ipaddress: Shared Address Space (100.64.0.0/10) is neither private nor global

Something I discovered when working on GH-65056 and GH-113171.

IPv4Address' and IPv4Network's is_global and is_private are false at the same time for addresses in this range:

>>> ipaddress.IPv4Address('100.64.0.0').is_global
False
>>> ipaddress.IPv4Address('100.64.0.0').is_private
False
>>> ipaddress.IPv4Network('100.64.0.0/10').is_global
False
>>> ipaddress.IPv4Network('100.64.0.0/10').is_private
False

I don't believe this is right and I'll explain why.

Historical context

Initial additions

is_private introduced in dc9b2555a875 ("Issue #14814: addition of the ipaddress module (stage 1 - code and tests)") on 2012-05-20.

It only handled the actual private-use ranges for IPv4 (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).

Documented as such.

100.64.0.0/10 handling and is_private semantics changes

There have been four ipaddress patches involving this range:

1. 22c31764262b ("#17400; ipaddress should make it easy to identify rfc6598 addresses")
2. be9c1b133b78 ("#17400: fix documentation, add cache to is_global and correctly handle 100.64.0.0/10")
3. e5019d518304 ("#17400: correct handling of 100.64.0.0/10, fixing the docs and updating NEWS")
4. 742192a4fea8 ("Issue #21386: Implement missing IPv4Address.is_global property")

The first three were part of GH-61602 (https://bugs.python.org/issue17400), the fourth one just adds a missing IPv4Address.is_global property to match the IPv4Network.is_global semantics and follows the semantics established in commits 1-3.

Commit 1 changed the semantics of is_private from "Is this a Private-use IPv4 address or a Unique-Local IPv6 address?" to roughly "Do [1] and [2] say globally reachable = false for this address". The documentation change was not quite precise

         A boolean, True if the address is reserved per
        iana-ipv4-special-registry or iana-ipv6-special-registry.

but the intent of the implementation is quite clear.

Commit 1 also added is_global that was effectively not is_private.

In commit 3 an exception for 100.64.0.0/10 is made and both is_global and is_private are false for that range.

is_global/is_private semantics clarification

As part of GH-65056 we changed the documentation to make it clear that we follow the "globally reachable" information from IANA (with some caveats, like special handling of IPv4-mapped IPv6 addresses, see 83f0f8d62f27 ("bpo-33433 Fix private address checking for IPv4 mapped IPv6. (GH-26172)")).

The problem

The motivation for handling 100.64.0.0/10 like this can be found here:

https://github.com/python/cpython/issues/61602#issuecomment-1093610325

The rationale for RFC 6598 is precisely that 100.64.0.0/10 is not private in the common sense, so it would deserve a different treatment in the ipaddress module as well.

I have to admit I don't find it convincing enough to make an exception for it.

[1] says the range is not globally reachable so in my opinion is_private should return true for it as it does for the rest of the not globally reachable address blocks (again, with the exception of IPv4-mapped IPv6 address handling).

I'd find is_private being false for the range surprising if I wasn't clearly aware of the semantics after reading this code multiple times. I believe it may lead to real-world issues.

Additionally the behavior where is_private is not the opposite of is_global is likely to be surprising too.

In short: IMO there should be no exception.

The downside of the proposed solution: this is technically breaking backwards compatibility if some code depends on the current semantics. I'm not sure I'd classify the proposed change strictly as a bug fix.

[1] https://www.iana.org/assignments/iana-ipv4-special-registry/iana-ipv4-special-registry.xhtml
[2] https://www.iana.org/assignments/iana-ipv6-special-registry/iana-ipv6-special-registry.xhtml

Linked PRs

• gh-119813

Start by reading the ipaddress implementation of IPv4Address and IPv4Network is_private and is_global, along with the discussion and linked PR gh-119813. Check how the 100.64.0.0/10 exception is represented and whether the proposed semantics are consistent across documentation and tests; done requires an agreed behavior, since the issue notes backward-compatibility concerns.