What is the impact of having more instance count in Application Gateway than the IP addresses in its subnet?

Gregorio Montaño 246 Reputation points
2022-03-03T23:26:31.527+00:00

Hi,

I have an Application Gateway subnet of /28.
This gives me an available 16 IP Addresses.

If "Azure also reserves 5 IP addresses in each subnet for internal use" as mentioned in https://learn.microsoft.com/en-us/azure/application-gateway/configuration-infrastructure then I am left to have 11 Maximum instance count in my Application Gateway configuration.

Why does Azure reserves 5 when you only need 1 for Network Address and another 1 for Broadcast Address? What are the 3 extra for?
What is the impact of having more instance count than available IP address in subnet (e.g. what happens if I increase Maximum instance count to 15)? Will my Application Gateway still work but only up to 11? etc.?

Thank you and have a nice day!

Azure Virtual Network
Azure Virtual Network
An Azure networking service that is used to provision private networks and optionally to connect to on-premises datacenters.
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Azure Application Gateway
Azure Application Gateway
An Azure service that provides a platform-managed, scalable, and highly available application delivery controller as a service.
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Accepted answer
  1. ChaitanyaNaykodi-MSFT 23,501 Reputation points Microsoft Employee
    2022-03-04T02:28:40.207+00:00

    Hello @Gregorio Montaño ,
    Hope you are having a nice day too.

    Based on your questions above.

    Why does Azure reserves 5 when you only need 1 for Network Address and another 1 for Broadcast Address? What are the 3 extra for?

    The other three IP addresses are reserved for these purposes. (Source:- https://learn.microsoft.com/en-us/azure/virtual-network/virtual-networks-faq#are-there-any-restrictions-on-using-ip-addresses-within-these-subnets)

    • x.x.x.1: Reserved by Azure for the default gateway
    • x.x.x.2, x.x.x.3: Reserved by Azure to map the Azure DNS IPs to the VNet space

    What is the impact of having more instance count than available IP address in subnet (e.g. what happens if I increase Maximum instance count to 15)? Will my Application Gateway still work but only up to 11? etc.?

    You cannot have more instance count then the available IP space. For example if you try and allocate Maximum instance count to 15 when you only have 11 addresses available in your myAGSubnet you will get an error <Your app Gateway> does not have enough capacity. Required capacity is at least 15, currently available capacity in subnet is 11.

    Hope this helps!


    ​​Please "Accept the answer" if the information helped you. This will help us and others in the community as well.

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