AWS Interview DropIntermediate TierScenario+25 XP on read
Two EC2 instances in the same VPC can't reach each other over a port that should be open. Walk through how you'd find out why.
Core Summary
Work outward from the instance rather than guessing at the network: confirm the app is actually listening, then check Security Groups, then NACLs, then the route table, then Flow Logs to see what AWS's own network actually did with the packet. Each layer either clears itself or points straight at the next thing to check.
Hints
Hint 1: Confirm the process is listening before blaming the network at all
Hint 2: Security Groups first, since they're the most common cause and the easiest to check
Hint 3: VPC Flow Logs will show you ACCEPT or REJECT, which tells you exactly which layer is involved