Plan an Azure VNet address space with the five reserved addresses per subnet and check the design actually fits.
Azure takes five addresses out of every subnet rather than the two an on-premises network loses, so a /29 yields three usable addresses and a /24 yields 251. Subnet capacity within the VNet is a simple power of two on the difference between the prefix lengths, and both constraints have to pass for the design to be viable. Subnets cannot be resized while resources are deployed in them, and services such as Application Gateway, Azure Firewall, Bastion and the VPN Gateway each demand their own dedicated subnet with a minimum size, so an address plan that is short by one bit becomes a redeployment.
Azure Virtual Network
usable addresses = 2^(32 − subnet prefix) − 5, because Azure reserves the network address, the default gateway, two DNS-mapping addresses and the broadcast address in every subnet.
usable addresses = 2^(32 − subnet prefix) − 5, because Azure reserves the network address, the default gateway, two DNS-mapping addresses and the broadcast address in every subnet. Azure takes five addresses out of every subnet rather than the two an on-premises network loses, so a /29 yields three usable addresses and a /24 yields 251. Subnet capacity within the VNet is a simple power of two on the difference between the prefix lengths, and both constraints have to pass for the design to be viable.
Subnets cannot be resized while resources are deployed in them, and services such as Application Gateway, Azure Firewall, Bastion and the VPN Gateway each demand their own dedicated subnet with a minimum size, so an address plan that is short by one bit becomes a redeployment.
This calculator takes 5 inputs: VNet address space, VNet prefix length, Subnet prefix length, Subnets required, Usable addresses needed per subnet. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.