diff --git a/Lib/test/test_urllib2.py b/Lib/test/test_urllib2.py
index 9db23e6ce04bd5..c604097fc91eca 100644
--- a/Lib/test/test_urllib2.py
+++ b/Lib/test/test_urllib2.py
@@ -808,12 +808,12 @@ def test_file(self):
             self.assertEqual(respurl, url)
 
         for url in [
-            "file://localhost:80%s" % urlpath,
+            "file://localhost:80/%s" % urlpath,
             "file:///file_does_not_exist.txt",
             "file://not-a-local-host.com//dir/file.txt",
-            "file://%s:80%s/%s" % (socket.gethostbyname('localhost'),
+            "file://%s:80/%s/%s" % (socket.gethostbyname('localhost'),
                                    os.getcwd(), TESTFN),
-            "file://somerandomhost.ontheinternet.com%s/%s" %
+            "file://somerandomhost.ontheinternet.com/%s/%s" %
             (os.getcwd(), TESTFN),
             ]:
             try:
diff --git a/Lib/test/test_urlparse.py b/Lib/test/test_urlparse.py
index dff9a8ede9b601..342cd300005135 100644
--- a/Lib/test/test_urlparse.py
+++ b/Lib/test/test_urlparse.py
@@ -608,9 +608,8 @@ def test_urlsplit_attributes(self):
 
         # Verify an illegal port raises ValueError
         url = b"HTTP://WWW.PYTHON.ORG:65536/doc/#frag"
-        p = urllib.parse.urlsplit(url)
         with self.assertRaisesRegex(ValueError, "out of range"):
-            p.port
+            urllib.parse.urlsplit(url)
 
     def test_urlsplit_remove_unsafe_bytes(self):
         # Remove ASCII tabs and newlines from input
@@ -660,10 +659,8 @@ def test_attributes_bad_port(self):
                         if bytes:
                             netloc = netloc.encode("ascii")
                             url = url.encode("ascii")
-                        p = parse(url)
-                        self.assertEqual(p.netloc, netloc)
                         with self.assertRaises(ValueError):
-                            p.port
+                            parse(url)
 
     def test_attributes_without_netloc(self):
         # This example is straight from RFC 3261.  It looks like it
@@ -1014,13 +1011,11 @@ def test_issue14072(self):
 
     def test_port_casting_failure_message(self):
         message = "Port could not be cast to integer value as 'oracle'"
-        p1 = urllib.parse.urlparse('http://Server=sde; Service=sde:oracle')
         with self.assertRaisesRegex(ValueError, message):
-            p1.port
+            urllib.parse.urlparse('http://Server=sde; Service=sde:oracle')
 
-        p2 = urllib.parse.urlsplit('http://Server=sde; Service=sde:oracle')
         with self.assertRaisesRegex(ValueError, message):
-            p2.port
+            urllib.parse.urlsplit('http://Server=sde; Service=sde:oracle')
 
     def test_telurl_params(self):
         p1 = urllib.parse.urlparse('tel:123-4;phone-context=+1-650-516')
diff --git a/Lib/urllib/parse.py b/Lib/urllib/parse.py
index bf16d0f42e5794..4fe73661b6a63e 100644
--- a/Lib/urllib/parse.py
+++ b/Lib/urllib/parse.py
@@ -32,6 +32,7 @@
 import re
 import sys
 import types
+import collections
 import warnings
 
 __all__ = ["urlparse", "urlunparse", "urljoin", "urldefrag",
@@ -123,6 +124,18 @@ def _coerce_args(*args):
         return args + (_noop,)
     return _decode_args(args) + (_encode_result,)
 
+def _parse_hostinfo(netloc):
+    _, _, hostinfo = netloc.rpartition('@')
+    _, have_open_br, bracketed = hostinfo.partition('[')
+    if have_open_br:
+        hostname, _, port = bracketed.partition(']')
+        _, _, port = port.partition(':')
+    else:
+        hostname, _, port = hostinfo.partition(':')
+    if not port:
+        port = None
+    return hostname, port
+
 # Result objects are more helpful than simple tuples
 class _ResultMixinStr(object):
     """Standard approach to encoding parsed results from str to bytes"""
@@ -167,13 +180,7 @@ def hostname(self):
     def port(self):
         port = self._hostinfo[1]
         if port is not None:
-            try:
-                port = int(port, 10)
-            except ValueError:
-                message = f'Port could not be cast to integer value as {port!r}'
-                raise ValueError(message) from None
-            if not ( 0 <= port <= 65535):
-                raise ValueError("Port out of range 0-65535")
+            port = _validate_port(port)
         return port
 
     __class_getitem__ = classmethod(types.GenericAlias)
@@ -197,16 +204,7 @@ def _userinfo(self):
     @property
     def _hostinfo(self):
         netloc = self.netloc
-        _, _, hostinfo = netloc.rpartition('@')
-        _, have_open_br, bracketed = hostinfo.partition('[')
-        if have_open_br:
-            hostname, _, port = bracketed.partition(']')
-            _, _, port = port.partition(':')
-        else:
-            hostname, _, port = hostinfo.partition(':')
-        if not port:
-            port = None
-        return hostname, port
+        return _parse_hostinfo(netloc)
 
 
 class _NetlocResultMixinBytes(_NetlocResultMixinBase, _ResultMixinBytes):
@@ -402,6 +400,16 @@ def _splitparams(url):
         i = url.find(';')
     return url[:i], url[i+1:]
 
+def _validate_port(port_str):
+    try:
+        port = int(port_str, 10)
+    except ValueError:
+        message = f'Port could not be cast to integer value as {port_str!r}'
+        raise ValueError(message) from None
+    if not (0 <= port <= 65535):
+        raise ValueError("Port out of range 0-65535: %r" % port_str)
+    return port
+
 def _splitnetloc(url, start=0):
     delim = len(url)   # position of end of domain part of url, default is end
     for c in '/?#':    # look for delimiters; the order is NOT important
@@ -410,9 +418,7 @@ 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 netloc.isascii():
-        return
+def _checknetloc_nfkc(netloc):
     # looking for characters like \u2100 that expand to 'a/c'
     # IDNA uses NFKC equivalence, so normalize for this check
     import unicodedata
@@ -428,6 +434,14 @@ def _checknetloc(netloc):
             raise ValueError("netloc '" + netloc + "' contains invalid " +
                              "characters under NFKC normalization")
 
+def _checknetloc(netloc):
+    if netloc and not netloc.isascii():
+        _checknetloc_nfkc(netloc)
+
+    _, port = _parse_hostinfo(netloc)
+    if port is not None:
+        _validate_port(port)
+
 # typed=True avoids BytesWarnings being emitted during cache key
 # comparison since this API supports both bytes and str input.
 @functools.lru_cache(typed=True)
diff --git a/Misc/ACKS b/Misc/ACKS
index b023bcb6d72fd3..b208fb74408719 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -227,6 +227,7 @@ Tom Bridgman
 Anthony Briggs
 Keith Briggs
 Tobias Brink
+Miguel Brito
 Dillon Brock
 Richard Brodie
 Michael Broghton
diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-05-01-10-22-18.bpo-43871.y2Cvah.rst b/Misc/NEWS.d/next/Core and Builtins/2021-05-01-10-22-18.bpo-43871.y2Cvah.rst
new file mode 100644
index 00000000000000..811278ea26307c
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2021-05-01-10-22-18.bpo-43871.y2Cvah.rst	
@@ -0,0 +1 @@
+Move port validation logic to parsing time. Patch by Miguel Brito.