To configure multiple NICs in an Azure VM, first confirm that the VM size supports the number of interfaces you need, then create or attach one Azure NIC resource per interface. Every NIC on the VM must connect to the same virtual network, although the NICs can use different subnets within it. After Azure attaches the interfaces, configure the guest operating system’s routes so traffic uses the intended NIC.
Check whether you need multiple NICs
A NIC is an Azure network interface resource. Multiple NICs give a VM separate interfaces, which can connect to different subnets in the same virtual network. By contrast, multiple IP configurations put additional IP addresses on one NIC; all of that NIC’s IP configurations must use the same subnet. Choose multiple IP configurations when the addresses belong to one subnet, and multiple NICs when you need interfaces on separate subnets or separate interface-level network configuration. See Microsoft’s guidance on network interfaces and IP configurations.
| Need | Use | Important constraint |
|---|---|---|
| More than one IP address on an interface in one subnet | Multiple IP configurations on one NIC | All IP configurations on that NIC must use the same subnet. |
| Interfaces in different subnets | Multiple NICs | All NICs attached to the VM must be in the same virtual network. |
Check Azure’s requirements before creating or attaching NICs
- Check the VM size. A VM needs at least one NIC, and the maximum supported number depends on its size. Check the current limit for the selected size in Microsoft’s VM sizes documentation; do not assume a universal maximum.
- Keep the NICs in one virtual network. They can connect to different subnets, but not different virtual networks. Each NIC must also be in the same Azure location and subscription as its VM.
- Use an unattached NIC. A NIC being added cannot already be attached to another VM.
- Plan guest routing. Azure attachment creates the cloud-side interface, but it does not ensure the operating system sends traffic over it. Microsoft explains this requirement in its guide to multiple NICs on a Linux VM.
Create a new VM with multiple NICs
The standard Azure portal VM creation path creates one NIC. For a new VM with multiple interfaces, create the NIC resources first and pass them into VM creation using Azure CLI or PowerShell.
- Choose the subnets. Decide which subnet each NIC should use and confirm that all of them belong to the same virtual network.
- Confirm NIC capacity. Verify that the VM size supports the number of interfaces you plan to attach.
- Create one NIC per interface. Set each NIC’s virtual network, subnet, and applicable network security group (NSG) settings. The Azure CLI NIC reference documents NIC creation options, including optional IP forwarding.
- Create the VM and attach the NICs. With Azure CLI, use
az vm create --nicsand provide the NIC resources. With PowerShell, build the VM configuration, add the NIC IDs usingAdd-AzVMNetworkInterface, and create the VM withNew-AzVM. See Microsoft’s CLI and Linux multi-NIC instructions and PowerShell and Windows multi-NIC instructions. - Set or confirm the primary NIC. The first attached NIC is primary by default. Use the primary-NIC option supported by your chosen workflow if you need a different interface to be primary.
- Configure the guest OS. After the VM starts, configure interface settings and routes for the additional NICs, then verify the route table and that the settings persist after a reboot.
Add a NIC to an existing VM
For an existing VM, deallocate it before attaching another NIC. Stopping the operating system without deallocating the VM is not the documented procedure. The new NIC must meet the same size, virtual-network, location, subscription, and attachment constraints described above. Microsoft’s multi-NIC guide covers the workflow.
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- Create the NIC. Place it on a subnet in the VM’s virtual network and apply the appropriate network settings.
- Deallocate the VM. In the Azure portal, open the VM and choose Stop; wait until its status indicates it is deallocated. If using CLI, run
az vm deallocatefor the VM. - Attach the NIC. Use the portal’s VM networking controls, Azure CLI
az vm nic add, or the applicable PowerShell VM network-interface cmdlets. Confirm that the VM size has capacity for the additional interface. - Start the VM. Start it from the portal or with
az vm start. - Configure guest networking. Set up the new interface and routes in the operating system, then verify the traffic path you intend to use.
Configure routing and primary-interface behavior
The first attached NIC is primary by default, and Azure’s default outbound path uses the primary NIC’s primary IP configuration. A secondary NIC can be present and connected without carrying the traffic you expect; guest routing determines how the operating system uses it. Configure routes deliberately for your workload and check the guest route table after setup.
Linux
Microsoft notes that persistent routes for multiple NICs on Linux must be added manually and that the exact steps vary by distribution. Follow the instructions for your distribution and networking stack; verify both the active routes and persistence after reboot. The Linux multi-NIC guide provides platform-specific guidance.
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Know when IP forwarding is relevant
Azure NIC IP forwarding is a separate setting from attaching multiple NICs. Enable it when the VM is acting as a network appliance and must forward packets whose source or destination is not one of its own assigned IP addresses. It is not required solely because the VM has more than one NIC. The Azure CLI NIC reference lists --ip-forwarding as an optional setting.
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Common setup failures to check
- The VM cannot accept another NIC: check the selected VM size’s current NIC limit.
- Azure rejects the NIC: verify it is not attached to another VM, and that it is in the same subscription, location, and virtual network as the VM.
- The VM starts, but traffic uses the wrong interface: inspect the guest route table and confirm which NIC is primary. Azure-side attachment alone does not configure secondary-NIC routing.
- Routes disappear after restart: configure persistent routes using the guest operating system’s supported method.
- You only need another address in the same subnet: consider another IP configuration on the existing NIC rather than adding a separate NIC.
Accelerated networking is a separate performance option, available only with supported VM sizes and operating systems; it is not a prerequisite for using multiple NICs. Check the selected size and OS compatibility in Microsoft’s VM size documentation.
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