ANS-C01 Network Design Practice Question
A company has a VPC with a CIDR of 10.0.0.0/16 and needs to add a second CIDR block of 10.1.0.0/16 for additional subnets. After adding the CIDR, the existing subnets cannot communicate with the new subnets. What is the most likely cause?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Route tables for the existing subnets do not have routes to the new CIDR.
When adding a secondary CIDR to a VPC, route tables for existing subnets must be updated to include routes to the new CIDR block; otherwise, traffic destined for the new subnets will not be routed correctly. Option A is wrong because security groups are stateful and automatically allow return traffic; they are not the cause of connectivity loss when adding a new CIDR. Option B is wrong because you can add a secondary CIDR to an existing VPC without creating a new VPC. Option D is wrong because Network ACLs are stateless but are not the primary issue; the problem is the missing routes in the route tables.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Security groups are blocking traffic between the old and new subnets.
Why it's wrong here
Security groups apply to instances, not subnet-to-subnet traffic.
- ✗
The VPC CIDR cannot be extended; a new VPC must be created.
Why it's wrong here
VPCs can have multiple CIDR blocks.
- ✓
Route tables for the existing subnets do not have routes to the new CIDR.
Why this is correct
Route tables must include routes for the new CIDR to enable communication.
- ✗
Network ACLs are blocking traffic by default.
Why it's wrong here
NACLs allow all traffic by default unless configured otherwise.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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