How do you design a VPC with public and private subnets?
⚡ Short Answer
Split into public subnets (route to an Internet Gateway — for load balancers/bastions) and private subnets (no direct internet; outbound via a NAT Gateway — for app servers/DBs). Spread subnets across AZs. Keep databases in private subnets, reachable only from the app tier's Security Group.
☕Coffee Chat Question
Concept Made Simple
“How do you design a VPC with public and private subnets?”
🧠Mind Map Answer
Remember It Faster
🔥What If?
Think Beyond the Expected
A private-subnet service needs to call an external API but has no internet route — how?
Route its outbound traffic through a NAT Gateway in a public subnet — it gets internet egress without being reachable from the internet. For AWS services specifically (S3/DynamoDB), a VPC endpoint keeps traffic on the AWS network and avoids NAT cost.
😂Real World
The public/private subnet + NAT + multi-AZ layout is the canonical VPC design; putting databases in private subnets with SG-restricted access is baseline security.
🎯Interviewer's Expectation
Keywords they're listening for:
⚠️Common Mistakes
- ✗Databases in public subnets
- ✗No NAT for private outbound
- ✗Single-AZ subnets (no HA)
✅Best Practices
- ✓Private subnets for app/data tiers
- ✓NAT for egress; VPC endpoints for AWS services
- ✓Subnets across multiple AZs
🔁Follow-up Questions
- 1NAT Gateway vs VPC endpoint for S3 access?
- 2Why place databases in private subnets?
- 3How do route tables enforce this?
🧩Related Technologies
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Plain-language foundations
I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner. Topic: VPC (AWS) Interview question: "How do you design a VPC with public and private subnets?" Please: 1. Explain the core idea in simple, plain language, using an everyday analogy. 2. Define any technical terms you use. 3. Walk through one small, concrete example. 4. Finish with a single sentence I can easily remember. Keep the tone friendly and assume I'm new to this topic.
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