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Network Engineering 9 min read Updated 2026-10-15

Understanding IPv4 Addressing and Subnetting for Network Engineers

Master CIDR notation, block sizes, usable host ranges, and Variable Length Subnet Masking (VLSM) using the fast binary-boundary method taught in our Harare networking labs.

SM

By S. Madondo

Senior CCNA, HCIA & Network Security Instructor at Majsystems

Why Subnetting Is Essential in Real Enterprise Networks

An unsegmented flat network places every workstation, printer, IP camera, and server into a single broadcast domain. As traffic grows, ARP broadcasts degrade performance and a single compromised host can scan the entire LAN.

Subnetting divides an IPv4 network block into smaller, logically isolated broadcast domains—such as separating Finance, Engineering, Voice (VoIP), Guest Wi-Fi, and Point-to-Point router links—while conserving IP address space.

The Block-Size Method for Fast CIDR Calculation

Every IPv4 address consists of 32 bits divided into four 8-bit octets. Within the interesting octet, the bit values from left to right are 128, 64, 32, 16, 8, 4, 2, and 1.

  • /24 (255.255.255.0): Block size 256 in the 3rd octet -> 254 usable hosts per subnet.
  • /25 (255.255.255.128): Block size 128 in the 4th octet -> 126 usable hosts per subnet.
  • /26 (255.255.255.192): Block size 64 -> 62 usable hosts per subnet (ideal for medium office VLANs).
  • /27 (255.255.255.224): Block size 32 -> 30 usable hosts per subnet.
  • /30 (255.255.255.252): Block size 4 -> 2 usable hosts per subnet (standard for point-to-point WAN router links).
Worked Example — Splitting 192.168.10.0/24 into /26 Subnets
Subnet 1: 192.168.10.0/26   | Usable: 192.168.10.1   - 192.168.10.62  | Broadcast: 192.168.10.63
Subnet 2: 192.168.10.64/26  | Usable: 192.168.10.65  - 192.168.10.126 | Broadcast: 192.168.10.127
Subnet 3: 192.168.10.128/26 | Usable: 192.168.10.129 - 192.168.10.190 | Broadcast: 192.168.10.191
Subnet 4: 192.168.10.192/26 | Usable: 192.168.10.193 - 192.168.10.254 | Broadcast: 192.168.10.255
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