CIDR Notation Explained: How to Read and Calculate Subnets

You've seen it everywhere: 192.168.1.0/24, 10.0.0.0/8, 203.0.113.5/32. The slash and number after an IP address is CIDR notation, and once you understand what it means, a whole layer of networking suddenly makes sense.

CIDR stands for Classless Inter-Domain Routing. It's the system used since 1993 to define network sizes and allocate IP address blocks efficiently. Before CIDR, IP addresses were carved into rigid Class A, B, and C blocks — a wasteful system that's now obsolete.

The Slash Number: What It Means

The number after the slash tells you how many bits of the IP address are the network portion — fixed across all addresses in that block. The remaining bits are available for individual hosts.

IPv4 addresses are 32 bits total. So:

The formula for usable hosts is 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.

Reading a Real Example: 192.168.1.0/24

This is the most common home and small office network range. Here's what /24 means in practice:

• Network address: 192.168.1.0 (identifies the network itself, not assignable)
• Broadcast address: 192.168.1.255 (sends to all hosts, not assignable)
• Usable range: 192.168.1.1 – 192.168.1.254 (254 hosts)
• Subnet mask: 255.255.255.0

The first 24 bits (192.168.1) are locked — that's your network. The last 8 bits (.0 through .255) are where you assign individual devices.

The Subnet Mask Connection

CIDR and subnet masks represent the same information in different formats. A /24 is the same as 255.255.255.0. They're interchangeable — CIDR is just more compact.

Converting between them: write out 32 bits, put 1s for network bits and 0s for host bits.

For /24:

11111111.11111111.11111111.00000000 = 255.255.255.0

For /28:

11111111.11111111.11111111.11110000 = 255.255.255.240

Common Slash Values and Where You See Them

/32 — Single host Used to refer to one specific IP address. Common in firewall rules ("allow traffic from 203.0.113.5/32"), static routes, and BGP route advertisements for individual IPs.

/30 — Point-to-point links 2 usable addresses. Used for router-to-router connections where you only need addresses for each end of the link.

/29 — Small segment 6 usable addresses. A common choice when you need a tiny subnet for a specific purpose (a DMZ with a few servers, a management network).

/28 to /26 — Small office / team networks 14 to 62 usable hosts. Useful for carving up a larger network into logical segments.

/24 — The workhorse 254 usable hosts. The standard for most home networks, small offices, and VLAN segments in enterprise environments. So common that people sometimes call it a "class C" out of habit (technically incorrect — class C is a pre-CIDR concept, but the range overlaps).

/22 to /20 — Medium network blocks Used by larger organisations for significant network segments or campus networks.

/16 — Large network 65,534 hosts. The 172.16.0.0/16 and 192.168.0.0/16 ranges are in this category. Often used as a VPC/cloud network supernet that gets subdivided.

/8 — Massive block Over 16 million addresses. The classic Class A ranges — 10.0.0.0/8 is the private range used by large enterprises and cloud providers.

Subnetting: Splitting a Block

CIDR lets you take a large address block and divide it into smaller subnets by increasing the prefix length.

Say you have 192.168.1.0/24 and need to split it into 4 equal subnets. You borrow 2 bits from the host portion, giving you /26:

Each subnet is independent — devices in different subnets need a router to communicate.

Supernetting: Combining Blocks

The reverse works too. Route summarisation (supernetting) combines multiple smaller prefixes into a single larger announcement, reducing the size of routing tables.

If you have:

• 192.168.4.0/24
• 192.168.5.0/24
• 192.168.6.0/24
• 192.168.7.0/24

These can be summarised as 192.168.4.0/22 — one route covers all four networks. This is how backbone routers stay manageable despite millions of routes.

Private vs. Public CIDR Ranges

Three ranges are reserved for private use (RFC 1918) — these addresses are never routed on the public internet:

Your router performs NAT to translate these private addresses to your public IP for internet traffic.

Calculating Subnets Without Math

For anything beyond quick mental arithmetic, use the CIDR Calculator. Enter any IP address and prefix length and it returns the network address, broadcast address, usable host range, subnet mask, and total host count — instantly.

It works in both directions: you can also enter a subnet mask and it returns the CIDR equivalent.

IPv6 CIDR

CIDR notation applies to IPv6 too, though the numbers are different. IPv6 addresses are 128 bits, so:

• /128 — single host (equivalent to /32 in IPv4)
• /64 — standard subnet size (18 quintillion host addresses)
• /48 — typical allocation to a site or organisation
• /32 — ISP allocation block

In practice, you'll rarely need to subnet IPv6 manually — the address space is large enough that most deployments just use /64 for every segment.

The Bottom Line

The slash number in CIDR notation tells you how many bits belong to the network — the rest belong to hosts. Larger prefix = smaller network. A /32 is one address; a /8 is sixteen million.

The key numbers to have memorised: /24 gives you 254 hosts, /16 gives you 65,534, /30 gives you 2 (point-to-point). Everything else you can calculate with the CIDR Calculator or the 2^n − 2 formula.