def METHOD_NAME(self):
    data = {self.field_name: "192.168.0.1", "namespace": self.namespace, "status": self.status}
    data.update(self.extra_data)
    form = self.form_class(data)
    self.assertFalse(form.is_valid())
    self.assertEqual("CIDR mask (e.g. /24) is required.", form.errors[self.field_name][0])