diff --git a/releasenotes/notes/add-subnet-id-config-option-fac3d6f12abfc171.yaml b/releasenotes/notes/add-subnet-id-config-option-fac3d6f12abfc171.yaml
new file mode 100644
index 0000000..a1bd4c5
--- /dev/null
+++ b/releasenotes/notes/add-subnet-id-config-option-fac3d6f12abfc171.yaml
@@ -0,0 +1,8 @@
+---
+features:
+  - A new config option 'subnet_id' is added to section
+    'network' to specify subnet which should be used for
+    allocation of IPs for VMs created during testing.
+    It should be used when the tested network contains more
+    than one subnet otherwise test of external connectivity
+    will fail. (Fixes bug #1856671)
diff --git a/tempest/config.py b/tempest/config.py
index 5a2d722..9685745 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -699,6 +699,11 @@
     cfg.StrOpt('floating_network_name',
                help="Default floating network name. Used to allocate floating "
                     "IPs when neutron is enabled."),
+    cfg.StrOpt('subnet_id',
+               default="",
+               help="Subnet id of subnet which is used for allocation of "
+                    "floating IPs. Specify when two or more subnets are "
+                    "present in network."),
     cfg.StrOpt('public_router_id',
                default="",
                help="Id of the public router that provides external "
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index cb7acbf..99dd653 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -1008,13 +1008,18 @@
             port_id, ip4 = self._get_server_port_id_and_ip4(thing)
         else:
             ip4 = None
-        result = client.create_floatingip(
-            floating_network_id=external_network_id,
-            port_id=port_id,
-            tenant_id=thing['tenant_id'],
-            fixed_ip_address=ip4
-        )
+
+        kwargs = {
+            'floating_network_id': external_network_id,
+            'port_id': port_id,
+            'tenant_id': thing['tenant_id'],
+            'fixed_ip_address': ip4,
+        }
+        if CONF.network.subnet_id:
+            kwargs['subnet_id'] = CONF.network.subnet_id
+        result = client.create_floatingip(**kwargs)
         floating_ip = result['floatingip']
+
         self.addCleanup(test_utils.call_and_ignore_notfound_exc,
                         client.delete_floatingip,
                         floating_ip['id'])
diff --git a/tempest/scenario/test_network_basic_ops.py b/tempest/scenario/test_network_basic_ops.py
index f46c7e8..d8584ec 100644
--- a/tempest/scenario/test_network_basic_ops.py
+++ b/tempest/scenario/test_network_basic_ops.py
@@ -346,10 +346,19 @@
                 network_id=CONF.network.public_network_id)['subnets']
             if s['ip_version'] == 4
         ]
-        self.assertEqual(1, len(v4_subnets),
-                         "Found %d IPv4 subnets" % len(v4_subnets))
 
-        external_ips = [v4_subnets[0]['gateway_ip']]
+        if len(v4_subnets) > 1:
+            self.assertTrue(
+                CONF.network.subnet_id,
+                "Found %d subnets. Specify subnet using configuration "
+                "option [network].subnet_id."
+                % len(v4_subnets))
+            subnet = self.os_admin.subnets_client.show_subnet(
+                CONF.network.subnet_id)['subnet']
+            external_ips = [subnet['gateway_ip']]
+        else:
+            external_ips = [v4_subnets[0]['gateway_ip']]
+
         self._check_server_connectivity(self.floating_ip_tuple.floating_ip,
                                         external_ips)