ALB, EC2 Instance must be in the same subnet?

Question

My Public ELB is connected to Public Subnet A, Public Subnet B.
And there is an EC2 instance in Private Subnet C in the same AZ.
In the above environment, the EC2 Instance in Private Subnet C is registered in the Target Group of this Public ELB and used well.
Here I want to add instances from Private Subnet D (same AZ) to Target Group of this Public ELB.
Request Timeout occurs, can you tell me why and how to solve it?

Answer

There should be no problem having an EC2 instance as a target in a different subnet to the ALB as per the diagrams on this page as long as they're all in the same VPC.

Check to make sure that any NACLs allow access between the subnets. Also check the security groups on the instances to make sure that they allow access on the appropriate ports.

ALB, EC2 Instance must be in the same subnet?

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