def get_subnet_cidr(self, subnet_id):
        """retrieve the CIDR associated with a subnet, given its ID. """
        try:
            subnet_list = self.neutronclient.list_subnets(id=subnet_id)
            subnet_dat = subnet_list.get('subnets')[0]
            return subnet_dat.get('cidr')
        except Exception as exc:
            LOG.error("Failed to list subnet for ID %(subnet)s, "
                      "exc %(exc)s", {'subnet': subnet_id, 'exc': exc})
            return None
retrieve the CIDR associated with a subnet, given its ID.