Takes a StringCounter of normalized URL and parses their hostnames

    N.B. this assumes that absolute URLs will begin with

    http://

    in order to accurately resolve the host name.
    Relative URLs will not have host names.

def host_names(urls):
    '''
    Takes a StringCounter of normalized URL and parses their hostnames

    N.B. this assumes that absolute URLs will begin with

    http://

    in order to accurately resolve the host name.
    Relative URLs will not have host names.
    '''
    host_names = StringCounter()
    for url in urls:
        host_names[urlparse(url).netloc] += urls[url]
    return host_names