added CIDR Guide

Author: DocGailZhou

infra/samples/network-subnet-design.bicep

@@ -76,6 +76,35 @@ param jumpboxConfiguration jumpBoxConfigurationType = {
 @description('Networking address prefix for the VNET.')
 param addressPrefixes array = ['10.0.0.0/20'] // 4096 addresses (enough for 8 /23 subnets or 16 /24 subnets)
 
+// Subnet Classless Inter-Doman Routing (CIDR)  Sizing Reference Table (Best Practices)
+// | CIDR      | # of Addresses | # of /24s | Notes                                 |
+// |-----------|---------------|-----------|----------------------------------------|
+// | /24       | 256           | 1         | Smallest recommended for Azure subnets |
+// | /23       | 512           | 2         | Good for 1-2 workloads per subnet      |
+// | /22       | 1024          | 4         | Good for 2-4 workloads per subnet      |
+// | /21       | 2048          | 8         |                                        |
+// | /20       | 4096          | 16        | Used for default VNet in this solution |
+// | /19       | 8192          | 32        |                                        |
+// | /18       | 16384         | 64        |                                        |
+// | /17       | 32768         | 128       |                                        |
+// | /16       | 65536         | 256       |                                        |
+// | /15       | 131072        | 512       |                                        |
+// | /14       | 262144        | 1024      |                                        |
+// | /13       | 524288        | 2048      |                                        |
+// | /12       | 1048576       | 4096      |                                        |
+// | /11       | 2097152       | 8192      |                                        |
+// | /10       | 4194304       | 16384     |                                        |
+// | /9        | 8388608       | 32768     |                                        |
+// | /8        | 16777216      | 65536     |                                        |
+//
+// Best Practice Notes:
+// - Use /24 as the minimum subnet size for Azure (smaller subnets are not supported for most services).
+// - Plan for future growth: allocate larger address spaces (e.g., /20 or /21 for VNets) to allow for new subnets.
+// - Avoid overlapping address spaces with on-premises or other VNets.
+// - Use contiguous, non-overlapping ranges for subnets.
+// - Document subnet usage and purpose in code comments.
+// - For AVM modules, ensure only one delegation per subnet and leave delegations empty if not required.
+
 import { subnetType } from '../modules/network/virtualNetwork.bicep'
 @description('Array of subnets to be created within the VNET.')
 param subnets subnetType[] = [