What is subnetting in networking?

Subnetting is the process of dividing a large IP network into smaller, more manageable sub-networks (subnets). Each subnet has its own network address, broadcast address, and range of usable host IPs. Subnetting improves network security, reduces broadcast traffic, and makes IP address management more efficient.

What is the magic number method for subnetting?

The magic number method is a shortcut for finding subnet boundaries without binary math. The magic number is 256 minus the interesting octet of the subnet mask. For a /24 (mask 255.255.255.0), the magic number in the fourth octet is 256 - 0 = 256 (i.e., subnets increment by 256, so only one /24 per third-octet value). For a /26 (mask 255.255.255.192), the magic number is 256 - 192 = 64, so subnets start at .0, .64, .128, .192.

What is VLSM in subnetting?

VLSM (Variable Length Subnet Masking) allows different subnets within the same network to have different prefix lengths. Instead of dividing a network into equal-sized subnets, VLSM lets you match subnet size to actual host requirements — using /28 for a 10-host segment and /22 for a 1,000-host segment from the same parent block.

How to Subnet IPv4

Last reviewed: May 2026

Subnetting is the skill of dividing a large IP address block into smaller networks. It's tested on every networking certification (CCNA, JNCIA, CompTIA Network+) and used daily by network engineers and cloud architects. This guide teaches you two methods: binary subnetting (exact, great for understanding) and the magic number method (fast, great for exams and real work).

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Prerequisites: IPv4 Addressing Basics

An IPv4 address is 32 bits written as four decimal octets, e.g. 192.168.10.5. Every address has two parts:

Network portion — identifies which network the address belongs to
Host portion — identifies a specific device within that network

The subnet mask (or CIDR prefix) divides the address into these two parts. All addresses with the same network portion are in the same subnet.

Method 1: The Magic Number Method (Fast)

The magic number method lets you find subnet ranges without binary math. It works in the "interesting octet" — the octet where the network-to-host boundary falls.

1 Find the interesting octet
Look at the subnet mask. The interesting octet is the one that is neither 255 (all network bits) nor 0 (all host bits).
Example: mask 255.255.255.192 — the fourth octet (192) is interesting.

2 Calculate the magic number
Magic number = 256 − interesting octet
Example: 256 − 192 = 64. Subnets increment by 64 in the fourth octet.

3 List the subnet boundaries
Starting from 0, add the magic number repeatedly: 0, 64, 128, 192, 256 (stop when you exceed 255).
These are your network addresses: .0, .64, .128, .192.

4 Find host range and broadcast for each subnet
Network address + 1 = first host. Next network address − 1 = broadcast.
Example for .0/26: hosts .1–.62, broadcast .63.

Worked Example: Divide 192.168.1.0/24 into /26 subnets

Mask: 255.255.255.192. Interesting octet: 4th. Magic number: 256 − 192 = 64.

Subnet	Network	First Host	Last Host	Broadcast	Usable
#1	192.168.1.0	192.168.1.1	192.168.1.62	192.168.1.63	62
#2	192.168.1.64	192.168.1.65	192.168.1.126	192.168.1.127	62
#3	192.168.1.128	192.168.1.129	192.168.1.190	192.168.1.191	62
#4	192.168.1.192	192.168.1.193	192.168.1.254	192.168.1.255	62

Method 2: Binary Subnetting (Exact)

Binary subnetting converts everything to bits. It's slower but builds deep understanding of how subnetting actually works.

Worked Example: Find the subnet for host 10.0.52.130/20

1 Write the IP address in binary

10.0.52.130 → 00001010.00000000.00110100.10000010

2 Write the subnet mask in binary (/20 = 20 ones, 12 zeros)

11111111.11111111.11110000.00000000

3 AND the IP with the mask to get the network address

IP: 00001010.00000000.00110100.10000010 Mask: 11111111.11111111.11110000.00000000 AND: 00001010.00000000.00110000.00000000 = 10.0.48.0

Network address: 10.0.48.0

4 Find the broadcast (flip all host bits to 1)

00001010.00000000.00111111.11111111 = 10.0.63.255

Broadcast: 10.0.63.255. Host range: 10.0.48.1 – 10.0.63.254.

