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Cannot allocate IP from prefix to Azure Firewall

Bryan Orsmond 5 Reputation points
2025-10-01T15:20:16.8533333+00:00

I have an existing IP Prefix with a /28 range. (Standard SKU)

I have an existing Azure firewall (PremiumSku)

Both are in the same region.

I have previously allocated IP's from the prefix to the Firewall Public IP's, however, I am not able to do that anymore.

From the Firewall Public IP Configuration - Add Public IP Configuration the pop up blade does not show the available IP from the prefix and nor dos it allow me to add one.

From the Prefix IP address side the Associate blade does not offer the Firewall under the list of Network Devices.

What am I missing - the prefix has available IP's and the IP is not currently associated with any device. Also the Firewall currently only has 9 IP's allocated to it.

Appreciate any advice - this is a production system

Many thanks

Azure Firewall
Azure Firewall
An Azure network security service that is used to protect Azure Virtual Network resources.
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  1. Jose Benjamin Solis Nolasco 6,051 Reputation points
    2025-10-01T15:43:04.79+00:00

    @Bryan Orsmond I hope you are doing well,

    In Azure, Firewall Public IPs cannot be allocated directly from an IP Prefix in the Firewall blade. The correct process is:

    Create a Public IP resource from your IP Prefix (Standard SKU, same region).

    Once the Public IP exists, go to your Firewall → Public IP Configuration → select that Public IP.

    That’s why the option doesn’t show in the Firewall blade — only Public IP resources are displayed there, not the Prefix itself.

    Also, for your /28, keep in mind Azure reserves 5 addresses, so you’ll have 11 usable IPs.

    If you get an error, please share with us a printscreen of it blurred any confidential information.

    😊 If my answer helped you resolve your issue, please consider marking it as the correct answer. This helps others in the community find solutions more easily. Thanks!

  2. Bryan Orsmond 5 Reputation points
    2025-10-01T19:59:30.98+00:00

    I managed to solve it using powershell.

    $pip = Get-AzPublicIpAddress -Name $pipName -ResourceGroupName $rg

    $fw = Get-AzFirewall -Name $fwName -ResourceGroupName $rg

    $fw.AddPublicIpAddress($pip)

    $fw | Set-AzFirewall

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