Gateway Subnet in Azure Virtual Networks
The Gateway Subnet is a special subnet in your virtual network (VNet) that you dedicate to host your VPN gateways and ExpressRoute gateways. When you deploy a gateway, you must specify this subnet. Azure uses this subnet to manage the gateway resources.
Purpose and Requirements
The Gateway Subnet serves a critical role in enabling connectivity to your Azure VNet from on-premises networks or other Azure VNets. Here are the key requirements and considerations:
- Dedicated Subnet: The Gateway Subnet must be a dedicated subnet within your VNet. You cannot deploy any other Azure resources within this subnet.
- Naming Convention: The subnet name must be
GatewaySubnet. Azure specifically looks for this name to deploy and manage gateway resources. - Address Space: While Azure manages the IP addresses within the Gateway Subnet, it's recommended to allocate a sufficiently large address range for it. A
/27or larger CIDR block is generally recommended, though smaller sizes like/28might be sufficient for some scenarios. This ensures you have enough IP addresses for future gateway scaling or upgrades. - No Network Security Groups (NSGs): Do not associate Network Security Groups (NSGs) with the Gateway Subnet. NSGs can interfere with gateway operations and connectivity.
- No Application Security Groups (ASGs): Do not associate Application Security Groups (ASGs) with the Gateway Subnet.
- No Route Tables: Do not associate custom route tables with the Gateway Subnet.
- IP Addressing: Azure assigns IP addresses from the Gateway Subnet to the gateway instances. You do not manually assign IP addresses to resources within this subnet.
Creating a Gateway Subnet
You can create the Gateway Subnet either during VNet creation or by adding it to an existing VNet. Here's a general outline:
Using Azure Portal
- Navigate to your virtual network in the Azure portal.
- Under "Settings", select "Subnets".
- Click "+ Gateway subnet".
- Azure will pre-populate the name as
GatewaySubnet. - Define the address range for the Gateway Subnet (e.g., 10.0.4.0/27).
- Click "Save".
Using Azure CLI
az network vnet subnet create \
--address-prefixes <your-gateway-subnet-cidr> \
--name GatewaySubnet \
--resource-group <your-resource-group-name> \
--vnet-name <your-vnet-name>
Using Azure PowerShell
$vnet = Get-AzVirtualNetwork -Name <your-vnet-name> -ResourceGroupName <your-resource-group-name>
Add-AzVirtualNetworkSubnetConfig -Name "GatewaySubnet" -AddressPrefix <your-gateway-subnet-cidr> -VirtualNetwork $vnet
Set-AzVirtualNetwork -VirtualNetwork $vnet
Gateway Subnet Size Recommendations
The required size of the Gateway Subnet depends on the type of gateway you deploy and its configuration. Here are general recommendations:
| Gateway Type | Minimum Recommended CIDR | Notes |
|---|---|---|
| VPN Gateway (Basic, VpnGw1-VpnGw5, VpnGw1AZ-VpnGw5AZ) | /27 | A /27 provides 32 IP addresses. |
| ExpressRoute Gateway | /28 | A /28 provides 16 IP addresses. For larger configurations or potential future scaling, consider /27. |
| VNet-to-VNet Gateway | /27 | Similar to VPN Gateways. |
Common Pitfalls and Best Practices
- Incorrect Subnet Name: Ensure the subnet is always named
GatewaySubnet. - Overlapping IP Addresses: Make sure the Gateway Subnet's IP range does not overlap with any other subnets in your VNet or any on-premises networks you intend to connect to.
- Associating NSGs/Route Tables: Avoid applying NSGs or route tables to the Gateway Subnet, as this can break gateway functionality.
- Insufficient Size: Underestimating the required size can lead to deployment failures or limitations when scaling your gateway.
Understanding and correctly configuring the Gateway Subnet is fundamental to successfully implementing VPN or ExpressRoute connectivity for your Azure resources.