def models(cls, api_version=DEFAULT_API_VERSION):
        """Module depends on the API version:

           * 2015-06-15: :mod:`v2015_06_15.models<azure.mgmt.network.v2015_06_15.models>`
           * 2016-09-01: :mod:`v2016_09_01.models<azure.mgmt.network.v2016_09_01.models>`
           * 2016-12-01: :mod:`v2016_12_01.models<azure.mgmt.network.v2016_12_01.models>`
           * 2017-03-01: :mod:`v2017_03_01.models<azure.mgmt.network.v2017_03_01.models>`
           * 2017-06-01: :mod:`v2017_06_01.models<azure.mgmt.network.v2017_06_01.models>`
           * 2017-08-01: :mod:`v2017_08_01.models<azure.mgmt.network.v2017_08_01.models>`
           * 2017-09-01: :mod:`v2017_09_01.models<azure.mgmt.network.v2017_09_01.models>`
           * 2017-10-01: :mod:`v2017_10_01.models<azure.mgmt.network.v2017_10_01.models>`
           * 2017-11-01: :mod:`v2017_11_01.models<azure.mgmt.network.v2017_11_01.models>`
           * 2018-01-01: :mod:`v2018_01_01.models<azure.mgmt.network.v2018_01_01.models>`
           * 2018-02-01: :mod:`v2018_02_01.models<azure.mgmt.network.v2018_02_01.models>`
           * 2018-04-01: :mod:`v2018_04_01.models<azure.mgmt.network.v2018_04_01.models>`
        """
        if api_version == '2015-06-15':
            from .v2015_06_15 import models
            return models
        elif api_version == '2016-09-01':
            from .v2016_09_01 import models
            return models
        elif api_version == '2016-12-01':
            from .v2016_12_01 import models
            return models
        elif api_version == '2017-03-01':
            from .v2017_03_01 import models
            return models
        elif api_version == '2017-06-01':
            from .v2017_06_01 import models
            return models
        elif api_version == '2017-08-01':
            from .v2017_08_01 import models
            return models
        elif api_version == '2017-09-01':
            from .v2017_09_01 import models
            return models
        elif api_version == '2017-10-01':
            from .v2017_10_01 import models
            return models
        elif api_version == '2017-11-01':
            from .v2017_11_01 import models
            return models
        elif api_version == '2018-01-01':
            from .v2018_01_01 import models
            return models
        elif api_version == '2018-02-01':
            from .v2018_02_01 import models
            return models
        elif api_version == '2018-04-01':
            from .v2018_04_01 import models
            return models
        raise NotImplementedError("APIVersion {} is not available".format(api_version))