Back to clean (https://docs.djangoproject.com/en/dev/ref/models/instances/#validating-objects)

coin/resources/models.py

@@ -15,15 +15,13 @@ class IPPool(models.Model):
     inet = CidrAddressField()
     objects = NetManager()
 
-    def clean_default_subnetsize(self):
-        if self.inet.version == 4:
+    def clean(self):
+        if self.inet is not None and self.inet.version == 4:
             if not self.inet.prefixlen <= self.default_subnetsize <= 32:
                 raise ValidationError('Invalid default subnet size')
-        elif self.inet.version == 6:
+        elif self.inet is not None and self.inet.version == 6:
             if not self.inet.prefixlen <= self.default_subnetsize <= 64:
                 raise ValidationError('Invalid default subnet size')
-        else:
-            pass
 
     def __unicode__(self):
         return self.name