diff --git a/tempest/api/network/admin/test_external_networks_negative.py b/tempest/api/network/admin/test_external_networks_negative.py
new file mode 100644
index 0000000..7dbb347
--- /dev/null
+++ b/tempest/api/network/admin/test_external_networks_negative.py
@@ -0,0 +1,53 @@
+# Copyright 2014 OpenStack Foundation
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from tempest.api.network import base
+from tempest import config
+from tempest import exceptions
+from tempest import test
+
+CONF = config.CONF
+
+
+class ExternalNetworksAdminNegativeTestJSON(base.BaseAdminNetworkTest):
+    _interface = 'json'
+
+    @test.attr(type=['negative'])
+    def test_create_port_with_precreated_floatingip_as_fixed_ip(self):
+        """
+        External networks can be used to create both floating-ip as well
+        as instance-ip. So, creating an instance-ip with a value of a
+        pre-created floating-ip should be denied.
+        """
+
+        # create a floating ip
+        client = self.admin_client
+        body = client.create_floatingip(
+            floating_network_id=CONF.network.public_network_id)
+        created_floating_ip = body['floatingip']
+        self.addCleanup(self._try_delete_resource,
+                        client.delete_floatingip,
+                        created_floating_ip['id'])
+        floating_ip_address = created_floating_ip['floating_ip_address']
+        self.assertIsNotNone(floating_ip_address)
+
+        # use the same value of floatingip as fixed-ip to create_port()
+        fixed_ips = [{'ip_address': floating_ip_address}]
+
+        # create a port which will internally create an instance-ip
+        self.assertRaises(exceptions.Conflict,
+                          client.create_port,
+                          network_id=CONF.network.public_network_id,
+                          fixed_ips=fixed_ips)