diff --git a/Doc/library/urlparse.rst b/Doc/library/urlparse.rst

index 22249da..0989c88 100644

--- a/Doc/library/urlparse.rst

+++ b/Doc/library/urlparse.rst

@@ -119,12 +119,22 @@ The :mod:`urlparse` module defines the following functions:

    See section :ref:`urlparse-result-object` for more information on the result

    object.

+   Characters in the :attr:`netloc` attribute that decompose under NFKC

+   normalization (as used by the IDNA encoding) into any of ``/``, ``?``,

+   ``#``, ``@``, or ``:`` will raise a :exc:`ValueError`. If the URL is

+   decomposed before parsing, or is not a Unicode string, no error will be

+   raised.

+

    .. versionchanged:: 2.5

       Added attributes to return value.

    .. versionchanged:: 2.7

       Added IPv6 URL parsing capabilities.

+   .. versionchanged:: 2.7.17

+      Characters that affect netloc parsing under NFKC normalization will

+      now raise :exc:`ValueError`.

+

 .. function:: parse_qs(qs[, keep_blank_values[, strict_parsing[, max_num_fields]]])

@@ -232,11 +242,21 @@ The :mod:`urlparse` module defines the following functions:

    See section :ref:`urlparse-result-object` for more information on the result

    object.

+   Characters in the :attr:`netloc` attribute that decompose under NFKC

+   normalization (as used by the IDNA encoding) into any of ``/``, ``?``,

+   ``#``, ``@``, or ``:`` will raise a :exc:`ValueError`. If the URL is

+   decomposed before parsing, or is not a Unicode string, no error will be

+   raised.

+

    .. versionadded:: 2.2

    .. versionchanged:: 2.5

       Added attributes to return value.

+   .. versionchanged:: 2.7.17

+      Characters that affect netloc parsing under NFKC normalization will

+      now raise :exc:`ValueError`.

+

 .. function:: urlunsplit(parts)

diff --git a/Lib/test/test_urlparse.py b/Lib/test/test_urlparse.py

index 4e1ded7..6fd1071 100644

--- a/Lib/test/test_urlparse.py

+++ b/Lib/test/test_urlparse.py

@@ -1,4 +1,6 @@

 from test import test_support

+import sys

+import unicodedata

 import unittest

 import urlparse

@@ -624,6 +626,35 @@ class UrlParseTestCase(unittest.TestCase):

         self.assertEqual(urlparse.urlparse("http://www.python.org:80"),

                 ('http','www.python.org:80','','','',''))

+    def test_urlsplit_normalization(self):

+        # Certain characters should never occur in the netloc,

+        # including under normalization.

+        # Ensure that ALL of them are detected and cause an error

+        illegal_chars = u'/:#?@'

+        hex_chars = {'{:04X}'.format(ord(c)) for c in illegal_chars}

+        denorm_chars = [

+            c for c in map(unichr, range(128, sys.maxunicode))

+            if (hex_chars & set(unicodedata.decomposition(c).split()))

+            and c not in illegal_chars

+        ]

+        # Sanity check that we found at least one such character

+        self.assertIn(u'\u2100', denorm_chars)

+        self.assertIn(u'\uFF03', denorm_chars)

+

+        # bpo-36742: Verify port separators are ignored when they

+        # existed prior to decomposition

+        urlparse.urlsplit(u'http://\u30d5\u309a:80')

+        with self.assertRaises(ValueError):

+            urlparse.urlsplit(u'http://\u30d5\u309a\ufe1380')

+

+        for scheme in [u"http", u"https", u"ftp"]:

+            for c in denorm_chars:

+                url = u"{}://netloc{}false.netloc/path".format(scheme, c)

+                if test_support.verbose:

+                    print "Checking %r" % url

+                with self.assertRaises(ValueError):

+                    urlparse.urlsplit(url)

+

 def test_main():

     test_support.run_unittest(UrlParseTestCase)

diff --git a/Lib/urlparse.py b/Lib/urlparse.py

index f7c2b03..f08e0fe 100644

--- a/Lib/urlparse.py

+++ b/Lib/urlparse.py

@@ -165,6 +165,24 @@ def _splitnetloc(url, start=0):

             delim = min(delim, wdelim)     # use earliest delim position

     return url[start:delim], url[delim:]   # return (domain, rest)

+def _checknetloc(netloc):

+    if not netloc or not isinstance(netloc, unicode):

+        return

+    # looking for characters like \u2100 that expand to 'a/c'

+    # IDNA uses NFKC equivalence, so normalize for this check

+    import unicodedata

+    n = netloc.rpartition('@')[2] # ignore anything to the left of '@'

+    n = n.replace(':', '')        # ignore characters already included

+    n = n.replace('#', '')        # but not the surrounding text

+    n = n.replace('?', '')

+    netloc2 = unicodedata.normalize('NFKC', n)

+    if n == netloc2:

+        return

+    for c in '/?#@:':

+        if c in netloc2:

+            raise ValueError("netloc '" + netloc + "' contains invalid " +

+                             "characters under NFKC normalization")

+

 def urlsplit(url, scheme='', allow_fragments=True):

     """Parse a URL into 5 components:

     <scheme>://<netloc>/<path>?<query>#<fragment>

@@ -193,6 +211,7 @@ def urlsplit(url, scheme='', allow_fragments=True):

                 url, fragment = url.split('#', 1)

             if '?' in url:

                 url, query = url.split('?', 1)

+            _checknetloc(netloc)

             v = SplitResult(scheme, netloc, url, query, fragment)

             _parse_cache[key] = v

             return v

@@ -216,6 +235,7 @@ def urlsplit(url, scheme='', allow_fragments=True):

         url, fragment = url.split('#', 1)

     if '?' in url:

         url, query = url.split('?', 1)

+    _checknetloc(netloc)

     v = SplitResult(scheme, netloc, url, query, fragment)

     _parse_cache[key] = v

     return v