/24 What is a Subnet?

What is a subnet?

A subnet (subnetwork) is a logical division of an IP network into smaller, isolated segments. Subnetting lets you organise devices into groups, control which devices can communicate directly with each other, and use IP address space efficiently.

Every subnet has three key addresses:

• Network address — the first address in the range, identifies the subnet itself. Not assignable to a device.
• Broadcast address — the last address, used to send data to all devices in the subnet simultaneously.
• Usable host addresses — everything in between. These can be assigned to devices.

Subnet masks

A subnet mask separates the network portion of an IP address from the host portion. It's written as four octets (like an IP address) where all network bits are 1 and all host bits are 0:

255.255.255.0  →  11111111.11111111.11111111.00000000
                   ↑ 24 network bits                ↑ 8 host bits

CIDR notation

CIDR (Classless Inter-Domain Routing) notation is a compact way to write a subnet. The prefix length after the slash tells you how many bits belong to the network:

192.168.1.0/24   ← network 192.168.1.x, 256 addresses
10.0.0.0/8       ← network 10.x.x.x, 16,777,216 addresses
172.16.0.0/16    ← network 172.16.x.x, 65,536 addresses

Common subnet sizes

Usable hosts = total addresses − 2 (network address + broadcast address), except /31 and /32 which are special cases.

Worked example

Given: 192.168.10.0/24

Use the subnet calculator to compute these values for any IP and prefix length.