The Subnetting Formula

These three formulas cover every subnetting question:

What to find	Formula	Example (/26)
Number of subnets from a /N split into /M	2^(M−N)	/24 → /26: 2² = 4 subnets
Total host addresses per subnet	2^(32−M)	/26: 2⁶ = 64 addresses
Usable host addresses per subnet	2^(32−M) − 2	/26: 64 − 2 = 62 hosts

VLSM — Variable Length Subnet Masking

VLSM lets you assign different-sized subnets to different segments, matching subnet size to actual need. This is how real networks are designed.

VLSM Rule: Always subnet largest-first

Allocate the biggest subnet first. Starting with the largest block prevents fragmentation and ensures each subnet stays within the parent block.

VLSM Example: Divide 10.1.0.0/24 for these requirements

Floor 1 LAN: 100 hosts
Floor 2 LAN: 50 hosts
Server VLAN: 20 hosts
Management: 6 hosts
WAN link: 2 hosts

Step 1 — Find minimum subnet sizes (need 2ⁿ − 2 ≥ required hosts):

Segment	Hosts Needed	Subnet Size	Usable Hosts
Floor 1 LAN	100	/25	126
Floor 2 LAN	50	/26	62
Server VLAN	20	/27	30
Management	6	/29	6
WAN link	2	/30	2

Step 2 — Assign largest-first from 10.1.0.0/24:

Segment	Assigned CIDR	Range	Used / Avail
Floor 1 LAN	10.1.0.0/25	10.1.0.1 – 10.1.0.126	100 / 126
Floor 2 LAN	10.1.0.128/26	10.1.0.129 – 10.1.0.190	50 / 62
Server VLAN	10.1.0.192/27	10.1.0.193 – 10.1.0.222	20 / 30
Management	10.1.0.224/29	10.1.0.225 – 10.1.0.230	6 / 6
WAN link	10.1.0.232/30	10.1.0.233 – 10.1.0.234	2 / 2

Total used: 10.1.0.0–10.1.0.235. Remaining: 10.1.0.236–10.1.0.255 (20 addresses for future growth).

Common Subnetting Mistakes

Starting a subnet on the wrong boundary — subnets must start on a multiple of their block size. A /26 must start at .0, .64, .128, or .192 — not .10.
Forgetting the −2 for reserved addresses — always subtract the network address and broadcast address when counting usable hosts.
Overlapping subnets — check that each subnet's range ends before the next one begins. Use a tool to verify.
Exceeding the parent block — subnets must fit within the parent block's boundary. 10.0.12.0/22 goes up to 10.0.15.255, so it's fine inside 10.0.0.0/16 but not inside 10.0.8.0/22.
Cloud reserved IPs — AWS and Azure reserve 5 IPs, GCP reserves 4. Remember to subtract these from usable counts in cloud environments.

Quick Reference: Subnet Sizes and Block Increments

CIDR	Mask (last 2 octets)	Magic Number (4th octet)	Block Size	Usable Hosts
/24	.255.0	256	256	254
/25	.255.128	128	128	126
/26	.255.192	64	64	62
/27	.255.224	32	32	30
/28	.255.240	16	16	14
/29	.255.248	8	8	6
/30	.255.252	4	4	2

Practice subnetting with SubnetSolver →

How to Subnet IPv4

Prerequisites: IPv4 Addressing Basics

Method 1: The Magic Number Method (Fast)

Worked Example: Divide 192.168.1.0/24 into /26 subnets

Method 2: Binary Subnetting (Exact)

Worked Example: Find the subnet for host 10.0.52.130/20

The Subnetting Formula

VLSM — Variable Length Subnet Masking

VLSM Example: Divide 10.1.0.0/24 for these requirements

Common Subnetting Mistakes

Quick Reference: Subnet Sizes and Block Increments

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