def get_fqdn(slice_id, server_id, site_id):
    """Returns an FQDN or None if the given slice_id is not valid."""

    slice_parts = slice_id.split('_')
    if len(slice_parts) != 2:
        return None
    return '.'.join([slice_parts[1], slice_parts[0], server_id, site_id,
                     'measurement-lab.org'])