diff --git a/Doc/library/urllib.parse.rst b/Doc/library/urllib.parse.rst

index d991254..647af61 100644

--- a/Doc/library/urllib.parse.rst

+++ b/Doc/library/urllib.parse.rst

@@ -121,6 +121,11 @@ or on combining URL components into a URL string.

    Unmatched square brackets in the :attr:`netloc` attribute will raise a

    :exc:`ValueError`.

+   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, no error will be raised.

+

    .. versionchanged:: 3.2

       Added IPv6 URL parsing capabilities.

@@ -133,6 +138,10 @@ or on combining URL components into a URL string.

       Out-of-range port numbers now raise :exc:`ValueError`, instead of

       returning :const:`None`.

+   .. versionchanged:: 3.6.9

+      Characters that affect netloc parsing under NFKC normalization will

+      now raise :exc:`ValueError`.

+

 .. function:: parse_qs(qs, keep_blank_values=False, strict_parsing=False, encoding='utf-8', errors='replace', max_num_fields=None)

@@ -256,10 +265,19 @@ or on combining URL components into a URL string.

    Unmatched square brackets in the :attr:`netloc` attribute will raise a

    :exc:`ValueError`.

+   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, no error will be raised.

+

    .. versionchanged:: 3.6

       Out-of-range port numbers now raise :exc:`ValueError`, instead of

       returning :const:`None`.

+   .. versionchanged:: 3.6.9

+      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 be50b47..68f633c 100644

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

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

@@ -1,3 +1,5 @@

+import sys

+import unicodedata

 import unittest

 import urllib.parse

@@ -984,6 +986,34 @@ class UrlParseTestCase(unittest.TestCase):

                 expected.append(name)

         self.assertCountEqual(urllib.parse.__all__, expected)

+    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 = '/:#?@'

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

+        denorm_chars = [

+            c for c in map(chr, 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('\u2100', denorm_chars)

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

+

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

+        # existed prior to decomposition

+        urllib.parse.urlsplit('http://\u30d5\u309a:80')

+        with self.assertRaises(ValueError):

+            urllib.parse.urlsplit('http://\u30d5\u309a\ufe1380')

+

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

+            for netloc in ["netloc{}false.netloc", "n{}user@netloc"]:

+                for c in denorm_chars:

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

+                    with self.subTest(url=url, char='{:04X}'.format(ord(c))):

+                        with self.assertRaises(ValueError):

+                            urllib.parse.urlsplit(url)

 class Utility_Tests(unittest.TestCase):

     """Testcase to test the various utility functions in the urllib."""

diff --git a/Lib/urllib/parse.py b/Lib/urllib/parse.py

index 85e68c8..fa8827a 100644

--- a/Lib/urllib/parse.py

+++ b/Lib/urllib/parse.py

@@ -391,6 +391,24 @@ 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 any(ord(c) > 127 for c in netloc):

+        return

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

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

+    import unicodedata

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

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

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

+    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>

@@ -420,6 +438,7 @@ 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 _coerce_result(v)

@@ -443,6 +462,7 @@ 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 _coerce_result(v)