Subnet Size Considerations for Azure Application Gateway v2 SKU

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Subnet Size Considerations for Azure Application Gateway v2 SKU

InfraSolutions 646

In the documentation for Application Gateway V2, it mentions that a /24 subnet is recommended for sufficient autoscaling and maintenance upgrades, allowing support for up to 125 instances. However, the current infrastructure only requires around 12 instances.

Is a /24 subnet still necessary in this scenario, or can smaller subnets such as /26 or /27 be utilized? If smaller subnets can be used, what is the maximum number of instances and connections they can support?

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Answer 1

Rohith Vinnakota 4,010 Microsoft External Staff

Hi @InfraSolutions,

I understand that you’re trying to figure out if you can get away with using a smaller subnet for your Azure Application Gateway v2 setup instead of the recommended /24.

Based on the documentation, while a /24 subnet is recommended for supporting up to 125 instances (including reserved addresses), it’s not strictly necessary if your infrastructure needs are smaller. You could indeed use smaller subnets like /26 or /27, but keep the following in mind:

/26 subnet: Provides up to 62 usable IP addresses, after accounting for Azure’s five reserved addresses. This subnet can support up to 32 Application Gateway instances, plus one for a private frontend IP if needed.
/27 subnet: Provides 30 usable IPs (62 total - 5 reserved = 57 available), which cannot fully support even the maximum number of instances for a v2 SKU but might work for smaller configurations with fewer instances.

To directly answer your question about the max instances and connections they can handle, your main constraints will be:

The number of IP addresses left after accounting for reserved IPs.
The actual instances you plan to run.

With that said, if you're only planning for around 12 instances, a /26 subnet should suffice. Just be careful to account for other configurations like private front end IPs and avoid scaling too close to your limits.

Kindly let us know if the above helps or you need further assistance on this issue.

Please do not forget to "Accept the answer” and “Yes” wherever the information provided helps you, this can be beneficial to other community members.

InfraSolutions 646 Reputation points

2025-04-29T22:10:21.87+00:00

Thank you for your quick response. Just so I understand this correctly, is a /24 subnet (highly recommended) required if the infrastructure needs large instances, but not strictly necessary for small or medium infrastructure? This isn't mentioned directly in the given screenshot,

Regarding a /27 subnet (32 IPs - 5 reserved = 27 usable) and 1 for private frontend IP, do you mean this is not compatible for V2 even for fewer instances?

When we refer to instances in Application Gateway v2, my understanding is that, it means the number of application gateway profiles available in the infrastructure. For example, if we have only one application gateway for my infrastructure and a frontend IP configured with one private IP, in this case, we are using only one instance. if this correct, then what is the minimum recommendation required - /26 or /27 subnets?

As of now, we don't know how many instances are actually required. Is there any way to find out?
Rohith Vinnakota 4,010 Reputation points Microsoft External Staff

2025-04-29T23:07:40.2933333+00:00

@InfraSolutions

is a /24 subnet (highly recommended) required if the infrastructure needs large instances, but not strictly necessary for small or medium infrastructure?

You are indeed correct.

Regarding a /27 subnet (32 IPs - 5 reserved = 27 usable) and 1 for private frontend IP, do you mean this is not compatible for V2 even for fewer instances?

We need to retain at least 32 IPs, excluding the application gateway instance.

When we refer to instances in Application Gateway v2, my understanding is that, it means the number of application gateway profiles available in the infrastructure. For example, if we have only one application gateway for my infrastructure and a frontend IP configured with one private IP, in this case, we are using only one instance. if this correct,

You're right! In the context of Application Gateway v2, the term "instances" refers to the number of Application Gateway profiles or configurations you have. If you are using one Application Gateway with one frontend IP, then you are using one instance.

what is the minimum recommendation required - /26 or /27 subnets?

/26 subnets.

As of now, we don't know how many instances are actually required. Is there any way to find out?

No, there are no options like that.

Kindly let us know if the above helps or you need further assistance on this issue.

Please do not forget to "Accept the answer” and “Yes” wherever the information provided helps you, this can be beneficial to other community members.
InfraSolutions 646 Reputation points

2025-04-29T23:22:51.5433333+00:00

Thanks for the clarification — your response is really helpful in understanding Application Gateway v2
Rohith Vinnakota 4,010 Reputation points Microsoft External Staff

2025-04-29T23:25:19.07+00:00

Glad to know it helped. Continue to use MS Q&A platform for any questions related to Azure!

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Subnet Size Considerations for Azure Application Gateway v2 SKU

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