Contents

• Abstract
• PEP Acceptance
• Motivation
• Rationale
• Background
• Specification
• Reference Implementation
• References
• Copyright

Abstract

This PEP proposes a design and for an IP address manipulation module for python.

PEP Acceptance

This PEP was accepted by Nick Coghlan on the 15th of May, 2012.

Motivation

Several very good IP address modules for python already exist. The truth is that all of them struggle with the balance between adherence to Pythonic principals and the shorthand upon which network engineers and administrators rely. ipaddress aims to strike the right balance.

Rationale

The existence of several Python IP address manipulation modules is evidence of an outstanding need for the functionality this module seeks to provide.

Background

PEP 3144 and ipaddr have been up for inclusion before. The version of the library specified here is backwards incompatible with the version on PyPI and the one which was discussed before. In order to avoid confusing users of the current ipaddr, I’ve renamed this version of the library ipaddress.

The main differences between ipaddr and ipaddress are:

• ipaddress *Network classes are equivalent to the ipaddr *Network class counterparts with the strict flag set to True.
• ipaddress *Interface classes are equivalent to the ipaddr *Network class counterparts with the strict flag set to False.
• The factory functions in ipaddress were renamed to disambiguate them from classes.
• A few attributes were renamed to disambiguate their purpose as well. (eg. network, network_address)
• A number of methods and functions which returned containers in ipaddr now return iterators. This includes subnets, address_exclude, summarize_address_range and collapse_address_list.

Due to the backwards incompatible API changes between ipaddress and ipaddr, the proposal is to add the module using the new provisional API status:

• http://docs.python.org/dev/glossary.html#term-provisional-package

Relevant messages on python-dev:

• https://mail.python.org/pipermail/python-dev/2012-January/116016.html
• https://mail.python.org/pipermail/python-dev/2012-February/116656.html
• https://mail.python.org/pipermail/python-dev/2012-February/116688.html

Specification

The ipaddr module defines a total of 6 new public classes, 3 for manipulating IPv4 objects and 3 for manipulating IPv6 objects. The classes are as follows:

• IPv4Address/IPv6Address - These define individual addresses, for example the IPv4 address returned by an A record query for www.google.com (74.125.224.84) or the IPv6 address returned by a AAAA record query for ipv6.google.com (2001:4860:4001:801::1011).
• IPv4Network/IPv6Network - These define networks or groups of addresses, for example the IPv4 network reserved for multicast use (224.0.0.0/4) or the IPv6 network reserved for multicast (ff00::/8, wow, that’s big).
• IPv4Interface/IPv6Interface - These hybrid classes refer to an individual address on a given network. For example, the IPV4 address 192.0.2.1 on the network 192.0.2.0/24 could be referred to as 192.0.2.1/24. Likewise, the IPv6 address 2001:DB8::1 on the network 2001:DB8::/96 could be referred to as 2001:DB8::1/96. It’s very common to refer to addresses assigned to computer network interfaces like this, hence the Interface name.

All IPv4 classes share certain characteristics and methods; the number of bits needed to represent them, whether or not they belong to certain special IPv4 network ranges, etc. Similarly, all IPv6 classes share characteristics and methods.

ipaddr makes extensive use of inheritance to avoid code duplication as much as possible. The parent classes are private, but they are outlined here:

• _IPAddrBase - Provides methods common to all ipaddr objects.
• _BaseAddress - Provides methods common to IPv4Address and IPv6Address.
• _BaseInterface - Provides methods common to IPv4Interface and IPv6Interface, as well as IPv4Network and IPv6Network (ipaddr treats the Network classes as a special case of Interface).
• _BaseV4 - Provides methods and variables (eg, _max_prefixlen) common to all IPv4 classes.
• _BaseV6 - Provides methods and variables common to all IPv6 classes.

Comparisons between objects of differing IP versions results in a TypeError 1. Additionally, comparisons of objects with different _Base parent classes results in a TypeError. The effect of the _Base parent class limitation is that IPv4Interface’s can be compared to IPv4Network’s and IPv6Interface’s can be compared to IPv6Network’s.

Reference Implementation

The current reference implementation can be found at:

http://code.google.com/p/ipaddress-py/source/browse/ipaddress.py

Or see the tarball to include the README and unittests. http://code.google.com/p/ipaddress-py/downloads/detail?name=ipaddress-1.0.tar.gz

More information about using the reference implementation can be found at: http://code.google.com/p/ipaddr-py/wiki/Using3144

References

1

Appealing to authority is a logical fallacy, but Vint Cerf is an authority who can’t be ignored. Full text of the email follows:

I have seen a substantial amount of traffic about IPv4 and IPv6 comparisons and the general consensus is that these are not comparable.

If we were to take a very simple minded view, we might treat these as pure integers in which case there is an ordering but not a useful one.

In the IPv4 world, “length” is important because we take longest (most specific) address first for routing. Length is determine by the mask, as you know.

Assuming that the same style of argument works in IPv6, we would have to conclude that treating an IPv6 value purely as an integer for comparison with IPv4 would lead to some really strange results.

All of IPv4 space would lie in the host space of 0::0/96 prefix of IPv6. For any useful interpretation of IPv4, this is a non-starter.

I think the only sensible conclusion is that IPv4 values and IPv6 values should be treated as non-comparable.

Vint

Copyright

This document has been placed in the public domain.