# Subnetting basics

> How to divide one network into smaller subnets, and why borrowing host bits is the whole trick.

Source: https://ronutz.com/en/learn/subnetting-basics  
Updated: 2026-06-23  
Related tools: https://ronutz.com/en/tools/cidr

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## What subnetting is for

A single large network is rarely what you want. Subnetting is the practice of taking one block of addresses and splitting it into several smaller blocks, **subnets**, each of which can be a separate broadcast domain, security boundary, or physical segment. A campus might give each building its own subnet; a cloud account might give each tier of an application its own.

The mechanism is simpler than it is usually made to sound: **subnetting borrows bits from the host portion and adds them to the network portion.**

## Borrowing bits

Start with `192.168.1.0/24`. The prefix is 24, so 8 bits are free for hosts, giving one block of 256 addresses.

Suppose you need four separate subnets instead of one. Four is 2², so you need **2 more network bits**. Borrow them from the host side: the prefix grows from /24 to /26.

- `/26` fixes 26 bits, leaves 6 free → 64 addresses per subnet.
- Four such subnets tile the original /24 exactly:
  - `192.168.1.0/26`, addresses .0 to .63
  - `192.168.1.64/26`, addresses .64 to .127
  - `192.168.1.128/26`, addresses .128 to .191
  - `192.168.1.192/26`, addresses .192 to .255

Notice the pattern: each subnet is 64 addresses wide, so each one starts on a multiple of 64. That "starts on a multiple of its size" rule is how valid subnet boundaries are found. A block must begin on an address that is a multiple of its own size, or it is not a valid CIDR (Classless Inter-Domain Routing) block.

## The trade-off

Every bit you borrow for the network halves the hosts per subnet:

| Prefix | Subnets of a /24 | Addresses each | Usable hosts each |
|---|---|---|---|
| /24 | 1 | 256 | 254 |
| /25 | 2 | 128 | 126 |
| /26 | 4 | 64 | 62 |
| /27 | 8 | 32 | 30 |
| /28 | 16 | 16 | 14 |

More subnets means fewer hosts in each. Network design is largely choosing where on this curve to sit: enough subnets for your segments, enough hosts in each for the devices that live there, with some room to grow.

## Variable-length subnetting

Subnets within the same parent do not all have to be the same size. **Variable-length subnet masking (VLSM)** lets you carve a large subnet for the segment that needs many hosts and small subnets for point-to-point links, all from the same parent block. The only rules are the constant ones: each subnet is a power of two in size, and each begins on a multiple of its size so the blocks never overlap.

The [CIDR calculator](https://ronutz.com/en/learn) shows the exact range, boundaries, and usable host count for any prefix, which makes checking a subnet plan a matter of typing in each block.