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Default VPC

🏗️ What is a Default VPC in AWS?

The default VPC is a pre-created, ready-to-use network that AWS sets up automatically in each AWS Region when you create an AWS account.

📌 It’s there to make it easier for users to launch EC2 instances and other resources quickly without needing to set up networking manually.

📦 Features of a Default VPC

Feature Description
CIDR Block 172.31.0.0/16 (can accommodate 65,536 IPs)
Subnets One public subnet per Availability Zone in the region
Internet Gateway Attached automatically, allowing internet access
Route Table Default route (0.0.0.0/0) points to Internet Gateway
Security Group Default SG allows inbound traffic from same SG and all outbound
Network ACL Stateless, allows all inbound/outbound by default
DNS Resolution Enabled (so instances can resolve names)

📁 Default VPC Structure (in a single region)

Default VPC: 172.31.0.0/16
│
├── Subnet A: 172.31.0.0/20 (AZ a)
├── Subnet B: 172.31.16.0/20 (AZ b)
├── Subnet C: 172.31.32.0/20 (AZ c)
│   └── All subnets are public (route to IGW)
│
├── Internet Gateway (attached)
├── Route Table: 0.0.0.0/0 → IGW
├── Default Security Group
└── Default Network ACL

✅ Use Cases for Default VPC

Use Case Why It's Useful
Quick EC2 deployment Launch instances with public IPs instantly
Testing and learning No need to create a custom VPC manually
Small dev/test environments Faster setup for demos or proof of concepts

⚠️ Limitations of Default VPC

Limitation Description
No private subnets All default subnets are public (not suitable for secure apps)
Limited customization CIDR range, subnet sizes, naming are fixed unless modified
Not production-ready Lacks NAT, private subnets, custom routing, etc.
Can be accidentally deleted While recoverable, it's best to avoid using it for production

🔁 Recovering a Deleted Default VPC

If you deleted it:

📌 Use AWS Console:

  1. Go to VPC dashboard

  2. Click "Actions" > "Create Default VPC"

📌 Use AWS CLI:

aws ec2 create-default-vpc

🔧 Should You Use the Default VPC?

Scenario Recommendation
Quick test/dev ✅ Yes, default VPC is fine
Production ❌ No, use custom VPC with proper design
Learning AWS ✅ Great for experimenting

🧱 Default VPC vs Custom VPC

Feature Default VPC Custom VPC
CIDR Block 172.31.0.0/16 You define
Subnets One per AZ (public) You define (public + private)
NAT Gateway ❌ Not included ✅ Optional
Custom Routing ❌ Limited ✅ Full control
Use Case Dev/Test Production

🧠 Summary

  • Default VPC = AWS-created network per region

  • CIDR: 172.31.0.0/16

  • Subnets: 1 public subnet per AZ

  • Includes: IGW, route table, SG, NACL

  • Great for quick use/testing

  • Avoid in production → build a custom VPC