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Subnetting

Subnetting is the process of dividing a larger IP network into smaller subnetworks.

It is used to organize networks, control traffic, improve address management, and separate systems by purpose, department, location, or security zone.

Why Subnetting Matters

Without subnetting, every device could end up in one large flat network.

That can create problems:

  • Too much broadcast traffic
  • Poor organization
  • Harder troubleshooting
  • Weak separation between departments
  • Inefficient IP address usage
  • Larger impact if one segment is compromised

Subnetting helps create smaller, more manageable network segments.

Examples:

Accounting subnet
HR subnet
Server subnet
Guest Wi-Fi subnet
Security camera subnet
Management subnet

Core Concepts

Every IPv4 subnet has important address types.

Address Type Purpose Example
Network address Identifies the subnet itself 192.168.1.0
Usable host addresses Assigned to devices 192.168.1.10
Default gateway Routes traffic to other networks 192.168.1.1
Broadcast address Sends traffic to all hosts in the subnet 192.168.1.255

Example subnet:

Network:        192.168.1.0/24
Usable hosts:   192.168.1.1 - 192.168.1.254
Broadcast:      192.168.1.255

Subnet Mask

A subnet mask defines which part of an IP address is the network portion and which part is the host portion.

Example:

IP address:     192.168.1.25
Subnet mask:    255.255.255.0
CIDR:           /24

In this example:

Network portion: 192.168.1
Host portion:    25

So 192.168.1.25/24 belongs to the network:

192.168.1.0/24

CIDR Notation

CIDR notation shows how many bits are used for the network portion.

192.168.1.0/24

The /24 means the first 24 bits are the network portion.

Common CIDR examples:

CIDR Subnet Mask Usable Hosts
/24 255.255.255.0 254
/25 255.255.255.128 126
/26 255.255.255.192 62
/27 255.255.255.224 30
/28 255.255.255.240 14
/29 255.255.255.248 6
/30 255.255.255.252 2

Note

The number of usable hosts is usually total addresses minus 2: one for the network address and one for the broadcast address.

Host Calculation

IPv4 addresses are 32 bits.

Formula:

Total addresses = 2^(host bits)
Usable hosts = 2^(host bits) - 2

Example:

/24 means 24 network bits
32 - 24 = 8 host bits
2^8 = 256 total addresses
256 - 2 = 254 usable hosts

Example: /24 Network

Network:        192.168.10.0/24
Subnet mask:    255.255.255.0
Broadcast:      192.168.10.255
Usable range:   192.168.10.1 - 192.168.10.254

This is common in home and small office networks.

Example: /26 Network

A /26 subnet has 64 total addresses and 62 usable hosts.

Network:        192.168.10.0/26
Subnet mask:    255.255.255.192
Broadcast:      192.168.10.63
Usable range:   192.168.10.1 - 192.168.10.62

The next /26 subnet starts at:

192.168.10.64/26

Subnet Increments

The subnet increment tells you where each subnet starts.

For a /26:

Subnet mask: 255.255.255.192
Block size:  256 - 192 = 64

So the subnets are:

192.168.10.0/26
192.168.10.64/26
192.168.10.128/26
192.168.10.192/26

Common Subnet Sizes

CIDR Total Addresses Usable Hosts Common Use
/24 256 254 Small LAN
/25 128 126 Split a /24 in half
/26 64 62 Department subnet
/27 32 30 Small team or lab segment
/28 16 14 Small service segment
/29 8 6 Network devices
/30 4 2 Point-to-point links

Default Gateway

The default gateway is the router interface used to reach other networks.

Example:

Host IP:         192.168.1.50
Subnet mask:     255.255.255.0
Default gateway: 192.168.1.1

If the host needs to reach another subnet or the internet, it sends traffic to the default gateway.

Host -> Default Gateway -> Other Network

Security Benefits

Subnetting can improve security by separating systems.

Example:

Subnet Purpose
192.168.10.0/24 User devices
192.168.20.0/24 Servers
192.168.30.0/24 Guest Wi-Fi
192.168.40.0/24 Cameras / IoT
192.168.50.0/24 Management

Benefits:

  • Guest devices can be isolated from internal systems.
  • Servers can be protected with firewall rules.
  • IoT devices can be restricted.
  • Sensitive systems can be placed in controlled segments.
  • Monitoring can be organized by subnet.

Important

Subnetting alone does not enforce security. You need routing rules, firewall rules, ACLs, VLANs, or other controls to restrict traffic between subnets.

Subnetting and VLANs

Subnets and VLANs are often used together.

Concept Layer Purpose
VLAN Layer 2 Separates broadcast domains
Subnet Layer 3 Defines IP network boundaries

Common design:

VLAN 10 -> 192.168.10.0/24 -> Users
VLAN 20 -> 192.168.20.0/24 -> Servers
VLAN 30 -> 192.168.30.0/24 -> Guest Wi-Fi

Traffic between VLANs usually requires routing.

Quick Method

When subnetting, ask:

  1. What network am I starting with?
  2. How many subnets do I need?
  3. How many hosts are needed per subnet?
  4. What CIDR size supports that number of hosts?
  5. What are the network, usable, and broadcast addresses?

Quick Reference

Need Suggested CIDR
Around 250 hosts /24
Around 125 hosts /25
Around 60 hosts /26
Around 30 hosts /27
Around 14 hosts /28
Around 6 hosts /29
2 hosts /30

Notes to Remember

  • Subnetting divides larger networks into smaller networks.
  • The subnet mask separates the network portion from the host portion.
  • CIDR notation shows how many bits belong to the network.
  • The network address identifies the subnet.
  • The broadcast address reaches all hosts in the subnet.
  • The default gateway routes traffic to other networks.
  • Smaller subnets reduce broadcast scope and improve organization.
  • Subnetting helps security, but firewall rules enforce security.