ANS-C01 Network Implementation Practice Question
A company is deploying a VPC with a CIDR block of 10.0.0.0/16. The VPC requires six subnets: three public and three private, each with a /24 CIDR. The company needs to ensure high availability across three Availability Zones. Which TWO of the following are valid subnet CIDR assignments that meet these requirements?
⚠ Common exam trap
It's easy for candidates to assume subnets can share the same base IP range as long as the subnet mask differs, but AWS requires all subnets in a VPC to have non-overlapping CIDR blocks regardless of prefix length.
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
✓
Public: 10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24; Private: 10.0.3.0/24, 10.0.4.0/24, 10.0.5.0/24
It assigns six non-overlapping /24 subnets (10.0.0.0/24 through 10.0.5.0/24) within the 10.0.0.0/16 VPC, allowing three public and three private subnets to be distributed across three Availability Zones for high availability. Option C is also correct as it uses the same six /24 subnets but interleaves public and private ranges (e.g., public: 10.0.0.0/24, 10.0.2.0/24, 10.0.4.0/24; private: 10.0.1.0/24, 10.0.3.0/24, 10.0.5.0/24), which still provides non-overlapping /24 CIDRs and supports the required multi-AZ architecture.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Public: 10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24; Private: 10.0.0.0/25, 10.0.1.0/25, 10.0.2.0/25
Why it's wrong here
Overlap with public subnets.
- ✓
Public: 10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24; Private: 10.0.3.0/24, 10.0.4.0/24, 10.0.5.0/24
Why this is correct
Non-overlapping, three AZs each.
- ✓
Public: 10.0.0.0/24, 10.0.2.0/24, 10.0.4.0/24; Private: 10.0.1.0/24, 10.0.3.0/24, 10.0.5.0/24
Why this is correct
Non-overlapping, each /24 in different AZ.
- ✗
Public: 10.0.0.0/24, 10.0.0.0/25, 10.0.0.128/25; Private: 10.0.1.0/24, 10.0.1.0/25, 10.0.1.128/25
Why it's wrong here
Overlapping CIDRs within same AZ.
- ✗
Public: 10.0.0.0/24, 10.0.1.0/24, 10.0.2.0/24; Private: 10.0.0.0/25, 10.0.1.0/25, 10.0.2.0/25
Why it's wrong here
Overlaps with public subnets.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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