'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.'

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))
