Error when requesting subdomain from localhost

Requesting a subdomain from localhost does not work.
As you can see requesting a url without subdomain from localhost works. When adding a subdomain "[Errno 8] nodename nor servname provided, or not known" is raised.

```python
import requests
requests.get('http://localhost:3000/highlights')
==> <Response [404]>

requests.get('http://api.localhost:3000/highlights')
==> Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/requests/packages/urllib3/connection.py", line 138, in _new_conn
    (self.host, self.port), self.timeout, **extra_kw)
  File "/usr/local/lib/python3.6/site-packages/requests/packages/urllib3/util/connection.py", line 75, in create_connection
    for res in socket.getaddrinfo(host, port, family, socket.SOCK_STREAM):
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/socket.py", line 743, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
socket.gaierror: [Errno 8] nodename nor servname provided, or not known

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/requests/packages/urllib3/connectionpool.py", line 594, in urlopen
    chunked=chunked)
  File "/usr/local/lib/python3.6/site-packages/requests/packages/urllib3/connectionpool.py", line 361, in _make_request
    conn.request(method, url, **httplib_request_kw)
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/http/client.py", line 1239, in request
    self._send_request(method, url, body, headers, encode_chunked)
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/http/client.py", line 1285, in _send_request
    self.endheaders(body, encode_chunked=encode_chunked)
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/http/client.py", line 1234, in endheaders
    self._send_output(message_body, encode_chunked=encode_chunked)
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/http/client.py", line 1026, in _send_output
    self.send(msg)
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/http/client.py", line 964, in send
    self.connect()
  File "/usr/local/lib/python3.6/site-packages/requests/packages/urllib3/connection.py", line 163, in connect
    conn = self._new_conn()
  File "/usr/local/lib/python3.6/site-packages/requests/packages/urllib3/connection.py", line 147, in _new_conn
    self, "Failed to establish a new connection: %s" % e)
requests.packages.urllib3.exceptions.NewConnectionError: <requests.packages.urllib3.connection.HTTPConnection object at 0x105a18470>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/requests/adapters.py", line 423, in send
    timeout=timeout
  File "/usr/local/lib/python3.6/site-packages/requests/packages/urllib3/connectionpool.py", line 643, in urlopen
    _stacktrace=sys.exc_info()[2])
  File "/usr/local/lib/python3.6/site-packages/requests/packages/urllib3/util/retry.py", line 363, in increment
    raise MaxRetryError(_pool, url, error or ResponseError(cause))
requests.packages.urllib3.exceptions.MaxRetryError: HTTPConnectionPool(host='api.localhost', port=3000): Max retries exceeded with url: /highlights (Caused by NewConnectionError('<requests.packages.urllib3.connection.HTTPConnection object at 0x105a18470>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known',))

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/site-packages/requests/api.py", line 70, in get
    return request('get', url, params=params, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/requests/api.py", line 56, in request
    return session.request(method=method, url=url, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/requests/sessions.py", line 488, in request
    resp = self.send(prep, **send_kwargs)
  File "/usr/local/lib/python3.6/site-packages/requests/sessions.py", line 609, in send
    r = adapter.send(request, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/requests/adapters.py", line 487, in send
    raise ConnectionError(e, request=request)
requests.exceptions.ConnectionError: HTTPConnectionPool(host='api.localhost', port=3000): Max retries exceeded with url: /highlights (Caused by NewConnectionError('<requests.packages.urllib3.connection.HTTPConnection object at 0x105a18470>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known',))
```

Any ideas how to make this work?

You need to add a hosts file entry. Sub domains of local host do not automatically resolve to the loop back address, and generally speaking are not well defined. This problem is really nothing to do with Requests: your OS is responsible for resolving names into IP addresses.