IP Subnetting: CIDR Notation, Network Masks, and Subnet Math

IPv4 Address Structure

An IPv4 address is 32 bits, written as four octets in decimal:

192.168.1.100
↓ in binary:
11000000.10101000.00000001.01100100

Every IP address has two parts: the network portion (identifies the subnet) and the host portion (identifies the device within the subnet). The subnet mask determines where the split happens.

CIDR Notation

CIDR (Classless Inter-Domain Routing) expresses the network prefix length as a number after a slash:

192.168.1.0/24
           ↑ 24 bits are the network portion
           → subnet mask: 255.255.255.0 (24 ones followed by 8 zeros)

Prefix /8   → mask 255.0.0.0       → 16,777,214 hosts
Prefix /16  → mask 255.255.0.0     → 65,534 hosts
Prefix /24  → mask 255.255.255.0   → 254 hosts
Prefix /28  → mask 255.255.255.240 → 14 hosts
Prefix /32  → mask 255.255.255.255 → 1 host (single IP)

Calculating Subnet Values

Given 192.168.10.0/26:

Network bits: 26
Host bits: 32 - 26 = 6
Total addresses: 2^6 = 64
Usable hosts: 64 - 2 = 62  (subtract network + broadcast)

Subnet mask: 255.255.255.192
  (.192 = 11000000 = 128+64 = first 2 bits of last octet)

Network address: 192.168.10.0   (all host bits = 0)
Broadcast address: 192.168.10.63  (all host bits = 1)
First usable: 192.168.10.1
Last usable: 192.168.10.62

Subnetting a /24 into Smaller Blocks

// Divide 192.168.1.0/24 into four /26 subnets:
192.168.1.0/26    hosts: .1 – .62
192.168.1.64/26   hosts: .65 – .126
192.168.1.128/26  hosts: .129 – .190
192.168.1.192/26  hosts: .193 – .254

Private IP Ranges (RFC 1918)

10.0.0.0/8       →  10.0.0.0 – 10.255.255.255  (16M addresses)
172.16.0.0/12    →  172.16.0.0 – 172.31.255.255 (1M addresses)
192.168.0.0/16   →  192.168.0.0 – 192.168.255.255 (65K addresses)

// Reserved for loopback
127.0.0.0/8      → 127.x.x.x (loopback — 127.0.0.1)

// Reserved for link-local (APIPA)
169.254.0.0/16   → auto-assigned when DHCP fails

AWS VPC CIDR Planning

// VPC CIDR: 10.0.0.0/16 (65,534 addresses)
// Divide into AZ subnets:

Public subnets (one per AZ):
  10.0.1.0/24  → us-east-1a  (254 hosts)
  10.0.2.0/24  → us-east-1b
  10.0.3.0/24  → us-east-1c

Private subnets:
  10.0.10.0/24 → us-east-1a  (254 hosts)
  10.0.11.0/24 → us-east-1b
  10.0.12.0/24 → us-east-1c

Database subnets (smaller, isolated):
  10.0.20.0/28 → us-east-1a  (14 hosts)
  10.0.20.16/28 → us-east-1b

Docker and Kubernetes Networks

# Docker default bridge: 172.17.0.0/16
# Docker Compose default: 172.x.0.0/20 (auto-assigned)
# Custom network:
networks:
  app-net:
    ipam:
      config:
        - subnet: 10.5.0.0/24

# Kubernetes pod CIDR: typically 10.244.0.0/16 (Flannel)
# Kubernetes service CIDR: typically 10.96.0.0/12

Calculate Subnets Instantly

Use ToolsVito's IP Subnet Calculator to compute network address, broadcast, host range, and CIDR breakdown for any IP/mask combination.