Understanding CIDR Notation

Ever wondered what those weird numbers after IP addresses mean? Like 192.168.1.0/24 or 10.0.0.0/8?

That’s CIDR notation, and it’s basically how the entire internet stays organized. Let me break it down for you.

Why You Should Care About CIDR

Back in the 1990s, the internet was literally running out of IP addresses. Routing tables were becoming massive nightmares. CIDR came to the rescue by:

• Making IP allocation way more efficient
• Simplifying routing (no more crazy long routing tables)
• Scaling the internet to handle billions of devices

Without CIDR? The internet would’ve crashed under its own weight years ago.

The Basics - What’s That Slash About?

192.168.1.0/24 breaks down like this:

• 192.168.1.0 = your network address
• /24 = how many bits are used for the network part

Think of it like your home address:

• Network part = your street name
• Host part = your house number

The Math Behind It (Don’t Worry, It’s Simple)

Every IP address has 32 bits total. With /24:

• Network gets 24 bits
• Hosts get the remaining 8 bits (32-24=8)

That /24 is shorthand for subnet mask 255.255.255.0:

11111111.11111111.11111111.00000000

1s = network portion, 0s = host portion

Real Numbers Time

Let’s work with 192.168.1.0/24:

• Network address: 192.168.1.0
• Broadcast address: 192.168.1.255
• Usable hosts: 192.168.1.1 to 192.168.1.254
• Total usable: 254 addresses (256 minus network and broadcast)

Common CIDR Blocks You’ll See

/8  → 16M+ hosts    (like 10.0.0.0/8)
/16 → 65K hosts     (like 172.16.0.0/16)
/24 → 254 hosts     (like 192.168.1.0/24)
/30 → 2 hosts       (perfect for router links)

The pattern? Bigger CIDR number = smaller network. Counter-intuitive but that’s how it works!

Visual Breakdown

192.168.1.0/24

Network: ←──── 24 bits ────→ ← 8 host bits →
Binary:  11000000.10101000.00000001.00000000
Decimal: 192     .168     .1      .0

Subnetting Made Simple

Got 192.168.1.0/24? You can split it into smaller networks:

192.168.1.0/26    → 62 hosts (1-62)
192.168.1.64/26   → 62 hosts (65-126)
192.168.1.128/26  → 62 hosts (129-190)
192.168.1.192/26  → 62 hosts (193-254)

Each /26 gives you 62 usable addresses instead of 254.

Don’t Make These Mistakes

Mistake #1: Thinking you can assign 192.168.1.0 to a device Nope! That’s the network address, not a host address.

Mistake #2: Forgetting about broadcast addresses 192.168.1.255 in a /24 network is for broadcast, not devices.

Mistake #3: Getting the math wrong /24 network = 2⁸ = 256 total addresses, but only 254 are usable.

Mistake #4: Thinking bigger numbers = bigger networks /30 is actually smaller than /24. More network bits = fewer host bits.

Where You’ll See This Stuff

• Your home router: Probably using 192.168.1.0/24
• Corporate networks: Departments get /24, router links get /30
• Cloud providers: AWS, Azure, GCP all use CIDR for VPCs
• ISPs: They get big blocks like /12 and carve them up

The Bottom Line

CIDR is like the postal system for the internet. Once you get the pattern, it’s pretty straightforward:

1. IP/number format
2. Bigger number = smaller network
3. Always lose 2 addresses (network + broadcast)
4. Start practicing with /24 networks

Master this and you’ll understand how 90% of networking actually works under the hood.

Quick Quiz

Q1: What’s the usable range for 10.50.0.0/16? A: 10.50.0.1 to 10.50.255.254

Q2: How many /24 subnets can you make from /22? A: 4 subnets (2²⁴⁻²² = 4)

Q3: Can you assign 172.16.5.255 to a device in 172.16.5.0/24? A: No way - that’s the broadcast address

Got questions? Drop them in the comments. CIDR clicks differently for everyone, so don’t worry if it takes a few tries to sink in!