How to Fix “CIDR Not Within VPC Ranges” Error When Adding AWS Subnets for RDS

When trying to create a new subnet with CIDR block 10.0.1.0/24 in your VPC (10.0.0.0/24), AWS throws this error because:

10.0.1.0/24 is outside your VPC's allocated address space (10.0.0.0/24)

A /24 CIDR gives you 256 IP addresses (10.0.0.0 - 10.0.0.255). Your attempt to create 10.0.1.0/24 would require:

10.0.1.0 - 10.0.1.255 (another 256 addresses)
This lies outside your current VPC's range

Option 1: Expand Your VPC CIDR

Modify your VPC to use a larger CIDR block that can accommodate both subnets:

Original: 10.0.0.0/24 (256 addresses)
New: 10.0.0.0/16 (65,536 addresses)

Using AWS CLI:

aws ec2 modify-vpc-cidr-block \
    --vpc-id vpc-12345678 \
    --cidr-block 10.0.0.0/16

Option 2: Create Secondary CIDR Block

Add a new CIDR range to your existing VPC (AWS allows up to 5 CIDR blocks per VPC):

aws ec2 associate-vpc-cidr-block \
    --vpc-id vpc-12345678 \
    --cidr-block 10.0.1.0/24

Option 3: Create New VPC with Proper Sizing

For production environments, consider creating a new VPC with proper CIDR planning:

aws ec2 create-vpc \
    --cidr-block 10.0.0.0/16 \
    --tag-specifications 'ResourceType=vpc,Tags=[{Key=Name,Value=Production-RDS-VPC}]'

Start with /16 for medium deployments (65k IPs)
Use contiguous /24 blocks for subnets
Leave room for future expansion
Avoid overlapping ranges with peered VPCs

When creating subnets for RDS:

# Create DB subnet group
aws rds create-db-subnet-group \
    --db-subnet-group-name my-rds-group \
    --db-subnet-group-description "RDS Subnets" \
    --subnet-ids subnet-12345678 subnet-87654321

Remember to:

Place RDS in private subnets
Ensure proper route table configuration
Consider multi-AZ deployments

When working with AWS VPCs, a common error beginners encounter is:

10.0.1.0/24 CIDR is not within the CIDR ranges of VPC

This occurs when you attempt to create a subnet whose IP range isn't contained within the parent VPC's CIDR block. In your case, your VPC uses 10.0.0.0/24 which only includes IPs from 10.0.0.0 to 10.0.0.255.

A /24 CIDR provides exactly 256 IP addresses. Your VPC's current allocation:

VPC CIDR:      10.0.0.0/24
Usable range:  10.0.0.1 - 10.0.0.254
(network: 10.0.0.0, broadcast: 10.0.0.255)

When you try to create 10.0.1.0/24, this represents an entirely different block of 256 addresses that doesn't overlap with your VPC's range.

Option 1: Expand your VPC CIDR

# Using AWS CLI
aws ec2 associate-vpc-cidr-block \
  --vpc-id vpc-12345678 \
  --cidr-block 10.0.0.0/23

# This gives you 10.0.0.0 - 10.0.1.255 (512 addresses)
# Now you can create both subnets:
# - 10.0.0.0/24 (original)
# - 10.0.1.0/24 (new)

Option 2: Create a new VPC with larger CIDR

aws ec2 create-vpc --cidr-block 10.0.0.0/16

# Now you can create multiple /24 subnets:
# 10.0.1.0/24, 10.0.2.0/24, etc.

When planning your VPC architecture:

Start with a larger CIDR (like /16) even if you don't need all addresses immediately
Leave room for future expansion between subnets
Use consistent subnet sizes for easier management (typically /24 or /26)

For RDS deployments, you'll typically need subnets in at least two AZs:

# Example: Creating RDS subnets in us-east-1
aws ec2 create-subnet --vpc-id vpc-12345678 \
  --cidr-block 10.0.1.0/24 --availability-zone us-east-1a

aws ec2 create-subnet --vpc-id vpc-12345678 \
  --cidr-block 10.0.2.0/24 --availability-zone us-east-1b

# Then create DB subnet group
aws rds create-db-subnet-group \
  --db-subnet-group-name my-rds-group \
  --db-subnet-group-description "RDS Subnet Group" \
  --subnet-ids subnet-12345678 subnet-87654321