Subnet Design: Six /24 Subnets Across Two AZs
A company is setting up a new VPC with a CIDR block of 10.0.0.0/16. They need to create subnets for different tiers: public (web servers), private (application servers), and database (RDS). They want to maximize the number of available IP addresses while ensuring each subnet has at least 256 IP addresses. Which subnet design meets these requirements?
Quick Answer
The answer is to create six /24 subnets across two Availability Zones, with three subnets per AZ. This design is correct because each /24 subnet provides exactly 256 IP addresses, meeting the minimum requirement of at least 256 IPs per subnet for the public, private, and database tiers, while also ensuring high availability by distributing the subnets across two AZs. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of subnet sizing and AZ redundancy—a common trap is choosing a /25 (128 IPs) or /26 (64 IPs) subnet, which fails the 256-IP requirement, or using only one AZ, which violates high-availability best practices. A useful memory tip: for “256 IPs, think /24—that’s the magic number for a full Class C block, and always pair it with two AZs for fault tolerance.
⚠ Common exam trap
The trap here is that candidates focus solely on the IP count requirement (at least 256 IPs) and overlook the high-availability requirement implied by 'maximize the number of available IP addresses' and best practices, leading them to choose Option D (three /24 subnets in one AZ) instead of the correct multi-AZ design.
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
✓
Create six /24 subnets (256 IPs each) across two Availability Zones (three per AZ).
It creates six /24 subnets (each with 256 IP addresses) across two Availability Zones, meeting the requirement of at least 256 IPs per subnet while maximizing total available IPs (1,536 IPs) and providing high availability by distributing subnets across multiple AZs. The /24 subnet mask provides exactly 256 IPs (254 usable) per subnet, which satisfies the minimum requirement without wasting address space.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Create six /24 subnets (256 IPs each) across two Availability Zones (three per AZ).
Why this is correct
/24 provides 256 IPs, and using two AZs provides high availability.
- ✗
Create three /25 subnets (128 IPs each) in one Availability Zone.
Why it's wrong here
/25 provides only 128 IPs, less than required 256, and single AZ is not resilient.
- ✗
Create six /26 subnets (64 IPs each) across two Availability Zones.
Why it's wrong here
/26 provides only 64 IPs, less than required 256.
- ✗
Create three /24 subnets (256 IPs each) in one Availability Zone.
Why it's wrong here
Single AZ is not highly available.
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Same concept, more angles
1 more way this is tested on ANS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
medium- A.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
- ✓ B.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
- ✓ C.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
- D.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
- E.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 B: 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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.