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PEP: 3144
Title: IP Address Manipulation Library for the Python Standard Library
Version: $Revision$
Last-Modified: $Date$
Author: Peter Moody <peter@hda3.com>
Discussions-To: ipaddr-py-dev@googlegroups.com
Status: Draft
Type: Standards Track
Content-Type: text/plain
Created: 13-Aug-2009
Python-Version: 3.2
Abstract:
This PEP proposes a design for a lightweight ip address manipulation module
for python.
Motivation:
Many network administrators use python in their day to day jobs. Finding a
library to assist with the common ip address manipulation tasks is easy.
Finding a good library for performing those tasks can be somewhat more
difficult. For this reason, I (like many before me) scratched an itch and
wrote my own with an emphasis on being easy to understand and fast for the
most common operations.
For context, a previous version of this library was up for inclusion in
python 3.1, see issue 3959 [1] for more information.
Rationale:
ipaddr was designed with the goal of abstracting out as much of the common
functionality as possible. As mentioned earlier, the similarities between
addresses and networks, IPV6 and IPV4 allows much code to be reused since
python allows for easy (and clean) multiple inheritance. Methods which are
specific to IPV4 or IPV6, addresses or networks are inherited from
appropriately named classes (Basev4, Basev6, BaseNet, BaseIP, etc) to
provide the full functionality of IPv4Address, IPv4Network, IPv6Address and
IPv6Network.
- Distinct IPV4 and IPV6 objects.
While there are many similarities, IPV4 and IPV6 objects are fundamentally
different. The similarities allow for easy abstraction of certain
operations which affect the bits from both in the same manner, but their
differences mean attempts to combine them into one object would be like
trying to force a round peg into a square hole (or visa versa).
- Distinct network and address objects.
Many people think of IP addresses and IP networks as synonymous, while they
are however, distinct. An IPV4 address is a single 32 bit number while the
IPV4 address assigned to a networked computer is a 32 bit address and
associated network. Similarly, an IPV6 address is a 128 bit number while
an IPV6 address assigned to a networked computer is a 128 bit number and
associated network information. The similarities leads to easy abstraction
of some methods and properties, but there are obviously a number of
address/network specific properties which require they be distinct. For
instance, IP networks contain network address (the base address of the
network), broadcast addresses (the upper end of the network, also the
address to which every machine on a given network is supposed listen, hence
the name broadcast), supernetworks and subnetworks, etc. The individual
property addresses in an IP network obviously don't have the same
properties, they're simply 32 or 128 bit numbers.
- Treat network elements as lists (in so far as it's possible).
Treating IP networks as lists is a natural extension from viewing the
network as a series of individual ip addresses. Most of the standard list
methods should be implemented and should behave in a manner that would be
consistent if the IP network object were actually a list of strings or
integers. The methods which actually modify a lists contents don't extend
as well to this model (__add__, __iadd__, __sub__, __isub__, etc) but
others (__contains__, __iter__, etc) work quite nicely. It should be noted
that __len__ doesn't work as expected since python internals has this
limited to a 32 bit integer and it would need to be at least 128 bits to
work with IPV6.
- Lightweight.
While some network programmers will undoubtedly want more than this library
provides, keeping the functionality to strictly what's required from a IP
address manipulation module is critical to keeping the code fast, easily
comprehensible and extensible. I've tried to provide enough options in
terms of functionality to allow the developer to easily do their work
without needlessly cluttering the library. Finally, It's important to note
that this design doesn't prevent subclassing or otherwise extending to meet
the unforeseen needs.
Specification:
A slightly more detailed look at the library follows.
