Add a comment on a non-trivial piece of code

coin/resources/models.py

@@ -86,6 +86,8 @@ class IPSubnet(models.Model):
             first_free = available.next()
         except StopIteration:
             raise ValidationError("Impossible d'allouer un sous-réseau : bloc d'IP rempli.")
+        # first_free is a subnet, but it might be too large for our needs.
+        # This selects the first sub-subnet of the right size.
         self.inet = first_free.subnet(self.ip_pool.default_subnetsize, 1).next()
 
     def validate_inclusion(self):