@classmethod
def _get_subscription_urls(cls, extension=''):
    """
        Gets the set of URLs used to subscribe to class changes
        in the APIC.

        :returns: Set of URL strings
        """
    resp = []
    for class_name in cls._get_apic_classes():
        url = '/api/class/%s.json?subscription=yes' % class_name
        url += extension
        resp.append(url)
    return resp
