subnets(new_prefix) & num_addresses

Split a network into subnets

Lesson 7 of 20 · 10 min · Module 2: Python’s ipaddress toolkit

Network engineers rarely use one big network. They split it into smaller subnets: one per floor, team, or service.

Free. No installation: you write and run the code in your browser, and it is checked automatically.

By the end, you’ll be able to…

Divide a network into equal subnets
Read subnet boundaries
Count addresses in a network

The idea

network.subnets(new_prefix=26) yields the smaller networks of the new size, in order. network.num_addresses is the total number of addresses. str(subnet) gives text such as "192.168.10.64/26".

import ipaddress
net = ipaddress.ip_network("192.168.10.0/24")
print([str(n) for n in net.subnets(new_prefix=25)])

01Each extra prefix bit halves the size

A /24 has 256 addresses. Two /25 networks have 128 each; four /26 networks have 64 each. Every subnet starts where the previous one ended.

import ipaddress
print(ipaddress.ip_network("192.168.10.0/26").num_addresses)

Output

02subnets() is a generator

It produces subnets one at a time instead of building a list immediately. Wrapping it in a list comprehension collects all of them, which is fine for a handful of subnets.

03Boundaries follow powers of two

/26 networks in a /24 always start at .0, .64, .128, and .192. Recognising these steps makes routing tables and firewall rules much easier to read.

Your turn

Write split_network(cidr, new_prefix) that returns the list of subnet strings. Also write usable_hosts(cidr) that returns the number of usable host addresses for IPv4 networks with a prefix up to /30.

What the checks look for

Splits a /24 into four /26 networks
Splits a /16 into /18 networks
Counts usable hosts

Try it in the editor →

Take this with you

Splitting a network is repeated halving; every subnet has the same size.

Go a little further

Return each subnet together with its usable host count as a tuple.