Learn IP Address Concepts With Python's ipaddress Module

Mechanics of IP Addresses

You saw above that an IP address boils down to an integer. A fuller definition is that an IPv4 address is a 32-bit integer used to represent a host on a network. The term host is sometimes used synonymously with an address.

It follows that there are 2³² possible IPv4 addresses, from 0 to 4,294,967,295 (where the upper bound is 2³² - 1). But this is a tutorial for human beings, not robots. No one wants to ping the IP address 0xdc0e0925.

The more common way to express an IPv4 address is using quad-dotted notation, which consists of four dot-separated decimal integers:

220.14.9.37

It’s not immediately obvious what underlying integer the address 220.14.9.37 represents, though. Formulaically, you can break the IP address 220.14.9.37 into its four octet components:

>>> (
...     220 * (256 ** 3) +
...      14 * (256 ** 2) +
...       9 * (256 ** 1) +
...      37 * (256 ** 0)
... )
3691907365

As shown above, the address 220.14.9.37 represents the integer 3,691,907,365. Each octet is a byte, or a number from 0 to 255. Given this, you can infer that the maximum IPv4 address is 255.255.255.255 (or FF.FF.FF.FF in hex notation), while the minimum is 0.0.0.0.

Next, you’ll see how Python’s ipaddress module does this calculation for you, allowing you to work with the human-readable form and let the address arithmetic occur out of sight.

The Python ipaddress Module

To follow along, you can fetch your computer’s external IP address to work with at the command line:

$ curl -sS ifconfig.me/ip
220.14.9.37

This requests your IP address from the site ifconfig.me, which can be used to show an array of details about your connection and network.

Now open up a Python REPL. You can use the IPv4Address class to build a Python object that encapsulates an address:

>>> from ipaddress import IPv4Address

>>> addr = IPv4Address("220.14.9.37")
>>> addr
IPv4Address('220.14.9.37')

Passing a str such as "220.14.9.37" to the IPv4Address constructor is the most common approach. However, the class can also accept other types:

>>> IPv4Address(3691907365)  # From an int
IPv4Address('220.14.9.37')

>>> IPv4Address(b"\xdc\x0e\t%")  # From bytes (packed form)
IPv4Address('220.14.9.37')

While constructing from a human-readable str is probably the more common way, you might see bytes input if you’re working with something like TCP packet data.

The conversions above are possible in the other direction as well:

>>> int(addr)
3691907365
>>> addr.packed
b'\xdc\x0e\t%'

In addition to allowing round-trip input and output to different Python types, instances of IPv4Address are also hashable. This means you can use them as keys in a mapping data type such as a dictionary:

>>> hash(IPv4Address("220.14.9.37"))
4035855712965130587

>>> num_connections = {
...     IPv4Address("220.14.9.37"): 2,
...     IPv4Address("100.201.0.4"): 16,
...     IPv4Address("8.240.12.2"): 4,
... }

On top of that, IPv4Address also implements methods that allow for comparisons using the underlying integer:

>>> IPv4Address("220.14.9.37") > IPv4Address("8.240.12.2")
True

>>> addrs = (
...     IPv4Address("220.14.9.37"),
...     IPv4Address("8.240.12.2"),
...     IPv4Address("100.201.0.4"),
... )
>>> for a in sorted(addrs):
...     print(a)
...
8.240.12.2
100.201.0.4
220.14.9.37

You can use any of the standard comparison operators to compare the integer values of address objects.

As you’ve seen above, the constructor itself for IPv4Address is short and sweet. It’s when you start lumping addresses into groups, or networks, that things become more interesting.

CIDR Notation

A network is defined using a network address plus a prefix in Classless Inter-Domain Routing (CIDR) notation:

>>> from ipaddress import IPv4Network
>>> net = IPv4Network("192.4.2.0/24")
>>> net.num_addresses
256

CIDR notation represents a network as <network_address>/<prefix>. The routing prefix (or prefix length, or just prefix), which is 24 in this case, is the count of leading bits used to answer questions such as whether a certain address is part of a network or how many addresses reside in a network. (Here leading bits refers to the first N bits counting from the left of the integer in binary.)

You can find the routing prefix with the .prefixlen property:

>>> net.prefixlen
24

Let’s jump right into an example. Is the address 192.4.2.12 in the network 192.4.2.0/24? The answer in this case is yes, because the leading 24 bits of 192.4.2.12 are the first three octets (192.4.2). With a /24 prefix, you can simply chop off the last octet and see that the 192.4.2.xxx parts match.

