Azure Visual Subnet Calculator
Interactive subnet planning tool. Divide and join subnets to design your Azure VNet architecture.
Plan Your Azure VNet Architecture
Proper subnet planning is the foundation of any Azure network architecture. This interactive calculator lets you visually divide a VNet address space into subnets — split them smaller or join them back together — while automatically accounting for Azure's 5 reserved IP addresses per subnet.
Use it for hub-spoke topologies, landing zone designs, or any scenario where you need to map out IP address allocation before deploying infrastructure. Start with a template or enter your own CIDR block.
Visual Subnet Calculator
Enter your VNet address space and interactively split it into subnets
Azure Reserved IP Addresses
Azure reserves 5 IP addresses in each subnet that cannot be assigned to resources:
The "Useable IPs" column shows available addresses after excluding these 5 reserved IPs.
Frequently Asked Questions
Why does Azure reserve 5 IP addresses in every subnet?
Azure reserves the first four addresses and the last address in each subnet: x.x.x.0 (network address), x.x.x.1 (default gateway), x.x.x.2 and x.x.x.3 (Azure DNS), and the last address (broadcast). This means a /28 subnet with 16 total addresses only provides 11 usable IPs for your resources.
What is the smallest subnet I can create in Azure?
The smallest supported subnet in Azure is a /29, which gives you 8 total addresses and 3 usable IPs after Azure's 5 reserved addresses. While /30 and /31 are valid CIDR notations, Azure requires at least a /29 for most services. Some services like Azure Application Gateway require even larger subnets (minimum /26).
Can I resize an Azure subnet after creation?
Yes, you can resize a subnet in Azure as long as the new range doesn't overlap with other subnets in the same VNet and no resources are using IP addresses that would fall outside the new range. You can expand or shrink subnets through the Azure Portal, CLI, or IaC tools like Terraform. However, it's much easier to plan correctly upfront than to resize later.
How should I plan subnets for VNet peering?
When planning for VNet peering, ensure that address spaces don't overlap between peered VNets. Use non-overlapping CIDR blocks for each VNet (e.g., 10.1.0.0/16 for production, 10.2.0.0/16 for development). This calculator helps you visualize how to divide each VNet's address space into subnets without overlap.
What is a good subnet sizing strategy for Azure landing zones?
A common approach is to use /16 VNets for each spoke and divide them into purpose-specific subnets: /24 for general workloads (251 usable IPs), /26 for Application Gateway (59 usable IPs), /27 for Azure Firewall subnets, and /28 for small management subnets. Always leave room for growth — it's easier to start with larger subnets than to expand later.