, but some of those services requires IP whitelisting. What we do now is to assign an EIP to an instance, go to the control panel of said service, add the new EIP and live on, but we're hitting a limit, both in numbers of EIPs available to our account (yes, I know it can be bumped, but still) and in how many IPs are whitelistable in those external services.
Given that we would like to keep things autoscalable I would like to ask for ideas on how to route all the instance generated traffic through a single EIPs without losing the current inner workings. A fixed special route towards those services is not really flexible unless it's going to use DNS, mostly because we can't know in advance all their IP pools.
I've read up about NAT gateways, but I'm not sure if the Load Balacing portion of the architecture will remain up, if the web servers will be able to respond back to requests, etc.