Subnetting Basics: How IP Address Ranges Work

What is an IP Address?

An IPv4 address is a 32-bit number that uniquely identifies a device on a network. We write it as four groups of 0–255 separated by dots — for example, 192.168.1.50. Each group is called an octet because it represents 8 bits of the address.

You do not need to work in binary to manage subnets day-to-day, but understanding that every address has exactly 32 bits is the key to everything that follows.

What is a Subnet?

A subnet (short for subnetwork) is a contiguous range of IP addresses treated as a single logical network. All devices within a subnet can communicate directly with each other without needing a router.

Dividing a large address block into smaller subnets lets you:

• Keep different parts of your business (office floor, CCTV, guest Wi-Fi) on separate networks for security
• Limit broadcast traffic so networks stay performant as they grow
• Apply firewall rules at a network boundary rather than on every device

CIDR Notation

You will see IP addresses written with a forward slash and a number: 192.168.1.0/24. The number after the slash is the prefix length — it tells you how many of the 32 bits identify the network, with the rest identifying individual hosts within it.

Notation	Network bits	Host bits	Total addresses	Usable hosts
/24	24	8	256	254
/25	25	7	128	126
/26	26	6	64	62
/27	27	5	32	30
/28	28	4	16	14
/30	30	2	4	2

The formula is: usable hosts = 2^(host bits) − 2. You subtract 2 because the first address is the network address and the last is the broadcast address — neither can be assigned to a device.

A Worked Example: 192.168.1.0/24

Take the subnet 192.168.1.0/24:

• Network address: 192.168.1.0 — identifies the subnet itself. Cannot be used by a device.
• Broadcast address: 192.168.1.255 — a packet sent here reaches every device on the subnet. Cannot be used by a device.
• Usable range: 192.168.1.1 through 192.168.1.254 — 254 addresses you can assign.
• Subnet mask: 255.255.255.0 — the traditional way to express /24. The 255s mark network bits; the 0 marks host bits.

Thinking in Blocks

Each time you increase the prefix length by 1, you halve the number of addresses. Going from /24 to /25 splits the 256-address block into two 128-address halves:

192.168.1.0/24 splits into:
  ├── 192.168.1.0/25    (hosts .1 – .126)
  └── 192.168.1.128/25  (hosts .129 – .254)

This is the foundation of subnetting — choosing the prefix length that fits your requirements, and splitting larger blocks into smaller ones as needed. The article on VLSM covers how to do this efficiently when segments need different sizes.

The Subnet Mask

Older equipment and some documentation use a subnet mask instead of CIDR notation. They are two ways of saying the same thing:

CIDR	Subnet mask
/24	255.255.255.0
/25	255.255.255.128
/26	255.255.255.192
/27	255.255.255.224
/16	255.255.0.0
/8	255.0.0.0

CIDR is almost universally used in modern networking because it is more compact. If you see a subnet mask, just count the consecutive 255s to get an approximate prefix length — or use the subnet calculator to convert.

Quick Reference

• Smaller prefix number → bigger subnet, more addresses (/8 has 16 million)
• Larger prefix number → smaller subnet, fewer addresses (/30 has 4)
• /32 is a single individual IP address — the smallest possible subnet
• /0 represents every IP address — the entire internet