- Design
ipaddr has four main classes most people will use:
1. IPv4Address. (eg, '192.168.1.1')
2. IPv4Network (eg, '192.168.0.0/16')
3. IPv6Address (eg, '::1')
4. IPv6Network (eg, '2001::/32')
Most of the operations a network administrator performs on networks are
similar for both IPv4 and IPv6 networks. Ie. finding subnets, supernets,
determining if an address is contained in a given network, etc. Similarly,
both addresses and networks (of the same ip version!) have much in common;
the process for turning a given 32 or 128 bit number into a human readable
string notation, determining if the ip is within the valid specified range,
etc. Finally, there are some pythonic abstractions which are valid for all
addresses and networks, both IPv4 and IPv6. In short, there is common
functionality shared between (ipaddr class names in parentheses):
1. all IP addresses and networks, both IPv4 and IPv6. (IPAddrBase)
2. all IP addresses of both versions. (BaseIP)
3. all IP networks of both version. (BaseNet)
4. all IPv4 objects, both addresses and networks. (BaseV4)
5. all IPv6 objects, both addresses and networks. (BaseV6)
Seeing this as a clear hierarchy is important for recognizing how much
code is common between the four main classes. For this reason, ipaddr uses
class inheritance to abstract out as much common code is possible and
appropriate. This lack of duplication and very clean layout also makes
the job of the developer much easier should they need to debug code (either
theirs or mine).
Knowing that there might be cases where the developer doesn't so much care
as to the types of IP they might be receiving, ipaddr comes with two
important helper functions, IPAddress() and IPNetwork(). These, as you
might guess, return the appropriately typed address or network objects for
the given argument.
Finally, this distinction between IPv4 and IPv6 IP versions means that
comparison operations on them return TypeError for py3k per Ordering
Comparisons [2]. In practice, this shouldn't pose a problem for the
developer who can easily write:
v4 = [x for x in mixed_list if x._version == 4]
v6 = [x for x in mixed_list if x._version == 6]
# perform operations on v4 and v6 here.
return v4_return + v6_return
- Multiple ways of displaying an IP Address.
Not everyone will want to display the same information in the same format;
IP addresses in cisco syntax are represented by network/hostmask, junipers
are (network/IP)/prefixlength and IPTables are (network/IP)/(prefixlength/
netmask). The ipaddr library provides multiple ways to display an address.
In [1]: IPNetwork('1.1.1.1').with_prefixlen
Out[1]: '1.1.1.1/32'
In [1]: IPNetwork('1.1.1.1').with_netmask
Out[1]: '1.1.1.1/255.255.255.255'
In [1]: IPNetwork('1.1.1.1').with_hostmask
Out[1]: '1.1.1.1/0.0.0.0'
the same applies to IPv6
- Lazy evaluation combined with aggressive caching of network elements.
(the following example is for IPv6Network objects but the exact same
properties apply to IPv6Network objects).
As mentioned, an IP network object is defined by a number of properties.
The object
In [1]: IPv4Network('1.1.1.0/24')
has a number of IPv4Address properties
In [1]: o = IPv4Network('1.1.1.0/24')
In [2]: o.network
Out[2]: IPv4Address('1.1.1.0')
In [3]: o.broadcast
Out[3]: IPv4Address('1.1.1.255')
In [4]: o.hostmask
Out[4]: IPv4Address('0.0.0.255')
If we were to compute them all at object creation time, we would incur a
non-negligible performance hit. Since these properties are required to
define the object completely but their values aren't always of interest to
the programmer, their computation should be done only when requested.
However, in order to avoid the performance hit in the case where one
attribute for a particular object is requested repeatedly (and continuously
recomputed), the results of the computation should be cached.
- Address list summarization.
ipaddr supports easy summarization of lists of possibly contiguous
addresses, as this is something network administrators constantly find
themselves doing. This currently works in a number of ways.
1. collapse_address_list([list]):
Given a list of networks, ipaddr will collapse the list into the smallest
possible list of networks that wholey contain the addresses supplied.
In [1]: collapse_address_list([IPNetwork('1.1.0.0/24'),
...: IPNetwork('1.1.1.0/24')])
Out[1]: [IPv4Network('1.1.0.0/23')]
more elaborately:
In [1]: collapse_address_list([IPNetwork(x) for x in
...: IPNetwork('1.1.0.0/23')])
Out[1]: [IPv4Network('1.1.0.0/23')]
2. summarize_address_range(first, last).
