ip_network & the in operator

Is it on my network?

Lesson 6 of 20 · 9 min · Module 2: Python’s ipaddress toolkit

Routers, firewalls, and VPNs constantly answer one question: which network does this address belong to?

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By the end, you’ll be able to…

Create network objects
Test whether an address belongs to a network
Understand first-match order

The idea

ipaddress.ip_network("10.1.0.0/16") creates a network object. The in operator tests membership: ip_address("10.1.2.3") in ip_network("10.1.0.0/16") is True. By default, the network text must have no host bits set (10.1.0.0/16, not 10.1.2.3/16).

import ipaddress
net = ipaddress.ip_network("192.168.1.0/24")
print(ipaddress.ip_address("192.168.1.42") in net)

01Networks are ranges

ip_network("192.168.1.0/24") represents 256 addresses, from 192.168.1.0 to 192.168.1.255. The in operator checks whether an address falls inside that range.

02Overlapping networks

10.1.0.0/16 is inside 10.0.0.0/8. A router prefers the most specific route, but a simple list check returns whichever matching entry comes first, so order matters.

import ipaddress
print(ipaddress.ip_network("10.1.0.0/16").subnet_of(ipaddress.ip_network("10.0.0.0/8")))

Output

True

03Mixing versions is safe

An IPv4 address is never inside an IPv6 network. The in operator simply returns False, so one list can mix both.

Your turn

Write find_network(address, networks). networks is a list of CIDR strings. Return the first network string that contains the address, or None if none does.

What the checks look for

Finds the matching network
Returns the first match when networks overlap
Returns None when nothing matches
Works with IPv6

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Take this with you

Convert once, then test membership with in.

Go a little further

Return the most specific matching network (the one with the longest prefix) instead of the first.