Added None as return value for IPv6Network /127 and /128

Author: David Groves

Doc/library/ipaddress.rst

@@ -780,7 +780,8 @@ dictionaries.
    .. attribute:: subnet_router_anycast_address
 
       The Subnet-Router anycast address for the network, which is the first address
-      in the network.
+      in the network or None if the network is a /127 or /128, which do not have
+      a subnet router, as defined in IETF RFC 6164, section 8.
 
       .. versionadded:: next
 

Lib/ipaddress.py

@@ -2370,6 +2370,8 @@ def is_site_local(self):
 
     @functools.cached_property
     def subnet_router_anycast_address(self):
+        if self._prefixlen == 127 or self._prefixlen == 128:
+            return None
         return self.first_address
 
     @functools.cached_property