import re


def unknown_hostname(ip):
    """
    Returns a custom  hostname for an unresolvable IP

    Args:
        ip (str): The unresolved IP for which to craft the hostname for

    Returns:
        str: The hostname returned is of the format: unknown-x-x-x-x

    """

    return u'unknown-' + re.sub(r'[.:]', u'-', ip)