firewall: add new IP allocation policy

fixes #88

circle/firewall/models.py

 # -*- coding: utf-8 -*-
 
-from itertools import islice
+from itertools import islice, chain
 import logging
 from netaddr import IPSet
 
@@ -298,13 +298,28 @@ class Vlan(AclBase, models.Model):
     def prefix6(self):
         return self.network6.prefixlen
 
+    def get_next_address(self, used_v4):
+        try:
+            last_address = list(used_v4)[-1]
+        except IndexError:
+            return []
+        next_address = last_address + 1
+
+        if next_address in self.network4.iter_hosts():
+            logger.debug("Found unused IPv4 address %s after %s.",
+                         next_address, last_address)
+            return [next_address]
+        else:
+            return []
+
     def get_new_address(self):
-        hosts = Host.objects.filter(vlan=self)
+        hosts = self.host_set
         used_v4 = IPSet(hosts.values_list('ipv4', flat=True))
         used_v6 = IPSet(hosts.exclude(ipv6__isnull=True)
                         .values_list('ipv6', flat=True))
 
-        for ipv4 in islice(self.network4.iter_hosts(), 10000):
+        for ipv4 in chain(self.get_next_address(used_v4),
+                          islice(self.network4.iter_hosts(), 10000)):
             ipv4 = str(ipv4)
             if ipv4 not in used_v4:
                 logger.debug("Found unused IPv4 address %s.", ipv4)