Subnet Calculator Guide — Understanding CIDR and IP Subnetting

IP subnetting is one of those foundational networking concepts that every DevOps engineer, network admin, and cloud architect needs to understand. Whether you are designing VPCs in AWS, configuring firewall rules, or splitting a network for different departments, subnetting is essential.

What is a Subnet?

A subnet (subnetwork) is a logical division of an IP network. Subnetting lets you split a large network into smaller, more manageable segments — improving security, performance, and organisation.

Understanding CIDR Notation

CIDR (Classless Inter-Domain Routing) notation represents an IP address and its network mask in one compact form: 192.168.1.0/24

192.168.1.0  → the network address
/24          → 24 bits are the network part
             → 8 bits remain for host addresses
             → 2^8 = 256 total addresses
             → 254 usable hosts (minus network + broadcast)

Common Subnet Sizes

CIDR   Subnet Mask       Total IPs   Usable Hosts
/8     255.0.0.0         16,777,216  16,777,214
/16    255.255.0.0       65,536      65,534
/24    255.255.255.0     256         254         ← most common
/25    255.255.255.128   128         126
/26    255.255.255.192   64          62
/27    255.255.255.224   32          30
/28    255.255.255.240   16          14
/29    255.255.255.248   8           6
/30    255.255.255.252   4           2           ← point-to-point
/32    255.255.255.255   1           0           ← single host

Key Subnet Addresses

For the network 192.168.1.0/24:

Network address:   192.168.1.0    ← identifies the subnet
First usable host: 192.168.1.1
Last usable host:  192.168.1.254
Broadcast address: 192.168.1.255  ← sends to all hosts on subnet
Subnet mask:       255.255.255.0
Wildcard mask:     0.0.0.255       ← inverse of subnet mask

AWS VPC Subnetting Example

A typical AWS VPC setup:

VPC CIDR:           10.0.0.0/16  (65,536 IPs)

Public subnet A:    10.0.1.0/24  (254 usable)
Public subnet B:    10.0.2.0/24  (254 usable)
Private subnet A:   10.0.10.0/24 (254 usable)
Private subnet B:   10.0.11.0/24 (254 usable)
Database subnet:    10.0.20.0/24 (254 usable)