Pictured differently, the /24 prefix translates to a netmask that, as its name implies, is used to mask bits in the addresses being compared:

>>> net.netmask
IPv4Address('255.255.255.0')

You compare leading bits to determine whether an address is part of a network. If the leading bits match, then the address is part of the network:

11000000 00000100 00000010 00001100  # 192.4.2.12  # Host IP address
11000000 00000100 00000010 00000000  # 192.4.2.0   # Network address
                          |
                          ^ 24th bit (stop here!)
|_________________________|
            |
      These bits match

Above, the final 8 bits of 192.4.2.12 are masked (with 0) and are ignored in the comparison. Once again, Python’s ipaddress saves you the mathematical gymnastics and supports idiomatic membership testing:

>>> net = IPv4Network("192.4.2.0/24")

>>> IPv4Address("192.4.2.12") in net
True
>>> IPv4Address("192.4.20.2") in net
False

This is made possible by the treasure that is operator overloading, by which IPv4Network defines __contains__() to allow membership testing using the in operator.

In the CIDR notation 192.4.2.0/24, the 192.4.2.0 part is the network address, which is used to identify the network:

>>> net.network_address
IPv4Address('192.4.2.0')

As you saw above, the network address 192.4.2.0 can be seen as the expected result when a mask is applied to a host IP address:

11000000 00000100 00000010 00001100  # Host IP address
11111111 11111111 11111111 00000000  # Netmask, 255.255.255.0 or /24
11000000 00000100 00000010 00000000  # Result (compared to network address)

When you think about it this way, you can see how the /24 prefix actually translates into a true IPv4Address:

>>> net.prefixlen
24
>>> net.netmask
IPv4Address('255.255.255.0')  # 11111111 11111111 11111111 00000000

In fact, if it strikes your fancy, you can construct an IPv4Network directly from two addresses:

>>> IPv4Network("192.4.2.0/255.255.255.0")
IPv4Network('192.4.2.0/24')

Above, 192.4.2.0 is the network address while 255.255.255.0 is the netmask.

At the other end of the spectrum in a network is its final address, or broadcast address, which is a single address that can be used to communicate to all the hosts on its network:

>>> net.broadcast_address
IPv4Address('192.4.2.255')

There’s one more point worth mentioning about the netmask. You’ll most often see prefix lengths that are multiples of 8:

However, any integer between 0 and 32 is valid, though less common:

>>> net = IPv4Network("100.64.0.0/10")
>>> net.num_addresses
4194304
>>> net.netmask
IPv4Address('255.192.0.0')

In this section, you saw how to construct an IPv4Network instance and test whether a certain IP address sits within it. In the next section, you’ll learn how to loop over the addresses within a network.

Looping Through Networks

The IPv4Network class supports iteration, meaning that you can iterate over its individual addresses in a for loop:

>>> net = IPv4Network("192.4.2.0/28")
>>> for addr in net:
...     print(addr)
...
192.4.2.0
192.4.2.1
192.4.2.2
...
192.4.2.13
192.4.2.14
192.4.2.15

Similarly, net.hosts() returns a generator that will yield the addresses shown above, excluding the network and broadcast addresses:

>>> h = net.hosts()
>>> type(h)
<class 'generator'>
>>> next(h)
IPv4Address('192.4.2.1')
>>> next(h)
IPv4Address('192.4.2.2')

In the next section, you’ll dive in to a concept closely related to networks: the subnet.

Subnets

A subnet is a subdivision of an IP network:

>>> small_net = IPv4Network("192.0.2.0/28")
>>> big_net = IPv4Network("192.0.0.0/16")
>>> small_net.subnet_of(big_net)
True
>>> big_net.supernet_of(small_net)
True

Above, small_net contains only 16 addresses, which is sufficient for you and a few cubicles around you. Conversely, big_net contains 65,536 addresses.

A common way to achieve subnetting is to take a network and increase its prefix length by 1. Let’s take this example from Wikipedia:

This example starts with a /24 network:

net = IPv4Network("200.100.10.0/24")

Subnetting by increasing the prefix length from 24 to 25 involves shifting bits around to break up the network into smaller parts. This is a bit mathematically hairy. Luckily, IPv4Network makes it a cinch because .subnets() returns an iterator over the subnets:

>>> for sn in net.subnets():
...     print(sn)
...
200.100.10.0/25
200.100.10.128/25

You can also tell .subnets() what the new prefix should be. A higher prefix means more and smaller subnets:

>>> for sn in net.subnets(new_prefix=28):
...     print(sn)
...
200.100.10.0/28
200.100.10.16/28
200.100.10.32/28
...
200.100.10.208/28
200.100.10.224/28
200.100.10.240/28

Besides addresses and networks, there’s a third core part of the ipaddress module that you’ll see next.

