diff --git a/neutron/tests/fullstack/base.py b/neutron/tests/fullstack/base.py
index 93989bf1c24..6fe81a37c87 100644
--- a/neutron/tests/fullstack/base.py
+++ b/neutron/tests/fullstack/base.py
@@ -15,6 +15,7 @@
 from concurrent import futures
 import os
 
+import netaddr
 from oslo_config import cfg
 from oslo_log import log as logging
 
@@ -121,3 +122,18 @@ class BaseFullStackTestCase(testlib_api.MySQLTestCaseMixin,
                 exception=RuntimeError("Could not ping the other VM, "
                                        "re-starting %s leads to network "
                                        "disruption" % agent_names))
+
+    def _find_available_ips(self, network, subnet, num):
+        ports = self.safe_client.list_ports(network_id=network['id'])
+        used_ips = [ip['ip_address']
+                    for port in ports for ip in port['fixed_ips']]
+        used_ips.append(subnet['gateway_ip'])
+        available_ips = []
+        for ip in netaddr.IPNetwork(subnet['cidr']).iter_hosts():
+            ip = str(ip)
+            if ip not in used_ips:
+                available_ips.append(ip)
+                if len(available_ips) >= num:
+                    return available_ips
+
+        self.fail("Cannot find enough free IP addresses.")
diff --git a/neutron/tests/fullstack/test_l3_agent.py b/neutron/tests/fullstack/test_l3_agent.py
index e0cbea07054..9b7fc0c9196 100644
--- a/neutron/tests/fullstack/test_l3_agent.py
+++ b/neutron/tests/fullstack/test_l3_agent.py
@@ -107,10 +107,7 @@ class TestL3Agent(base.BaseFullStackTestCase):
         gateway_port = self.safe_client.list_ports(
             device_id=router['id'],
             device_owner=constants.DEVICE_OWNER_ROUTER_GW)[0]
-        ip_1 = str(netaddr.IPNetwork(
-            ext_sub['gateway_ip']).next(100)).split('/')[0]
-        ip_2 = str(netaddr.IPNetwork(
-            ext_sub['gateway_ip']).next(101)).split('/')[0]
+        ip_1, ip_2 = self._find_available_ips(ext_net, ext_sub, 2)
         self.safe_client.update_port(gateway_port['id'], fixed_ips=[
             {'ip_address': ip_1},
             {'ip_address': ip_2}])