diff --git a/GanetiCluster.py b/GanetiCluster.py
index 9c35a3d..821b8b5 100755
--- a/GanetiCluster.py
+++ b/GanetiCluster.py
@@ -130,8 +130,23 @@ class GanetiCluster(object):
 	images = json.loads(r.content)
 	return images
 
+    def get_occupied_dns_names(self):
+	occupied = []
+	kwargs = {
+	    "headers": headers,
+	    "auth": (self.username, self.password),
+	    "verify": False,
+	    "params":"bulk=1"
+	}
+	r = requests.request("get", self.base_url+"instances", **kwargs)
+	list_answer = json.loads(r.content)
+	for i in range(0,len(list_answer)):
+	   occupied.append(str(list_answer[i]['name']));
+	return occupied
+	
     def run_instances(self, instance_count, os, disk, cpu, ram):
-	return self.run_instances_names(self.utils.getFree_dns_names(int(instance_count)), os, disk, cpu, ram)
+	occupied = self.get_occupied_dns_names();
+	return self.run_instances_names(self.utils.getFree_dns_names(int(instance_count),occupied), os, disk, cpu, ram)
     
     def run_instances_names(self, names, os, disk, cpu, ram): 
         instances = []
diff --git a/Utils.py b/Utils.py
index 4faa0e3..0abfc2d 100644
--- a/Utils.py
+++ b/Utils.py
@@ -55,8 +55,11 @@ class Utils(object):
 	}
 	self.free_ips = self.mac_ip_mapping.keys()
 
-    def getFree_dns_names(self, count):
+    def getFree_dns_names(self, count, occupied=[]):
 	names = []
+	for ip in occupied:
+		if ip in self.free_ips:
+			self.free_ips.remove(ip)
 	if (count <= len(self.free_ips)):
 		for i in range(0,count):
 			names.append(self.free_ips.pop())