I know we can set up application with internal or external ip address using load balancer. If I use external Ip address I can reserve it in Azure beforehand as public. Now my question is what if I don't want that ip address to be visible from outside the cluster ?

Configuration for internal ip address in kubernetes yaml would be:

apiVersion: v1
kind: Service
metadata:
  name: internal-app
  annotations:
    service.beta.kubernetes.io/azure-load-balancer-internal: "true"
spec:
  loadBalancerIP: 10.240.1.90
  type: LoadBalancer
  ports:
  - port: 80
  selector:
    app: internal-app

Now I've read that the specified IP address must reside in the same subnet as the AKS cluster and must not already be assigned to a resource.

If I have ip address for my aks agentpool set up as X.X.0.0/16 and I use for example X.X.0.1 as Ip address for my internal load balancer I'm getting error: 'Private IP address is in reserved subnet range'

I see I also have something like internal endpoints in AKS. Can those be used for internal application-to-application communication ?

I'm just looking for any way for my apps to talk with each other internally with out exposing them to outside world. Also I'd like for that to be repeatable that means that something like dynamic ip addresses wouldn't be too good. I need the set up to be repeatable so I don't have to change all of the apps internal settings every time Ip address changes accidentally.