DEV Community

Er. Bhupendra
Er. Bhupendra

Posted on

VPC AND SUBNETS CIDR CLACUALTION

#aws #beginners #networking #tutorial

Yes 👍 bilkul le sakte ho — /16 VPC ke andar /18 subnet valid hai ✅
Bas 2 rules follow hone chahiye:

✅ 1. Rule Check

🔹 VPC: 

172.31.0.0/16
Enter fullscreen mode Exit fullscreen mode

🔹 Subnet: 

172.31.0.0/18  ✅ VALID
Enter fullscreen mode Exit fullscreen mode

✔ Smaller (more specific) than /16
✔ Inside VPC range
✔ No overlap (agar dusra subnet same range use na kare)

🧠 2. /18 ka matlab kya hota hai?

  • /18 = 4096 IPs
  • Range example: 
172.31.0.0/18
→ 172.31.0.0  to  172.31.63.255
Enter fullscreen mode Exit fullscreen mode

📊 3. /16 ko /18 me divide karte hain

Ek /16 VPC ko tum 4 subnets (/18) me tod sakte ho:

1. 172.31.0.0/18     → 0 – 63
2. 172.31.64.0/18    → 64 – 127
3. 172.31.128.0/18   → 128 – 191
4. 172.31.192.0/18   → 192 – 255
Enter fullscreen mode Exit fullscreen mode

👉 Ye sab non-overlapping hai ✅

❌ 4. Kab problem aayega?

Agar tum already ek subnet bana chuke ho:

172.31.16.0/20
Enter fullscreen mode Exit fullscreen mode

👉 Then:

172.31.0.0/18 ❌ (overlap karega)
Enter fullscreen mode Exit fullscreen mode

Kyuki:

  • /18 range: 0 – 63
  • /20 range: 16 – 31

👉 Overlap ho gaya → AWS reject karega

🔥 5. Real Interview Tip

👉 Technically /18 allowed hai
👉 BUT real world me:

✔ /24 → best practice
✔ /18 → too large (waste of IPs)

🎯 Final Answer (Short)

✔ Yes, /18 le sakte ho inside /16 VPC
❗ But:

  • overlap nahi hona chahiye
  • mostly /24 use hota hai practical me

Agar chaho toh main tumhe CIDR calculation ka shortcut (2 min me master) sikha deta hoon — interview me 100% kaam aayega 🚀

===============================
Great 👍 let’s build a clear visual + practical VPC subnet structure so this never confuses you again.

🧠 1. Full VPC Structure (Simple Visualization)

Image

Image

Image

Image

📦 2. Your VPC Setup (Example)

🔹 VPC 

CIDR: 172.31.0.0/16
Range: 172.31.0.0 → 172.31.255.255
Enter fullscreen mode Exit fullscreen mode

🧩 3. Divide into Subnets (Correct Way)

👉 Option 1: Easy & Safe (Recommended)

Use /24 subnets (256 IPs each)

Subnet 1 → 172.31.0.0/24
Subnet 2 → 172.31.1.0/24
Subnet 3 → 172.31.2.0/24
Subnet 4 → 172.31.3.0/24
...
Subnet N → 172.31.255.0/24
Enter fullscreen mode Exit fullscreen mode

👉 Total = 256 subnets possible

🌍 4. Real AWS Design (Interview Level)

Multi-AZ Setup (Best Practice) 

VPC: 172.31.0.0/16
│
├── AZ-1 (ap-south-1a)
│   ├── Public Subnet → 172.31.0.0/24
│   └── Private Subnet → 172.31.1.0/24
│
├── AZ-2 (ap-south-1b)
│   ├── Public Subnet → 172.31.2.0/24
│   └── Private Subnet → 172.31.3.0/24
│
├── AZ-3 (ap-south-1c)
│   ├── Public Subnet → 172.31.4.0/24
│   └── Private Subnet → 172.31.5.0/24
Enter fullscreen mode Exit fullscreen mode

