def extract_netmask(ip_address_with_netmask, separator="/") -> str:
    """
    This function will extract netmask from an
    'IP address with netmask' receive in parameter.
    Separator is the char that separate ip_address of netmask. Example :

        a) 192.168.1.1/24               - separator='/'
        b) 192.168.1.1 24               - separtor =' '
        c) 192.168.1.1 255.255.255.0    - separtor =' '

    Separtor value can not be '.' or a digit...

    :param ip_address_with_netmask: IP address with the netmask (1.1.1.1/24)
    :param separator: Char that separate ip_address of netmask
    :return: Netmask value can be (255.255.255.0 or 24)
    """
    if separator.isdigit() is False and separator != ".":
        index = (ip_address_with_netmask.find(separator) + 1)
        return ip_address_with_netmask[index:]
    else:
        raise Exception