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You created a VPC but your instances cannot communicate with each other. What is the most likely cause?

medium📝 Debug Q14 of 15
AWS - VPC Fundamentals
You created a VPC but your instances cannot communicate with each other. What is the most likely cause?
ASecurity groups block all inbound and outbound traffic.
BThe route table has a route to the local network.
CThe subnet CIDR block overlaps with another VPC.
DThe VPC has no internet gateway attached.
Step-by-Step Solution
Solution:

  1. Step 1: Analyze communication issue

    Instances in a VPC communicate if security groups allow traffic.

  2. Step 2: Check options

    No internet gateway affects external access, overlapping CIDR causes conflicts but not internal block, route to local network is needed for communication.

  3. Final Answer:

    Security groups block all inbound and outbound traffic. -> Option A

  4. Quick Check:

    Blocked security groups = no communication [OK]
Quick Trick: Check security group rules first for communication issues [OK]
Common Mistakes:
  • Assuming internet gateway affects internal traffic
  • Ignoring security group rules
  • Thinking route table with local route blocks traffic

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