How Azure subnet sizing works
Azure virtual networks use CIDR notation to define address spaces and subnets. A CIDR prefix describes how many bits identify the network. A smaller prefix number creates a larger address range: a /24 contains 256 IPv4 addresses, while a /27 contains 32.
Azure reserves five IPv4 addresses in every subnet: the first four addresses and the final address. That means a /24 provides 251 addresses for resources, while a /29 provides only three. Plan for platform components, private endpoints, scaling, upgrades, and future services before selecting the smallest possible subnet.
- Use non-overlapping ranges for connected VNets and on-premises networks.
- Leave capacity for scale-out events and rolling upgrades.
- Check service-specific subnet delegation and minimum-size requirements.
- Document ownership and intended use before allocating a new range.
Reading the calculator results
The network address is the first address in the calculated block. The broadcast address is the last mathematical address, although Azure does not use traditional broadcast traffic. The first Azure-usable address begins after the four addresses Azure reserves at the start of the range, and the last usable address is immediately before the final reserved address.
The calculator normalizes any host address to its actual network boundary. For example, entering 10.20.4.57/24 produces the network 10.20.4.0/24. Use the normalized result in infrastructure code and network documentation.
Azure VNet planning guidance
Subnet calculations are only one part of network design. Confirm routing, network security groups, private DNS, service endpoints, private endpoints, firewalls, peering, and hybrid connectivity. Address space is difficult to change after many resources and peerings depend on it.
For AKS, Application Gateway, Azure Firewall, private endpoints, and delegated platform services, consult the current Microsoft requirements before deployment. This calculator provides address mathematics and Azure reservation guidance; it does not validate a complete Azure architecture.
Frequently asked questions
How many IP addresses does Azure reserve in a subnet?
Azure reserves five IPv4 addresses in each subnet: the first four addresses and the final address in the range.
What is the smallest IPv4 subnet supported by Azure?
Azure generally requires at least a /29 subnet, but individual services can require a substantially larger range. Always check the service requirements.
Does this calculator send my network ranges to a server?
No. The IPv4 calculation is performed locally in your browser and is not uploaded by the tool.