IPRangeExplorer
How-To 6 min read

Subnetting for Beginners: From /24 to /28 in 10 Minutes

Subnetting divides a larger network into smaller, more manageable segments. This step-by-step tutorial walks through the maths using a /24 as the starting point.

subnetting beginners tutorial cidr networking

Subnetting is the process of dividing a single IP network into multiple smaller networks — subnets. If you have been handed a /24 block and need to carve it into smaller pieces for different departments, server tiers, or security zones, this tutorial walks through the mechanics step by step. When you are done, verify your work with the CIDR calculator.

Starting Point: The /24

Let us start with 192.168.10.0/24. This block contains 256 total IPs — 254 usable hosts. The network address is 192.168.10.0 and the broadcast address is 192.168.10.255. See the /24 subnet reference for the full breakdown.

Splitting into Two /25s

Each time you increase the prefix length by 1, you halve the address space and double the number of subnets. A /24 splits into two /25 subnets:

  • 192.168.10.0/25 — addresses .0 to .127 (126 usable hosts)
  • 192.168.10.128/25 — addresses .128 to .255 (126 usable hosts)

The boundary between the two subnets is at .128 — exactly halfway through the /24.

Splitting into Four /26s

Increase the prefix by one more to get four /26 subnets (62 usable hosts each):

  • 192.168.10.0/26 — .0 to .63
  • 192.168.10.64/26 — .64 to .127
  • 192.168.10.128/26 — .128 to .191
  • 192.168.10.192/26 — .192 to .255

Each boundary is a multiple of 64 — the block size for a /26.

Splitting into Sixteen /28s

A /28 has 16 total addresses (14 usable). A /24 contains sixteen /28 subnets. Each starts at a multiple of 16:

  • 192.168.10.0/28 — .0 to .15
  • 192.168.10.16/28 — .16 to .31
  • 192.168.10.32/28 — .32 to .47
  • ... continuing in steps of 16 ...
  • 192.168.10.240/28 — .240 to .255

The rule: the block size is always 2(32-prefix). For a /28: 24 = 16. Each subnet starts at a multiple of 16.

The Golden Rule of Subnetting

A subnet's network address must be divisible by its block size. 192.168.10.16/28 is valid because 16 is divisible by 16. 192.168.10.18/28 is not a valid network address — 18 is not divisible by 16. The CIDR calculator automatically corrects to the nearest valid network address when you enter a host address with a prefix.

Practical Application

Following our subnetting guide will deepen your understanding of the underlying maths. For day-to-day work, use the CIDR calculator to verify boundaries quickly. Subnetting by hand is a skill worth having — but not a task worth doing repeatedly in production.

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