Subnetting Explained

Before you subnet — understand CIDR notation

Every IP address you see in subnetting problems comes with a prefix length written as a slash followed by a number — for example 192.168.1.0/24. This is called CIDR notation (Classless Inter-Domain Routing), and the number after the slash tells you how many bits are used for the network portion of the address.

An IPv4 address has 32 bits total. If 24 bits are the network, the remaining 8 bits are available for hosts. The larger the prefix number, the smaller the network.

192.168.1.0/24
          ↑
          24 bits = network portion
          32 - 24 = 8 bits left for hosts
          2⁸ = 256 addresses, 254 usable hosts

The binary foundation — why the math works

Subnetting math is based on binary. Each octet of an IP address is 8 bits, and each bit has a value: 128, 64, 32, 16, 8, 4, 2, 1. You don't need to memorize binary to subnet, but understanding where the numbers come from makes the patterns stick.

A subnet mask works by setting network bits to 1 and host bits to 0. Here's what 255.255.255.0 looks like in binary:

Teal bits = network (fixed). Grey bits = host (variable — these are where your device addresses live).

The CIDR reference table — memorise /24 to /30

For A+ you need a solid understanding of /24. For Network+ you need to be comfortable across the full range, especially /25 through /30. This table is your foundation:

Usable hosts = 2ⁿ − 2, where n = host bits. Two addresses are always reserved: the network address (all host bits 0) and the broadcast address (all host bits 1).

How to calculate a subnet — the method

Given any IP address and prefix length, you need to be able to find four things: the network address, the first usable host, the last usable host, and the broadcast address. Here's the reliable method that works on every problem:

Worked example 1 — /24 (A+ level)

Worked example 2 — /26 (Network+ level)

Worked example 3 — /27 (Network+ level)

The two formulas you must know

Usable hosts per subnet  =  2ⁿ − 2
where n = number of host bits (32 − prefix length)

Number of subnets        =  2ˢ
where s = number of subnet bits borrowed from the host portion

Block size               =  256 − last non-255 subnet mask octet
also equals: 2ⁿ where n = host bits in the interesting octet

Private IP ranges — what they are and why they matter

Most subnetting you encounter in the real world uses private IP address ranges — addresses that are not routed on the public internet. You'll see these constantly in both exam questions and actual networks:

10.0.0.0/8       → 10.0.0.0 – 10.255.255.255    (Class A, 16M+ addresses)
172.16.0.0/12    → 172.16.0.0 – 172.31.255.255  (Class B, 1M+ addresses)
192.168.0.0/16   → 192.168.0.0 – 192.168.255.255 (Class C, 65K addresses)

Exam scenarios

Key things to memorise for exam day

/24  →  255.255.255.0    →  254 hosts   block: 256
/25  →  255.255.255.128  →  126 hosts   block: 128
/26  →  255.255.255.192  →  62 hosts    block: 64
/27  →  255.255.255.224  →  30 hosts    block: 32
/28  →  255.255.255.240  →  14 hosts    block: 16
/29  →  255.255.255.248  →  6 hosts     block: 8
/30  →  255.255.255.252  →  2 hosts     block: 4

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