Azure VNet Calculator
Plan Azure Virtual Network address space, subnet sizing, and reserved IPs for delegation.
Inputs
Subnets delegated to services like App Service, ACI, or Azure NetApp Files often need extra reserved capacity
Max Subnets of This Size
256
Usable IPs per Subnet
251
Total VNet Address Space
65,536
Reserved IPs per Subnet (Azure)
5
Step by step
Values used
VNet Address Space Prefix = 16 /; Subnet Prefix = 24 /; Subnets Requiring Delegation = 1
Usable IPs per Azure subnet
usable = 2^(32 − subnet_prefix) − 5
Max Subnets of This Size
= 256
Usable IPs per Subnet
= 251
Total VNet Address Space
= 65,536
Reserved IPs per Subnet (Azure)
= 5
How it works
Azure VNets, like AWS VPCs, reserve 5 IP addresses in every subnet: the network address, a default gateway address, two addresses reserved for mapping Azure DNS, and the broadcast address. Azure supports VNet address spaces considerably larger than AWS's /16 cap (down to /8 in principle, though most real deployments use /16 to /24), and subnets delegated to platform services like App Service Environments, Azure NetApp Files, or Azure Container Instances typically need a larger minimum subnet size (often /24 or larger) to leave room for the service's own scale-out address consumption.
Formula
Usable IPs per Azure subnet
usable = 2^(32 − subnet_prefix) − 5
Frequently Asked Questions
What are Azure's 5 reserved IPs used for?
The network address (first), the default gateway (second), two addresses reserved to map Azure's DNS servers into the VNet space (third and second-to-last), and the broadcast address (last) — identical in count, though not identical in exact purpose, to AWS's 5 reserved addresses.
Why do delegated subnets need extra room?
Services like Azure App Service Environment or Azure NetApp Files provision their own internal infrastructure IPs within the delegated subnet as they scale, so a too-small subnet can block the service from scaling out or even from being created in the first place.
Can I add more address space to a VNet later?
Yes — unlike some cloud providers, Azure lets you add additional address ranges to an existing VNet after creation, and you can also add/resize subnets as long as the new ranges don't overlap with existing subnets or peered VNets.
Does VNet peering require non-overlapping address spaces?
Yes — two VNets with overlapping IP address ranges cannot be peered together, which is why address space planning across an organization's full set of VNets (and any on-premises RFC 1918 ranges) matters from the start, not just within a single VNet.