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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _list_subnets_by_identifier(self, identifier):
 """Returns a list of IDs of the subnet matching the identifier. :param string identifier: The identifier to look up :returns: List of matching IDs """

        identifier = identifier.split('/', 1)[0]

        results = self.list_subnets(identifier=identifier, mask='id')
        return [result['id'] for result in results]