How to assign a private IP dynamically generated with AWS Cloudformation?

Question

I have an AWS Cloudformation, and I want two EC2 instances A and B on a private subnet under the same VPC.
In the application, I need A to access B.
How can I configure A with the dynamically allocated private IP of B?

I believe I can configure A with an environment variable (e.g., with aws:elasticbeanstalk:application:environment).
The problem is that I don't know how to get the IP of B. I assume it cannot be an elastic IP because it's private, hence it may be changed between deployments.

Answer 1

You use security groups so you don't have to worry about IP addresses. Create a sec group for instance in subnet A and one for instance in subnet b and then allow traffic between the sec groups. Any instance in sec group A can talk to an instance in sec group b over the port you define or all traffic.

instanceA:
  Type: AWS::EC2::Instance
  Properties:
    ImageId: !Ref imageID
    SecurityGroupIds:
    - !Ref SGroupA
    SubnetId: !Ref subnetA
instanceB:
  Type: AWS::EC2::Instance
  Properties:
    ImageId: !Ref imageID
    SecurityGroupIds:
    - !Ref SGroupb
    SubnetId: !Ref subnetb
SGroupA:
  Type: AWS::EC2::SecurityGroup
  Properties:
    GroupDescription: EC2 Instance access
SGroupB:
  Type: AWS::EC2::SecurityGroup
  Properties:
    GroupDescription: EC2 Instance access
SGroupAIngress:
  Type: AWS::EC2::SecurityGroupIngress
  Properties:
    GroupName: !Ref SGroupA
    IpProtocol: tcp
    ToPort: 80
    FromPort: 80
    SourceSecurityGroupName: !Ref SGroupB
SGroupBIngress:
  Type: AWS::EC2::SecurityGroupIngress
  Properties:
    GroupName: !Ref SGroupB
    IpProtocol: tcp
    ToPort: 80
    FromPort: 80
    SourceSecurityGroupName: !Ref SGroupA

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/quickref-ec2.html