🔥 5. Public vs Private Subnet

Type CIDR Example Purpose
🌐 Public 172.31.0.0/24 Internet access (EC2, Load Balancer)
🔒 Private 172.31.1.0/24 DB, Backend, Secure apps

⚙️ 6. Key Components Mapping 

Internet Gateway → attached to VPC

Public Subnet:
   Route → 0.0.0.0/0 → IGW

Private Subnet:
   Route → 0.0.0.0/0 → NAT Gateway
Enter fullscreen mode Exit fullscreen mode

❌ 7. What You Did Wrong (Now Clear) 

Your CIDR: 172.31.0.0/15 ❌
Enter fullscreen mode Exit fullscreen mode

👉 Why wrong:

  • Bigger than VPC (/16)
  • Overlaps everything
  • AWS rejects immediately

🧠 8. CIDR Trick (VERY IMPORTANT 🔥)

CIDR Size Use
/16 65,536 IPs VPC
/24 256 IPs Subnet (BEST)
/20 4096 IPs Large subnet

👉 Golden Rule:

Subnet CIDR > VPC CIDR  → ❌ WRONG
Subnet overlap          → ❌ WRONG
Enter fullscreen mode Exit fullscreen mode

🎯 9. Interview Ready Answer

If interviewer asks:

👉 "How do you design subnets in VPC?"

You say:

"I divide VPC CIDR (/16) into smaller non-overlapping subnets like /24, distribute them across multiple AZs, and separate public and private subnets for scalability and security."

🚀 Want Next Level?

I can also teach you:
✅ CIDR calculation tricks (2 min mastery)
✅ How many IPs AWS actually gives (5 IP reserved rule)
✅ Real-world architecture (EKS / Kubernetes + VPC)

Just say 👍

========================================
You’re getting this error because:

👉 Your subnet CIDR overlaps with an existing subnet inside the VPC

🔍 Problem Breakdown

  • VPC CIDR: 172.31.0.0/16
    → Range: 172.31.0.0 – 172.31.255.255

  • Existing Subnet: 172.31.16.0/20
    → Range: 172.31.16.0 – 172.31.31.255

  • Your Subnet: 172.31.0.0/15 ❌
    → Range: 172.30.0.0 – 172.31.255.255

👉 Issue:

  • /15 is larger than the VPC range
  • It also overlaps existing subnet (172.31.16.0/20)

🚫 Rule You Violated

Subnet CIDR must:

  1. ✅ Be inside VPC CIDR
  2. ❌ NOT overlap with any existing subnet
  3. ✅ Be smaller (more specific) than VPC

✅ Correct Fix

Use a valid smaller CIDR block, like:

✔️ Good Examples:

  • 172.31.0.0/20
  • 172.31.32.0/20
  • 172.31.48.0/20
  • 172.31.64.0/20

👉 Just make sure:

  • It does NOT fall in 172.31.16.0 – 172.31.31.255

💡 Simple Trick (Exam + Interview)

  • /16 → VPC
  • /24 → Most common subnet size (safe choice)

👉 So easiest solution:

Use: 172.31.X.0/24
Enter fullscreen mode Exit fullscreen mode

Example:

  • 172.31.50.0/24 ✅
  • 172.31.100.0/24 ✅

🧠 Pro Tip (Real-world)

Run this mentally:

👉 If subnet CIDR is bigger than VPC (/15 vs /16)
→ ❌ Always wrong

👉 If CIDRs overlap
→ ❌ AWS will reject

🔥 Final Answer (What you should do)

👉 Replace:

172.31.0.0/15 ❌
Enter fullscreen mode Exit fullscreen mode

👉 With something like:

172.31.32.0/20 ✅
Enter fullscreen mode Exit fullscreen mode

or safest:

172.31.50.0/24 ✅
Enter fullscreen mode Exit fullscreen mode

If you want, I can:
✅ Draw full VPC + subnet structure
✅ Give CIDR shortcut tricks (super useful for interviews like CKA/AWS)

Top comments (0)