I think Subnet Size /24 = 256 IP addresses, not 255.

- Add a link for reserved IP addresses on each subnet.
- Recalculate /24 subnet.

Author: mazgi

articles/application-gateway/configuration-infrastructure.md

@@ -24,7 +24,7 @@ An application gateway is a dedicated deployment in your virtual network. Within
 
 Application Gateway uses one private IP address per instance, plus another private IP address if a private front-end IP is configured.
 
-Azure also reserves five IP addresses in each subnet for internal use: the first four and the last IP addresses. For example, consider 15 application gateway instances with no private front-end IP. You need at least 20 IP addresses for this subnet: five for internal use and 15 for the application gateway instances.
+Azure also reserves [five IP addresses in each subnet for internal use: the first four and the last IP addresses](../virtual-network/virtual-networks-faq.md#are-there-any-restrictions-on-using-ip-addresses-within-these-subnets). For example, consider 15 application gateway instances with no private front-end IP. You need at least 20 IP addresses for this subnet: five for internal use and 15 for the application gateway instances.
 
 Consider a subnet that has 27 application gateway instances and an IP address for a private front-end IP. In this case, you need 33 IP addresses: 27 for the application gateway instances, one for the private front end, and five for internal use.
 
@@ -40,10 +40,10 @@ For example, here's how to calculate the available addressing for a subnet with
 - Gateway 3: Maximum of 15 instances; utilizes a private frontend IP configuration
 - Subnet Size: /24
 
-Subnet Size /24 = 255 IP addresses - 5 reserved from the platform = 250 available addresses.
-250 - Gateway 1 (10) - 1 private frontend IP configuration = 239
-239 - Gateway 2 (2) = 237
-237 - Gateway 3 (15) - 1 private frontend IP configuration = 221
+Subnet Size /24 = 256 IP addresses - 5 reserved from the platform = 251 available addresses.
+251 - Gateway 1 (10) - 1 private frontend IP configuration = 240
+240 - Gateway 2 (2) = 238
+238 - Gateway 3 (15) - 1 private frontend IP configuration = 222
 
 > [!IMPORTANT]
 > Although a /24 subnet isn't required per Application Gateway v2 SKU deployment, it is highly recommended. This is to ensure that Application Gateway v2 has sufficient space for autoscaling expansion and maintenance upgrades. You should ensure that the Application Gateway v2 subnet has sufficient address space to accommodate the number of instances required to serve your maximum expected traffic. If you specify the maximum instance count, then the subnet should have capacity for at least that many addresses. For capacity planning around instance count, see [instance count details](understanding-pricing.md#instance-count).