Returns unassigned port ranges according to IANA.

def _iana_unassigned_port_ranges():
    """
    Returns unassigned port ranges according to IANA.
    """
    page = urllib2.urlopen(IANA_DOWNLOAD_URL).read()
    xml = ElementTree.fromstring(page)
    records = xml.findall('{%s}record' % IANA_NS)
    for record in records:
        description = record.find('{%s}description' % IANA_NS).text
        if description == 'Unassigned':
            numbers = record.find('{%s}number' % IANA_NS).text
            yield numbers