In addition to the method of counting instance IP backref's inside a VN object, then adding it with the number of services and gateway IP, there is a better way to obtain the total number of used IP addresses in a subnet, as discussed in this article.
The following VN has 11 instance IP backrefs in the object, plus gateway and service IP, there are 13 IP addresses in use right now. We can easily get this number with the uuid of the VN and subnet along with the following command:
curl -u admin:Juniper -X POST http:// <config api ip> :8095/virtual-network/ <vn uuid> /subnet-ip-count -H "Content-Type: application/json" -d '{"subnet_list": [" <subnet uuid> "]}'
root@cont101:~# curl -u admin:Juniper http://localhost:8095/virtual-network/bccebf94-c6b7-48fb-86aa-a98faa39140e | python -m json.tool % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 100 7409 100 7409 0 0 894k 0 --:--:-- --:--:-- --:--:-- 904k { "virtual-network": { "access_control_lists": [ { "href": "http://localhost:8095/access-control-list/4d4bbb11-94f5-4489-940c-e8eb5018eebd", "to": [ "default-domain", "jjzhang", "jjzhang-testvn", "jjzhang-testvn" ], "uuid": "4d4bbb11-94f5-4489-940c-e8eb5018eebd" } ], "display_name": "jjzhang-testvn", "ecmp_hashing_include_fields": {}, "export_route_target_list": { "route_target": [] }, "flood_unknown_unicast": false, "fq_name": [ "default-domain", "jjzhang", "jjzhang-testvn" ], "href": "http://localhost:8095/virtual-network/bccebf94-c6b7-48fb-86aa-a98faa39140e", "id_perms": { "created": "2018-10-03T23:11:58.214254", "creator": null, "description": null, "enable": true, "last_modified": "2019-04-06T02:19:59.755006", "permissions": { "group": "_member_", "group_access": 7, "other_access": 7, "owner": "admin", "owner_access": 7 }, "user_visible": true, "uuid": { "uuid_lslong": 9703754781624439822, "uuid_mslong": 13605022170042812667 } }, "import_route_target_list": { "route_target": [] }, "instance_ip_back_refs": [ { "attr": null, "href": "http://localhost:8095/instance-ip/4c8438e4-65bf-43e3-83ac-40dcc120a6ef", "to": [ "4c8438e4-65bf-43e3-83ac-40dcc120a6ef" ], "uuid": "4c8438e4-65bf-43e3-83ac-40dcc120a6ef" }, { "attr": null, "href": "http://localhost:8095/instance-ip/3e2f256e-cb9f-4774-8197-7a330827e150", "to": [ "3e2f256e-cb9f-4774-8197-7a330827e150" ], "uuid": "3e2f256e-cb9f-4774-8197-7a330827e150" }, { "attr": null, "href": "http://localhost:8095/instance-ip/71a5c306-77be-47d6-bd4f-0d3951c5c941", "to": [ "71a5c306-77be-47d6-bd4f-0d3951c5c941" ], "uuid": "71a5c306-77be-47d6-bd4f-0d3951c5c941" }, { "attr": null, "href": "http://localhost:8095/instance-ip/63e70f8a-ca0a-40f0-9a75-f31258a783fc", "to": [ "63e70f8a-ca0a-40f0-9a75-f31258a783fc" ], "uuid": "63e70f8a-ca0a-40f0-9a75-f31258a783fc" }, { "attr": null, "href": "http://localhost:8095/instance-ip/b0a62b31-dd68-445b-8add-2719fef21e64", "to": [ "b0a62b31-dd68-445b-8add-2719fef21e64" ], "uuid": "b0a62b31-dd68-445b-8add-2719fef21e64" }, { "attr": null, "href": "http://localhost:8095/instance-ip/90e0ce45-c6d8-416e-90b0-62c49edf550f", "to": [ "90e0ce45-c6d8-416e-90b0-62c49edf550f" ], "uuid": "90e0ce45-c6d8-416e-90b0-62c49edf550f" }, { "attr": null, "href": "http://localhost:8095/instance-ip/0537a8b6-039f-48d6-8d16-d3febfce7e1d", "to": [ "0537a8b6-039f-48d6-8d16-d3febfce7e1d" ], "uuid": "0537a8b6-039f-48d6-8d16-d3febfce7e1d" }, { "attr": null, "href": "http://localhost:8095/instance-ip/617b4106-6028-4743-9fdc-df780b28aad3", "to": [ "617b4106-6028-4743-9fdc-df780b28aad3" ], "uuid": "617b4106-6028-4743-9fdc-df780b28aad3" }, { "attr": null, "href": "http://localhost:8095/instance-ip/5eec7ab1-1455-4ea1-bdb1-92387d2067fa", "to": [ "5eec7ab1-1455-4ea1-bdb1-92387d2067fa" ], "uuid": "5eec7ab1-1455-4ea1-bdb1-92387d2067fa" }, { "attr": null, "href": "http://localhost:8095/instance-ip/f2136b5d-6d8d-4ae9-90a9-e81b39934707", "to": [ "f2136b5d-6d8d-4ae9-90a9-e81b39934707" ], "uuid": "f2136b5d-6d8d-4ae9-90a9-e81b39934707" }, { "attr": null, "href": "http://localhost:8095/instance-ip/e06c53e1-0321-40b4-9c8c-94fb1167cc27", "to": [ "e06c53e1-0321-40b4-9c8c-94fb1167cc27" ], "uuid": "e06c53e1-0321-40b4-9c8c-94fb1167cc27" } ], "is_shared": false, "multi_policy_service_chains_enabled": false, "name": "jjzhang-testvn", "network_ipam_refs": [ { "attr": { "ipam_subnets": [ { "addr_from_start": true, "default_gateway": "111.111.111.1", "dns_server_address": "111.111.111.2", "enable_dhcp": true, "subnet": { "ip_prefix": "111.111.111.0", "ip_prefix_len": 24 }, "subnet_name": "0a4bca7e-bdd9-4fba-bb8d-06887aba43d9", "subnet_uuid": "0a4bca7e-bdd9-4fba-bb8d-06887aba43d9" } ] }, "href": "http://localhost:8095/network-ipam/8b143a9e-745d-4010-884a-c9e78255dd6b", "to": [ "default-domain", "admin", "admin-ipam" ], "uuid": "8b143a9e-745d-4010-884a-c9e78255dd6b" } ], "network_policy_refs": [ { "attr": { "sequence": { "major": 0, "minor": 0 }, "timer": null }, "href": "http://localhost:8095/network-policy/6a5acc21-c853-4e12-a34d-d7efc40bfd2a", "to": [ "default-domain", "jjzhang", "jjzhang-policy" ], "uuid": "6a5acc21-c853-4e12-a34d-d7efc40bfd2a" } ], "parent_href": "http://localhost:8095/project/7fccf71a-7e7d-4c79-8413-513ee809435a", "parent_type": "project", "parent_uuid": "7fccf71a-7e7d-4c79-8413-513ee809435a", "perms2": { "global_access": 0, "owner": "7fccf71a7e7d4c798413513ee809435a", "owner_access": 7, "share": [] }, "provider_properties": null, "route_target_list": { "route_target": [ "target:60111:800111" ] }, "router_external": false, "routing_instances": [ { "href": "http://localhost:8095/routing-instance/418154b1-f1ed-43a5-b4d8-73ce3a63fbf4", "to": [ "default-domain", "jjzhang", "jjzhang-testvn", "jjzhang-testvn" ], "uuid": "418154b1-f1ed-43a5-b4d8-73ce3a63fbf4" } ], "uuid": "bccebf94-c6b7-48fb-86aa-a98faa39140e", "virtual_machine_interface_back_refs": [ { "attr": null, "href": "http://localhost:8095/virtual-machine-interface/87589a4b-92d2-4b44-a7a5-71338ec63f1c", "to": [ "default-domain", "jjzhang", "87589a4b-92d2-4b44-a7a5-71338ec63f1c" ], "uuid": "87589a4b-92d2-4b44-a7a5-71338ec63f1c" }, { "attr": null, "href": "http://localhost:8095/virtual-machine-interface/df2efdf9-51c7-4fb4-ad61-a8523d4451da", "to": [ "default-domain", "jjzhang", "df2efdf9-51c7-4fb4-ad61-a8523d4451da" ], "uuid": "df2efdf9-51c7-4fb4-ad61-a8523d4451da" }, { "attr": null, "href": "http://localhost:8095/virtual-machine-interface/9ad9e9aa-b02c-4a60-ade9-6a082ddec791", "to": [ "default-domain", "jjzhang", "9ad9e9aa-b02c-4a60-ade9-6a082ddec791" ], "uuid": "9ad9e9aa-b02c-4a60-ade9-6a082ddec791" }, { "attr": null, "href": "http://localhost:8095/virtual-machine-interface/11476728-56ed-45ae-884d-5e965188882e", "to": [ "default-domain", "jjzhang", "test_tap" ], "uuid": "11476728-56ed-45ae-884d-5e965188882e" }, { "attr": null, "href": "http://localhost:8095/virtual-machine-interface/44ce2b4e-8de6-4843-9cdf-b4f2d26990a5", "to": [ "default-domain", "jjzhang", "44ce2b4e-8de6-4843-9cdf-b4f2d26990a5" ], "uuid": "44ce2b4e-8de6-4843-9cdf-b4f2d26990a5" }, { "attr": null, "href": "http://localhost:8095/virtual-machine-interface/5a1f8ab9-e173-4cb8-8bb9-523af1667aaf", "to": [ "default-domain", "jjzhang", "test1" ], "uuid": "5a1f8ab9-e173-4cb8-8bb9-523af1667aaf" }, { "attr": null, "href": "http://localhost:8095/virtual-machine-interface/edacf650-c845-4cf7-a99b-aa11343a2db8", "to": [ "default-domain", "jjzhang", "edacf650-c845-4cf7-a99b-aa11343a2db8" ], "uuid": "edacf650-c845-4cf7-a99b-aa11343a2db8" }, { "attr": null, "href": "http://localhost:8095/virtual-machine-interface/e2c8dadf-5f58-48b0-b52a-0cef70f6f2eb", "to": [ "default-domain", "jjzhang", "e2c8dadf-5f58-48b0-b52a-0cef70f6f2eb" ], "uuid": "e2c8dadf-5f58-48b0-b52a-0cef70f6f2eb" }, { "attr": null, "href": "http://localhost:8095/virtual-machine-interface/e792a99b-0990-4774-bf9a-330a5d67deae", "to": [ "default-domain", "jjzhang", "e792a99b-0990-4774-bf9a-330a5d67deae" ], "uuid": "e792a99b-0990-4774-bf9a-330a5d67deae" }, { "attr": null, "href": "http://localhost:8095/virtual-machine-interface/0d5bd133-e5c1-48e7-be5c-b361ee11d2fb", "to": [ "default-domain", "jjzhang", "test" ], "uuid": "0d5bd133-e5c1-48e7-be5c-b361ee11d2fb" }, { "attr": null, "href": "http://localhost:8095/virtual-machine-interface/06ba9e7c-9a68-475c-a64e-6be704c346d2", "to": [ "default-domain", "jjzhang", "06ba9e7c-9a68-475c-a64e-6be704c346d2" ], "uuid": "06ba9e7c-9a68-475c-a64e-6be704c346d2" } ], "virtual_network_network_id": 56, "virtual_network_properties": { "allow_transit": false, "mirror_destination": false, "rpf": "enable" } } } root@cont101:~# curl -u admin:Juniper -X POST http://localhost:8095/virtual-network/bccebf94-c6b7-48fb-86aa-a98faa39140e/subnet-ip-count -H "Content-Type: application/json" -d '{"subnet_list": ["0a4bca7e-bdd9-4fba-bb8d-06887aba43d9"]}' {"ip_count_list": [13]} root@cont101:~#