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Azad Shukor
Azad Shukor

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Understanding CIDR Notation

#beginners #computerscience #networking #tutorial

Understanding IPv4 CIDR Blocks

A CIDR block looks like:

10.0.0.0/16
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It has two parts:

  • IP address → starting point
  • Prefix (/16) → how many bits are fixed

Core Idea (Memorize This First)

  • IPv4 = 32 bits total
  • CIDR /N = N bits are fixed (network)
  • Remaining bits = host bits (free to change) 
Total addresses = 2^(32 - N)
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IPv4 Structure

An IPv4 address:

10.0.0.0
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Breakdown:

10       . 0       . 0       . 0
8 bits     8 bits    8 bits    8 bits

Total = 32 bits
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Each block:

  • 8 bits = values from 0 to 255

  • Example:

    • 0 → 00000000
    • 255 → 11111111

Think in Terms of LOCKED vs FREE

Instead of memorizing formulas, think:

  • Locked bits = define the network
  • Free bits = generate all possible addresses

Example: 10.0.0.0/16

Step 1: Count free bits 

32 - 16 = 16 free bits
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Step 2: Calculate total addresses 

2^16 = 65,536
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Step 3: Visualize 

10       . 0       . 0       . 0
[LOCKED]   [LOCKED]   [FREE]    [FREE]
  8 bits     8 bits     8 bits    8 bits
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  • First 2 blocks fixed → 10.0
  • Last 2 blocks vary → 0.0 to 255.255

Range: 

10.0.0.0 → 10.0.255.255
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How to Read Any CIDR Quickly

Rule:

Move left to right, filling 8 bits per block, until you reach /N.

Example: /10 

10.0.0.0/10
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10       . 0       . 0       . 0
[8 LOCK]   [2 LOCK]   [FREE]    [FREE]
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  • 8 bits from first block
  • 2 bits from second block 
Total locked = 10 bits
Free = 22 bits
Total addresses = 2^22 = 4,194,304
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Example: /20 

10.0.0.0/20
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10       . 0       . 0       . 0
[8 LOCK]   [8 LOCK]   [4 LOCK]   [FREE]
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Locked = 20 bits
Free = 12 bits
Total = 2^12 = 4,096
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Example: /26 

10.0.0.0/26
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10       . 0       . 0       . 0
[8 LOCK]   [8 LOCK]   [8 LOCK]   [2 LOCK]
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Locked = 26 bits
Free = 6 bits
Total = 2^6 = 64
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Example: /30 

10.0.0.0/30
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10       . 0       . 0       . 0
[8 LOCK]   [8 LOCK]   [8 LOCK]   [6 LOCK]
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Free bits = 2
Total = 2^2 = 4
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Range:

10.0.0.0 → 10.0.0.3
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Shortcut Patterns (High Value)

Instead of recalculating every time:

CIDR Free Bits Total IPs
/16 16 65,536
/24 8 256
/26 6 64
/30 2 4

Important Practical Note

Not all addresses are usable:

  • First IP → network address
  • Last IP → broadcast address

Example /30:

10.0.0.0 → network
10.0.0.1 → usable
10.0.0.2 → usable
10.0.0.3 → broadcast
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Mental Model (Best Way to Think)

Treat CIDR like a binary lock:

  • /N = how many switches are locked
  • Remaining switches = combinations

More free bits → bigger network
Fewer free bits → smaller network

One-Line Summary

CIDR is just: how many bits are fixed vs how many bits can vary.

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