[Python-Dev] Request for comments: [issue22941] IPv4Interface arithmetic changes subnet mask

[Scott Dial]

On 2015-03-12 10:46 AM, Eric V. Smith wrote:
> On 3/12/2015 10:00 AM, Antoine Pitrou wrote:
>> Related issue:
>>
>>>>> ipaddress.IPv4Interface('10.0.0.255/8') + 1
>> IPv4Interface('10.0.1.0/32')
>>
>> Should the result be IPv4Interface('10.0.0.0/8')
>> (i.e. wrap around)? Should OverflowError be raised?
> 
> I think it should be an OverflowError. 10.0.1.0/8 makes no sense, and
> 10.0.0.0/8 is unlikely to be desirable.
> 

No, you are both imbuing meaning to the dot-notation that is not there.
A.B.C.D is just an ugly way to represent a single 32-bit unsigned
integer. The only contentious part would be when your addition would
overlfow the host-portion of the address:

>>> ipaddress.IPv4Interface('10.255.255.255/8') + 1
IPv4Interface('11.0.0.0/8')

However, if you take the perspective that the address is just a 32-bit
unsigned integer, then it makes perfect sense. I would argue it's likely
to be a source of bugs, but I can't say either way because I never
adopted using this library due to issues that are cataloged in the
mailing list archives. (In the end, I wrote my own and have never found
the need to support such operands.)

-- 
Scott Dial
scott at scottdial.com