Network+ Academy · Lesson

Reading CIDR Slash Notation

Learn how /24 and similar notations describe a subnet quickly.

Reading CIDR Slash Notation is a free Network+ Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Network+ Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What CIDR Means

CIDR stands for Classless Inter-Domain Routing. It replaced rigid address classes with flexible masks of any length, written using slash notation like /24.

The number after the slash is simply the count of 1 bits in the subnet mask — that is, how many bits belong to the network portion.

The Slash Is a Bit Count

In 192.168.1.0/24, the /24 means the first 24 bits are network. Since 24 bits fill the first three octets, the mask equals 255.255.255.0.

The remaining 8 bits are for hosts. The slash format is shorter and clearer than writing the full dotted-decimal mask.

/24 = 255.255.255.0
/16 = 255.255.0.0
/8  = 255.0.0.0

Converting Slash to Mask

To convert a prefix to a mask, fill in that many 1 bits from the left, then zeros:

/8 → 255.0.0.0
/16 → 255.255.0.0
/24 → 255.255.255.0

These three are the easy "octet boundary" prefixes because each falls neatly at the end of an octet.

Prefixes Inside an Octet

Prefixes do not have to land on octet boundaries. A /26 uses 26 network bits, which is 24 (three full octets) plus 2 bits into the fourth octet.

Two 1 bits in an octet give 11000000 = 192, so /26 equals 255.255.255.192. These mid-octet prefixes are where subnetting math gets interesting.

Partial-Octet Mask Values

Each extra 1 bit in an octet produces a specific value: 1 bit = 128, 2 = 192, 3 = 224, 4 = 240, 5 = 248, 6 = 252, 7 = 254, 8 = 255.

So /25 = 255.255.255.128, /27 = 255.255.255.224, and /30 = 255.255.255.252. Memorizing this short list lets you convert any prefix instantly.

More Bits, Smaller Network

A larger prefix means more network bits and fewer host bits, so each network holds fewer hosts. A /30 leaves only 2 host bits, giving just 4 addresses.

A smaller prefix like /16 leaves 16 host bits and tens of thousands of addresses. Remember: bigger slash number = smaller network.

CIDR Spans Old Classes

CIDR ignores the old class boundaries. You can summarize several Class C networks into one route, such as 192.168.0.0/22, covering four /24 blocks.

This route summarization shrinks routing tables and is a major reason CIDR was adopted across the internet backbone.

Reading a CIDR Block

When you read 172.16.8.0/22, interpret it as: network 172.16.8.0, with 22 network bits and 10 host bits. The mask is 255.255.252.0.

The block contains 2 to the 10th power = 1024 addresses. Quickly decoding such notation is essential for both the exam and real configuration work.

CIDR in Everyday Tools

You will meet CIDR everywhere: firewall rules, cloud security groups, routing entries, and DHCP scopes all use slash notation.

A rule allowing 10.0.0.0/8 permits the entire private Class A range, while 10.0.5.0/24 permits just one subnet. Reading the slash tells you the scope of any rule at a glance.

Common Prefix Lengths

A few prefixes appear constantly:

/24: 256 addresses, the classic LAN subnet.
/30: 4 addresses, used for point-to-point links between routers.
/32: a single host address, used for specific allow rules.

Recognizing these on sight speeds up your work.

From Slash to Understanding

CIDR notation packs the whole network-host split into one tidy number. Convert it to a mask when you need to AND, but for quick reasoning the slash alone tells you network size and host count.

With this, you are ready to count exactly how many hosts a subnet provides — the focus of the next lesson.

Quick Check

Convert the prefix.

Recap

You learned to read CIDR slash notation.

CIDR uses a slash plus a number = count of network (1) bits.
/8, /16, /24 land on octet boundaries (255.0.0.0, etc.).
Partial-octet bits: 128, 192, 224, 240, 248, 252, 254, 255.
Bigger slash = smaller network (fewer hosts).
CIDR enables route summarization across old classes.

Frequently asked questions

Is the “Reading CIDR Slash Notation” lesson free?

Yes — the full text of “Reading CIDR Slash Notation” is free to read here on the web, and the Network+ Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Network+ Academy course, upgrade to CoddyKit PRO.

What will I learn in “Reading CIDR Slash Notation”?

Learn how /24 and similar notations describe a subnet quickly. You practise Network+ Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Network+ Academy?

No prior experience is required. Network+ Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Reading CIDR Slash Notation” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Network+ Academy lesson?

Yes. Every Network+ Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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