Very simple check of the cidr format in no_proxy variable.

    :rtype: bool

def is_valid_cidr(string_network):
    """
    Very simple check of the cidr format in no_proxy variable.

    :rtype: bool
    """
    if string_network.count('/') == 1:
        try:
            mask = int(string_network.split('/')[1])
        except ValueError:
            return False

        if mask < 1 or mask > 32:
            return False

        try:
            socket.inet_aton(string_network.split('/')[0])
        except socket.error:
            return False
    else:
        return False
    return True