Given a start and end address, ipaddr will provide the smallest number of
networks to cover the given range.
In [1]: summarize_address_range(IPv4Address('1.1.1.0'),
...: IPv4Address('2.2.2.0'))
Out[1]:
[IPv4Network('1.1.1.0/24'),
IPv4Network('1.1.2.0/23'),
IPv4Network('1.1.4.0/22'),
IPv4Network('1.1.8.0/21'),
IPv4Network('1.1.16.0/20'),
IPv4Network('1.1.32.0/19'),
IPv4Network('1.1.64.0/18'),
IPv4Network('1.1.128.0/17'),
IPv4Network('1.2.0.0/15'),
IPv4Network('1.4.0.0/14'),
IPv4Network('1.8.0.0/13'),
IPv4Network('1.16.0.0/12'),
IPv4Network('1.32.0.0/11'),
IPv4Network('1.64.0.0/10'),
IPv4Network('1.128.0.0/9'),
IPv4Network('2.0.0.0/15'),
IPv4Network('2.2.0.0/23'),
IPv4Network('2.2.2.0/32')]
- Address Exclusion.
Used somewhat less often, but all the more annoying, is the case where an
programmer would want "all of the addresses in a newtork *except* these".
ipaddr performs this exclusion equally well for IPv4 and IPv6 networks
and collapses the resulting address list.
In [1]: IPNetwork('1.1.0.0/15').address_exclude(IPNetwork('1.1.1.0/24'))
Out[1]:
[IPv4Network('1.0.0.0/16'),
IPv4Network('1.1.0.0/24'),
IPv4Network('1.1.2.0/23'),
IPv4Network('1.1.4.0/22'),
IPv4Network('1.1.8.0/21'),
IPv4Network('1.1.16.0/20'),
IPv4Network('1.1.32.0/19'),
IPv4Network('1.1.64.0/18'),
IPv4Network('1.1.128.0/17')]
In [1]: IPNewtork('::1/96').address_exclude(IPNetwork('::1/112'))
Out[1]:
[IPv6Network('::1:0/112'),
IPv6Network('::2:0/111'),
IPv6Network('::4:0/110'),
IPv6Network('::8:0/109'),
IPv6Network('::10:0/108'),
IPv6Network('::20:0/107'),
IPv6Network('::40:0/106'),
IPv6Network('::80:0/105'),
IPv6Network('::100:0/104'),
IPv6Network('::200:0/103'),
IPv6Network('::400:0/102'),
IPv6Network('::800:0/101'),
IPv6Network('::1000:0/100'),
IPv6Network('::2000:0/99'),
IPv6Network('::4000:0/98'),
IPv6Network('::8000:0/97')]
- IPv6 address compression. (in a pending changelist [3])
By default, IPv6 addresses are compressed internally (see the method
BaseV6._compress_hextets), but ipaddr makes both the compressed and the
exploded representations available.
In [1]: IPNetwork('::1').compressed
Out[1]: '::1/128'
In [2]: IPNetwork('::1').exploded
Out[2]: '0000:0000:0000:0000:0000:0000:0000:1/128'
In [3]: IPv6Address('::1').exploded
Out[3]: '0000:0000:0000:0000:0000:0000:0000:0001'
In [4]: IPv6Address('::1').compressed
Out[4]: '::1'
(the same methods exist for IPv4 networks and addresses, but they're
just stubs for returning the normal __str__ representation).
Reference Implementation:
A reference implementation is available at:
http://ipaddr-py.googlecode.com/svn/branches/2.0.x
References:
[1] http://bugs.python.org/issue3959
[2] http://docs.python.org/dev/3.0/whatsnew/3.0.html#ordering-comparisons
[3] http://codereview.appspot.com/110044
Copyright:
This document has been placed in the public domain.
Local Variables:
mode: indented-text
indent-tabs-mode: nil
sentence-end-double-space: t
fill-column: 70
coding: utf-8
End:
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