How do you ensure that your Azure SQL Server only accepts connections from a specific Subnet of some VNET?

You need to allow access to Azure services and configure a virtual network rule for the SQL Server.

• Sign in to Azure Portal
• Select your SQL Server
• Click Firewalls and virtual networks
• in the Virtual networks section, click on Add existing virtual network (this will open the rule editor)
• Fill in the rule name, select Virtual network, select Subnet name/address prefix
• Click Save

Note that the Service Endpoint needs to be created. If it doesn't exist, Azure will create one for you automatically on the VNET so that the VNET can communicate with the SQL Server.