code: def _validate_subnet_cidr(context, network_id, new_subnet_cidr):
      
      if neutron_cfg.cfg.CONF.allow_overlapping_ips:
          return
  
      try:
          new_subnet_ipset = netaddr.IPSet([new_subnet_cidr])
      except TypeError:
          LOG.exception("Invalid or missing cidr: %s" % new_subnet_cidr)
          raise n_exc.BadRequest(resource="subnet",
                                 msg="Invalid or missing cidr")
  
      filters = {
          'network_id': network_id,
          'shared': [False]
      }
      # Using admin context here, in case we actually share networks later
      subnet_list = db_api.subnet_find(context=context.elevated(), **filters)
  
      for subnet in subnet_list:
          if (netaddr.IPSet([subnet.cidr]) & new_subnet_ipset):
              # don't give out details of the overlapping subnet
              err_msg = (_("Requested subnet with cidr: %(cidr)s for "
                           "network: %(network_id)s overlaps with another "
                           "subnet") %
                         {'cidr': new_subnet_cidr,
                          'network_id': network_id})
              LOG.error(_("Validation for CIDR: %(new_cidr)s failed - "
                          "overlaps with subnet %(subnet_id)s "
                          "(CIDR: %(cidr)s)"),
                        {'new_cidr': new_subnet_cidr,
                         'subnet_id': subnet.id,
                         'cidr': subnet.cidr})
              raise n_exc.InvalidInput(error_message=err_msg)
docs: Validate the CIDR for a subnet.
  
      Verifies the specified CIDR does not overlap with the ones defined
      for the other subnets specified for this network, or with any other
      CIDR if overlapping IPs are disabled.