[Network] az network vnet subnet list-available-ips: Get list of available IPs for subnet (#23109)

* add new command `az network vnet subnet list-available-ips`

* update _help.py

Author: ZengTaoxu

src/azure-cli/azure/cli/command_modules/network/_help.py

@@ -5925,6 +5925,15 @@
     crafted: true
 """
 
+helps['network vnet subnet list-available-ips'] = """
+type: command
+short-summary: List some available ips in the subnet.
+examples:
+  - name: List some available ips in the subnet.
+    text: |
+        az network vnet subnet list-available-ips --resource-group MyResourceGroup --vnet-name MyVNet -n MySubnet
+"""
+
 helps['network vnet subnet show'] = """
 type: command
 short-summary: Show details of a subnet.

src/azure-cli/azure/cli/command_modules/network/commands.py

@@ -1385,6 +1385,7 @@ def _make_singular(value):
         g.generic_update_command('update', setter_name='begin_create_or_update', setter_arg_name='subnet_parameters',
                                  custom_func_name='update_subnet')
         g.custom_command('list-available-delegations', 'list_avail_subnet_delegations', min_api='2018-08-01', validator=process_list_delegations_namespace)
+        g.custom_command('list-available-ips', 'subnet_list_available_ips', min_api='2016-09-01', is_preview=True)
     # endregion
 
     # region VirtualNetworkGateways

src/azure-cli/azure/cli/command_modules/network/custom.py

@@ -7263,6 +7263,18 @@ def list_available_ips(cmd, resource_group_name, virtual_network_name):
                                                          ip_address=start_ip)
     return available_ips.available_ip_addresses
 
+
+def subnet_list_available_ips(cmd, resource_group_name, virtual_network_name, subnet_name):
+    client = network_client_factory(cmd.cli_ctx)
+    subnet = client.subnets.get(resource_group_name=resource_group_name,
+                                virtual_network_name=virtual_network_name,
+                                subnet_name=subnet_name)
+    if subnet.address_prefix is not None:
+        start_ip = subnet.address_prefix.split('/')[0]
+    available_ips = client.virtual_networks.check_ip_address_availability(resource_group_name=resource_group_name,
+                                                                          virtual_network_name=virtual_network_name,
+                                                                          ip_address=start_ip)
+    return available_ips.available_ip_addresses
 # endregion