code: def _ip_string_from_prefix(self, prefixlen=None):
          
          if not prefixlen:
              prefixlen = self._prefixlen
          return self._string_from_ip_int(self._ip_int_from_prefix(prefixlen))
docs: Turn a prefix length into a dotted decimal string.
  
          Args:
              prefixlen: An integer, the netmask prefix length.
  
          Returns:
              A string, the dotted decimal netmask string.