New Post: Why different behavior of localhost and 127.0.0.1?

Why does localhost:3901 work ok, but 127.0.0.1:3901 doesn't work (hangs browser before timing out)? Shouldn't they operate the same way? I'm using Windows 8.1. I'm new so maybe I'm missing something obvious.
Works:

    m_listener = web::http::experimental::listener::http_listener(L"http://localhost:3901");

No work:

    m_listener = web::http::experimental::listener::http_listener(L"http://127.0.0.1:3901");

Hosts file fragment:

# localhost name resolution is handled within DNS itself.
#   127.0.0.1       localhost
#   ::1             localhost