Perfect.
Today we’ll demystify AWS networking in a way that is useful, minimal, and permanent for a Data Engineer / Python developer.

You will not become a network engineer —
you will become network-aware enough to design & debug real AWS systems.

📅 DAY 3 — AWS NETWORKING (ONLY WHAT DATA ENGINEERS MUST MASTER)

VPC, Subnets, Public vs Private, and Why Things “Can’t Connect”

Amazon VPC

🎯 DAY 3 GOAL

By the end of today, you will:

  • Understand VPC, Subnets, Routing clearly
  • Know why some services need VPC and some don’t
  • Never panic when someone says “network issue”
  • Answer AWS networking interview questions confidently

🧠 PART 1 — WHAT A VPC REALLY IS (NO JARGON)

❌ Wrong understanding

VPC = networking service

✅ Correct understanding

VPC = your private data center inside AWS

Everything you launch (EC2, EMR, RDS) lives inside a VPC.

AWS Region
 └── VPC
      ├── Subnets
      │     ├── EC2
      │     ├── EMR
      │     └── RDS

📌 Glue & Lambda can run outside VPC (we’ll explain why)

🧩 PART 2 — SUBNETS (PUBLIC vs PRIVATE)

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What is a Subnet?

A subnet is a slice of a VPC, usually inside one AZ.

🔓 Public Subnet

Has access to the internet

Used for:

  • Bastion host
  • Load balancer
  • Public APIs

🔒 Private Subnet

❌ No direct internet access

Used for:

  • EMR
  • RDS
  • Backend services

📌 Data workloads should live in private subnets

🧠 PART 3 — ROUTING (THIS CAUSES 80% OF ISSUES)

Internet Gateway (IGW)

  • Allows public subnet → internet

NAT Gateway

  • Allows private subnet → internet (outbound only)
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Real-Life Example

EMR in private subnet
→ Needs to download Spark libs
→ Uses NAT Gateway

📌 No NAT = EMR bootstrap fails

🧠 PART 4 — SECURITY GROUPS vs NACLs (INTERVIEW FAVORITE)

FeatureSecurity GroupNACL
LevelInstanceSubnet
Stateful✅ Yes❌ No
Commonly used✅ YES❌ Rare

📌 Security Groups = main firewall you care about

🧠 PART 5 — WHY SOME AWS SERVICES DON’T NEED VPC

❓ Why Glue / Athena work without VPC?

Because:

  • They are AWS-managed
  • They access S3 via AWS backbone
  • No customer-managed network required

❓ Why EMR NEEDS VPC?

Because:

  • EMR runs on EC2
  • EC2 must live in a VPC
  • You control networking

🧠 PART 6 — DATA ENGINEER NETWORKING PATTERNS (REAL LIFE)

🔹 Pattern 1 — EMR (PRIVATE)

Private Subnet
 + NAT Gateway
 + S3 access

Used in 90% companies.

🔹 Pattern 2 — Lambda (NO VPC)

Lambda
 → S3
 → Glue

Faster, cheaper, simpler.

🔹 Pattern 3 — Lambda INSIDE VPC (RARE)

Used only when:

  • Needs RDS
  • Needs private service

⚠️ Adds complexity & cold start

🧠 PART 7 — COMMON REAL-WORLD FAILURES (VERY IMPORTANT)

❌ EMR can’t access S3
➡ IAM role or NAT missing

❌ Glue job stuck
➡ VPC misconfiguration

❌ Lambda timeout
➡ VPC ENI cold start

🎤 INTERVIEW STATEMENTS (MEMORIZE)

✔ “VPC provides network isolation within an AWS region.”
✔ “Private subnets are used for data workloads like EMR and RDS.”
✔ “Glue and Athena don’t require VPC because AWS manages networking.”
✔ “NAT Gateway allows outbound internet access for private subnets.”

🧪 DAY 3 THINKING EXERCISE

Answer mentally:

  1. Why is EMR placed in private subnets?
  2. Why is NAT needed for EMR but not for Glue?
  3. What happens if a private subnet has no route to NAT?

🧠 DAY 3 MEMORY MAP (SAVE THIS)

VPC = Private Data Center

Public Subnet  → Internet
Private Subnet → No Internet

IGW → Public access
NAT → Private outbound access

Security Group > NACL

⏭️ DAY 4 PREVIEW — EC2 DEEP DIVE (WHY IT STILL MATTERS)

Tomorrow:

  • EC2 vs Lambda vs Containers
  • Instance families
  • EBS vs instance store
  • Why EC2 still exists in serverless world
  • Bastion host concept

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DAY 4

We continue building mastery 🚀

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