Saturday, January 13, 2024

Azure: Subnets with multiple IP address spaces?

 We were having a look today at the Azure documentation for virtual networks and subnets specifically. Both for Bicep and ARM there are two options to specify the addressprefix (address space) for a subnet. The first one is "addressPrefix" which takes a string as input and the second one is "addressPrefixes" which takes an array, see below. This leads one to expect that you can provide multiple IP address ranges for the subnet in the array in the same way that it can be done for VNets.

If you try to manually add an additional IP range to an existing subnet via the portal, it will show an error.

If you try to deploy multiple ranges via an ARM template, it still throws an error but we get a bit more information in the error message, see below:

The error states that the subscription is not registered for the following feature:


This is usually handled under Resource Providers for the subscription. If you go to subscriptions -> resource providers in the Portal, this feature is not there to enable, though.

It is possible however, to register the feature via Azure CLI. But when you run the command, this feature goes from "not registered" to "pending" and then it will just stay like that and never move to registered.

It looks like below:

The commands are:

az feature register --namespace Microsoft.Network -n AllowMultipleAddressPrefixesOnSubnet --subscription <subscriptionId>


az feature show --namespace Microsoft.Network -n AllowMultipleAddressPrefixesOnSubnet --subscription <subscriptionId>

It turns out that this feature is available only to MSFT developers and is not available either in public or private preview. There is not much info around this, as I suppose it is not really a sought after feature.

I found this explanation and also response from MSFT, see link here.

And then another person had the same issue as late as Jan 8, 2024, see link here.

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