What this calculator does
Start with a VPC CIDR, choose how many equal subnets you want, and get a subnet-by-subnet breakdown of CIDR, mask, network, broadcast, first IP, last IP, and usable host counts.
Instructions / Networking
AWS VPC subnet planner instructions
Use this guide to understand how a VPC CIDR is split into equal subnets and how to interpret each subnet's usable range.
Who should use it
Cloud architects, AWS learners, network engineers, and anyone planning public or private subnets inside a VPC.
Input definitions
- VPC CIDR: the parent IPv4 range, such as 10.0.0.0/16.
- Subnet count: the number of equal subnets to split the VPC into.
- Mode: AWS VPC or standard IPv4 math.
Formula explanation
- Subnet prefix = parent prefix + log2(subnet count)
- Subnet size = 2^(32 - subnet prefix)
- Each subnet starts where the previous one ends plus one address.
Example calculation
10.0.0.0/16 split into 4 equal subnets gives 4 /18 blocks:
- 10.0.0.0/18
- 10.0.64.0/18
- 10.0.128.0/18
- 10.0.192.0/18
How to interpret the result
Use the network address to identify the subnet, the broadcast address to see the end of the block, and the first and last usable IPs to plan instances, NAT gateways, or other resources.
Common mistakes
- Using a subnet count that is not a power of two.
- Forgetting that AWS reserves five IP addresses in each subnet.
- Choosing a CIDR that leaves no room for future growth.
- Mixing up the parent VPC CIDR with the child subnet CIDRs.
Limitations and disclaimers
- This MVP focuses on equal-sized IPv4 subnet planning.
- It does not generate multi-AZ placement rules.
- It does not replace architecture review for production networks.