When you implement a landing zone or deploy workloads on Azure, Virtual Network (VNet) is usually the very fundamental Azure resource that you need to plan and deploy first before other resources. Among all other important aspects that you need to plan for a VNet, there is a basic one: the address space of the VNet and its subnets. You need to ensure the VNet has the plenty of the address space for your workloads and the future growth. At the same time, you should also try not to waste IP addresses. When you integrate Azure resources with the VNet, different resources may have different scaling patterns and therefore require different address space for the subnet. So, it’s important to plan the subnets based on the requirements of the resources that you will deploy.
Quite often I got asked by customers about what kind of address space they need to plan for the VNet and subnets to run their workloads. To simplify this planning task, I created a tool, Azure Virtual Network Capacity Planner. With this tool, you create subnets for the Azure resources that you need to deploy. It helps you calculate the address space the subnets need and eventually the address space of the VNet. You can then export the result into the ARM template or CSV file for the actual deployment.
The following is a screenshot of the planner.