<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _is_valid_netmask(self, netmask):
 """Verify that the netmask is valid. Args: netmask: A string, either a prefix or dotted decimal netmask. Returns: A boolean, True if the prefix represents a valid IPv4 netmask. """

        mask = netmask.split('.')
        if len(mask) == 4:
            try:
                for x in mask:
                    if int(x) not in self._valid_mask_octets:
                        return False
            except ValueError:
                # Found something that isn't an integer or isn't valid
                return False
            for idx, y in enumerate(mask):
                if idx > 0 and y > mask[idx - 1]:
                    return False
            return True
        try:
            netmask = int(netmask)
        except ValueError:
            return False
        return 0 <= netmask <= self._max_prefixlen