If I have the following VPC in AWS:
10.0.0.0/16 and I provision an application load balancer (internal) and AWS selects the following ip addresses for me 10.0.0.9 and 10.0.0.12 inside the subnets I choose.
Question: Do the internal addresses (10.0.0.9 and 10.0.0.12) that are picked ever change for the life of the load balancer?
I understand if I delete the load balancer, it will pick new ones. I also understand that an internet application load balancer IP changes regularly (and thats why people use Network load balancers for static ips) but not much is said about the private internal ips associated with the load balancers.
Any information would be great.
Yes, they could change for application load balancers.
As the application load balancer scales with traffic it will "launch" more instances behind the scenes and use more IPs in your subnets (ENI creation). You don't see those instances in the console but you can have a look at the elastic network interfaces in the console. When it scales down, it's not guaranteed that you get the same IPs. This is why you always need some free space in the subnets used by your application load balancer.
Network load balancers have static private IPs (one ENI per availability zone). Those can be used in the security group of your instances, see Target Security Groups.