Host Interfaces

Last but certainly not least, Python’s ipaddress module exports an IPv4Interface class for representing a host interface. A host interface is a way to describe, in a single compact form, both a host IP address and a network that it sits in:

>>> from ipaddress import IPv4Interface

>>> ifc = IPv4Interface("192.168.1.6/24")
>>> ifc.ip  # The host IP address
IPv4Address('192.168.1.6')
>>> ifc.network  # Network in which the host IP resides
IPv4Network('192.168.1.0/24')

Above, 192.168.1.6/24 means “the IP address 192.168.1.6 in the network 192.168.1.0/24.”

Put differently, an IP address alone doesn’t tell you which network(s) that address sits in, and a network address is a group of IP addresses rather than a single one. The IPv4Interface gives you a way of simultaneously expressing, through CIDR notation, a single host IP address and its network.

Composition’s Core Role

The ipaddress module takes advantage of an object-oriented pattern called composition. Its IPv4Address class is a composite that wraps a plain Python integer. IP addresses are fundamentally integers, after all.

Each IPv4Address instance has a quasi-private ._ip attribute that is itself an int. Many of the other properties and methods of the class are driven by the value of this attribute:

>>> addr = IPv4Address("220.14.9.37")
>>> addr._ip
3691907365

The ._ip attribute is actually what’s responsible for producing int(addr). The chain of calls is that int(my_addr) calls my_addr.__int__(), which IPv4Address implements as just my_addr._ip:

If you asked the CPython developers about this, then they might tell you that ._ip is an implementation detail. While nothing is truly private in Python, the leading underscore denotes that ._ip is quasi-private, not part of the public ipaddress API, and subject to change without notice. That’s why it’s more stable to extract the underlying integer with int(addr).

With all that said, though, it’s the underlying ._ip that gives the IPv4Address and IPv4Network classes their magic.

Extending IPv4Address

You can demonstrate the power of the underlying ._ip integer by extending the IPv4 address class:

from ipaddress import IPv4Address

class MyIPv4(IPv4Address):
    def __and__(self, other: IPv4Address):
        if not isinstance(other, (int, IPv4Address)):
            raise NotImplementedError
        return self.__class__(int(self) & int(other))

Adding .__and__() lets you use the binary AND (&) operator. Now you can directly apply a netmask to a host IP:

>>> addr = MyIPv4("100.127.40.32")
>>> mask = MyIPv4("255.192.0.0")  # A /10 prefix

>>> addr & mask
MyIPv4('100.64.0.0')

>>> addr & 0xffc00000  # Hex literal for 255.192.0.0
MyIPv4('100.64.0.0')

Above, .__and__() allows you to use either another IPv4Address or an int directly as the mask. Because MyIPv4 is a subclass of IPv4Address, the isinstance() check will return True in that case.

Besides operator overloading, you could add brand new properties as well:

 1import re
 2from ipaddress import IPv4Address
 3
 4class MyIPv4(IPv4Address):
 5    @property
 6    def binary_repr(self, sep=".") -> str:
 7        """Represent IPv4 as 4 blocks of 8 bits."""
 8        return sep.join(f"{i:08b}" for i in self.packed)
 9
10    @classmethod
11    def from_binary_repr(cls, binary_repr: str):
12        """Construct IPv4 from binary representation."""
13        # Remove anything that's not a 0 or 1
14        i = int(re.sub(r"[^01]", "", binary_repr), 2)
15        return cls(i)

In .binary_repr (line 8), using .packed transforms the IP address into an array of bytes that is then formatted as the string representation of its binary form.

In .from_binary_repr, the call to int(re.sub(r"[^01]", "", binary_repr), 2) on line 14 has two parts:

1. It removes anything besides 0s and 1s from the input string.
2. It parses the result, assuming base 2, with int(<string>, 2).

Using .binary_repr() and .from_binary_repr() allows you to convert to and construct from a str of 1s and 0s in binary notation:

>>> MyIPv4("220.14.9.37").binary_repr
'11011100.00001110.00001001.00100101'
>>> MyIPv4("255.255.0.0").binary_repr  # A /16 netmask
'11111111.11111111.00000000.00000000'

>>> MyIPv4.from_binary_repr("11011100 00001110 00001001 00100101")
MyIPv4('220.14.9.37')

These are just a few ways demonstrating how taking advantage of the IP-as-integer pattern can help you extend the functionality of IPv4Address with a small amount of additional code.