def _make_netmask(cls, arg):
        """Make a (netmask, prefix_len) tuple from the given argument.

        Argument can be:
        - an integer (the prefix length)
        - a string representing the prefix length (e.g. "24")
        - a string representing the prefix netmask (e.g. "255.255.255.0")
        """
        if arg not in cls._netmask_cache:
            if isinstance(arg, _compat_int_types):
                prefixlen = arg
            else:
                try:
                    # Check for a netmask in prefix length form
                    prefixlen = cls._prefix_from_prefix_string(arg)
                except NetmaskValueError:
                    # Check for a netmask or hostmask in dotted-quad form.
                    # This may raise NetmaskValueError.
                    prefixlen = cls._prefix_from_ip_string(arg)
            netmask = IPv4Address(cls._ip_int_from_prefix(prefixlen))
            cls._netmask_cache[arg] = netmask, prefixlen
        return cls._netmask_cache[arg]
Make a (netmask, prefix_len) tuple from the given argument.

        Argument can be:
        - an integer (the prefix length)
        - a string representing the prefix length (e.g. "24")
        - a string representing the prefix netmask (e.g. "255.255.255.0")