ipaddress is_private misleading for IPv4 mapped IPv6 addresses

[ThomasKriechbaumer]

BPO	33433
Nosy	@gpshead, @miss-islington, @JamoBox
PRs	bpo-33433 Fix private address checking for IPv4 mapped IPv6. #26172 [3.10] bpo-33433 Fix private address checking for IPv4 mapped IPv6. (GH-26172) #26189

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-05-17.19:43:01.443>
created_at = <Date 2018-05-05.13:34:54.127>
labels = ['type-feature', 'library', '3.10', '3.11']
title = 'ipaddress is_private misleading for IPv4 mapped IPv6 addresses'
updated_at = <Date 2021-05-17.19:43:01.442>
user = 'https://bugs.python.org/ThomasKriechbaumer'

bugs.python.org fields:

activity = <Date 2021-05-17.19:43:01.442>
actor = 'gregory.p.smith'
assignee = 'none'
closed = True
closed_date = <Date 2021-05-17.19:43:01.443>
closer = 'gregory.p.smith'
components = ['Library (Lib)']
creation = <Date 2018-05-05.13:34:54.127>
creator = 'Thomas Kriechbaumer'
dependencies = []
files = []
hgrepos = []
issue_num = 33433
keywords = ['patch']
message_count = 5.0
messages = ['316213', '393749', '393789', '393823', '393824']
nosy_count = 4.0
nosy_names = ['gregory.p.smith', 'miss-islington', 'Thomas Kriechbaumer', 'Wicken']
pr_nums = ['26172', '26189']
priority = 'normal'
resolution = 'fixed'
stage = 'commit review'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue33433'
versions = ['Python 3.10', 'Python 3.11']

[ThomasKriechbaumer]

ipaddress.IPv4Address and ipaddress.IPv6Address provide a is_private function that indicates if the address (or network) is part of a "private" IP range, as designated by the IANA special-use registry.

The current documentation is as follows:

is_private
True if the address is allocated for private networks. See iana-ipv4-special-registry (for IPv4) or iana-ipv6-special-registry (for IPv6).

However, IPv4-mapped IPv6 addresses are currently being reported as "private" by this function (see 22c3176#diff-0fc57874e463b95dbdfe6f80ae918ea1R1869).

I consider this a bug or at least counter-intuitive, because e.g. ::ffff:8.8.8.8 is most definitely NOT a private IP address, and yet the ipaddress.is_private function reports True.
The IANA special-use registry does not contain any references to "being private" for the ::ffff region. It just designates it to be used for IPv4-mapped address.

[JamoBox]

I've opened a PR that should hopefully address this issue.

[gpshead]

Questionable as to if this counts as a bug or not, but I agree it is counter-intuitive given the purpose of actually checking the .is_private property of an address. So updating the behavior makes sense, but I'll be conservative w.r.t. behavior changes and not apply this to a 3.9 or earlier.

[miss-islington]

New changeset a44bb6d by Miss Islington (bot) in branch '3.10':
bpo-33433 Fix private address checking for IPv4 mapped IPv6. (GH-26172)
a44bb6d

[gpshead]

thanks!