Subnetting·Lesson 1 of 10
Subnetting
What Is Subnetting?
Subnetting means taking one big network and splitting it into several smaller networks, called subnets (short for 'sub-networks'). Instead of having every device sit in one enormous pool, you carve that pool into tidy, separate groups. Each subnet behaves like its own little network, even though they all started from the same larger block of addresses.
The one-line takeaway
Subnetting divides one network into smaller subnets by borrowing part of the address. It makes networks tidier, safer, and more efficient — without needing to buy more addresses.
One big office vs floors
Imagine a company crammed into one huge open room — noisy, chaotic, and hard to manage. Now imagine splitting that same space into floors, one per department. Nothing new was built; the space was just divided sensibly. Subnetting does the same to a network: it partitions one address space into organised groups.
First, a Refresher: What Is an IP Address?
To understand subnetting you first need to picture an IP address. Every device on a network — a laptop, phone, or printer — gets an (Internet Protocol address), which is its unique number on the network, a bit like a postal address for data. The most common form is (Internet Protocol version 4), which is a 32-bit number. A 'bit' is a single 0 or 1, so an IPv4 address is 32 zeros and ones.
Because 32 zeros and ones are hard to read, we split them into four groups of 8 bits. Each 8-bit group is called an , and we write it as a normal decimal number from 0 to 255, separating the four with dots. That gives the familiar dotted form like 192.168.1.10. Here is the same address in both decimal and its underlying binary:
Decimal: 192 . 168 . 1 . 10
Binary: 11000000 . 10101000 . 00000001 . 00001010
(8 bits) (8 bits) (8 bits) (8 bits) = 32 bits total
Every Address Has Two Parts
Here's the key idea that makes subnetting possible: an IP address is really two pieces joined together. The first part identifies the network, and the second part identifies the specific device (the ) on that network. Devices on the same network share the same network part and differ only in the host part.
Network Part
Identifies which network the address belongs to. Like a street name — shared by everyone living on that street.
Host Part
Identifies the individual device on that network. Like a house number — unique to each home on the street.
So How Does Subnetting Work?
Subnetting works by borrowing some bits from the host part and treating them as extra network bits. Each new combination of those borrowed bits becomes a separate subnet. If you borrow enough bits to make, say, four patterns, you get four subnets — each a smaller slice of the original network. The tool that marks where the network part ends and the host part begins is the , which is the very next topic.
Don't worry if the bit-borrowing feels abstract right now — the later lessons show it step by step with real numbers. For now, just hold on to the picture: one network, sliced into smaller subnets by moving the line between the network part and the host part.
“"Subnetting is only for huge companies with thousands of computers."”
Q:In plain terms, what is subnetting?
A: Subnetting is dividing one larger network into several smaller networks called subnets. It works by borrowing bits from the host part of the IP address to create extra network groups. The result is a network that's easier to manage, more secure, and more efficient with its addresses.
Quick Revision Cheat Sheet
Subnetting: Splitting one network into smaller subnets
IPv4 address: A 32-bit number, shown as four octets (0-255)
Octet: One group of 8 bits
Two parts: Network part + host part
How: Borrow bits from the host part to make subnets