gh-136832: Add fec0::/10 to IPv6 reserved networks and deprecate is_site_local

Doc/library/ipaddress.rst

@@ -246,8 +246,9 @@ write code that handles both IP versions correctly.  Address objects are
       .. note:: For IPv4, ``is_reserved`` is not related to the address block value of the
         ``Reserved-by-Protocol`` column in iana-ipv4-special-registry_.
 
-      .. caution:: For IPv6, ``fec0::/10`` a former Site-Local scoped address prefix is
-         currently excluded from that list (see :attr:`~IPv6Address.is_site_local` & :rfc:`3879`).
+      .. versionchanged:: next
+         For IPv6, ``fec0::/10`` (the former Site-Local scoped address prefix
+         deprecated by :rfc:`3879`) is now included in the reserved range.
 
    .. attribute:: is_loopback
 
@@ -365,6 +366,10 @@ write code that handles both IP versions correctly.  Address objects are
       :attr:`~IPv4Address.is_private` to test if this address is in the
       space of unique local addresses as defined by :RFC:`4193`.
 
+      .. deprecated-removed:: next 3.21
+         Site-local addresses (``fec0::/10``) were deprecated by :rfc:`3879`.
+         Use :attr:`~IPv6Address.is_private` instead.
+
    .. attribute:: ipv4_mapped
 
       For addresses that appear to be IPv4 mapped addresses in the range
@@ -800,6 +805,10 @@ dictionaries.
       This attribute is true for the network as a whole if it is true
       for both the network address and the broadcast address.
 
+      .. deprecated-removed:: next 3.21
+         Site-local addresses (``fec0::/10``) were deprecated by :rfc:`3879`.
+         Use :attr:`~IPv6Network.is_private` instead.
+
 
 Operators
 ^^^^^^^^^

Lib/ipaddress.py

@@ -10,6 +10,9 @@
 
 import functools
 
+lazy import warnings
+
+
 IPV4LENGTH = 32
 IPV6LENGTH = 128
 
@@ -2083,6 +2086,13 @@ def is_site_local(self):
             A boolean, True if the address is reserved per RFC 3513 2.5.6.
 
         """
+        warnings._deprecated(
+            "IPv6Address.is_site_local",
+            "{name!r} is deprecated and slated for removal in Python "
+            "{remove}; site-local addresses (fec0::/10) were deprecated by "
+            "RFC 3879, use is_private instead",
+            remove=(3, 21),
+        )
         return self in self._constants._sitelocal_network
 
     @property
@@ -2363,8 +2373,16 @@ def is_site_local(self):
             A boolean, True if the address is reserved per RFC 3513 2.5.6.
 
         """
-        return (self.network_address.is_site_local and
-                self.broadcast_address.is_site_local)
+        warnings._deprecated(
+            "IPv6Network.is_site_local",
+            "{name!r} is deprecated and slated for removal in Python "
+            "{remove}; site-local addresses (fec0::/10) were deprecated by "
+            "RFC 3879, use is_private instead",
+            remove=(3, 21),
+        )
+        sitelocal_network = self._constants._sitelocal_network
+        return (self.network_address in sitelocal_network and
+                self.broadcast_address in sitelocal_network)
 
 
 class _IPv6Constants:
@@ -2408,7 +2426,7 @@ class _IPv6Constants:
         IPv6Network('8000::/3'), IPv6Network('A000::/3'),
         IPv6Network('C000::/3'), IPv6Network('E000::/4'),
         IPv6Network('F000::/5'), IPv6Network('F800::/6'),
-        IPv6Network('FE00::/9'),
+        IPv6Network('FE00::/9'), IPv6Network('FEC0::/10'),
     ]
 
     _sitelocal_network = IPv6Network('fec0::/10')

Lib/test/test_ipaddress.py

@@ -2418,12 +2418,13 @@ def testReservedIpv6(self):
         self.assertEqual(True, ipaddress.ip_network('ff00::').is_multicast)
         self.assertEqual(False, ipaddress.ip_network('fdff::').is_multicast)
 
-        self.assertEqual(True, ipaddress.ip_network('fecf::').is_site_local)
-        self.assertEqual(True, ipaddress.ip_network(
-                'feff:ffff:ffff:ffff::').is_site_local)
-        self.assertEqual(False, ipaddress.ip_network(
-                'fbf:ffff::').is_site_local)
-        self.assertEqual(False, ipaddress.ip_network('ff00::').is_site_local)
+        with self.assertWarns(DeprecationWarning):
+            self.assertEqual(True, ipaddress.ip_network('fecf::').is_site_local)
+            self.assertEqual(True, ipaddress.ip_network(
+                    'feff:ffff:ffff:ffff::').is_site_local)
+            self.assertEqual(False, ipaddress.ip_network(
+                    'fbf:ffff::').is_site_local)
+            self.assertEqual(False, ipaddress.ip_network('ff00::').is_site_local)
 
         self.assertEqual(True, ipaddress.ip_network('fc00::').is_private)
         self.assertEqual(True, ipaddress.ip_network(
@@ -2457,12 +2458,13 @@ def testReservedIpv6(self):
         self.assertEqual(True, ipaddress.ip_address('ff00::').is_multicast)
         self.assertEqual(False, ipaddress.ip_address('fdff::').is_multicast)
 
-        self.assertEqual(True, ipaddress.ip_address('fecf::').is_site_local)
-        self.assertEqual(True, ipaddress.ip_address(
-                'feff:ffff:ffff:ffff::').is_site_local)
-        self.assertEqual(False, ipaddress.ip_address(
-                'fbf:ffff::').is_site_local)
-        self.assertEqual(False, ipaddress.ip_address('ff00::').is_site_local)
+        with self.assertWarns(DeprecationWarning):
+            self.assertEqual(True, ipaddress.ip_address('fecf::').is_site_local)
+            self.assertEqual(True, ipaddress.ip_address(
+                    'feff:ffff:ffff:ffff::').is_site_local)
+            self.assertEqual(False, ipaddress.ip_address(
+                    'fbf:ffff::').is_site_local)
+            self.assertEqual(False, ipaddress.ip_address('ff00::').is_site_local)
 
         self.assertEqual(True, ipaddress.ip_address('fc00::').is_private)
         self.assertEqual(True, ipaddress.ip_address(
@@ -2503,6 +2505,9 @@ def testReservedIpv6(self):
         # some generic IETF reserved addresses
         self.assertEqual(True, ipaddress.ip_address('100::').is_reserved)
         self.assertEqual(True, ipaddress.ip_network('4000::1/128').is_reserved)
+        # gh-136832: fec0::/10 (former site-local prefix) is reserved
+        self.assertEqual(True, ipaddress.ip_address('fec0::1').is_reserved)
+        self.assertEqual(True, ipaddress.ip_network('fec0::/10').is_reserved)
 
     def testIpv4Mapped(self):
         self.assertEqual(

Misc/NEWS.d/next/Library/2026-06-22-22-11-06.gh-issue-136832.JvKyu4.rst

@@ -0,0 +1,7 @@
+The IPv6 address block ``fec0::/10`` (the former Site-Local scoped address
+prefix deprecated by :rfc:`3879`) is now reported as reserved by
+:attr:`ipaddress.IPv6Address.is_reserved`.  The
+:attr:`~ipaddress.IPv6Address.is_site_local` and
+:attr:`~ipaddress.IPv6Network.is_site_local` attributes are now deprecated
+and slated for removal in Python 3.21; use
+:attr:`~ipaddress.IPv6Address.is_private` instead.