Subnetting 101

Step 1 — Binary Fundamentals

Binary is the foundation of subnetting. Before touching IP addresses, you need to understand how binary works. This step teaches you from absolute zero — no prior knowledge required.

What is Binary?

Binary is a number system that uses only two digits: 0 and 1. Every number we use in networking can be represented in binary.

Computers use binary because they store information as on/off switches. Each switch is a bit — either 0 (off) or 1 (on).

Why 8 Bits = 255

An octet is exactly 8 bits. With 8 switches, you can create 256 different combinations (2⁸ = 256). Since we start counting at 0, the range is 0 to 255.

Minimum: 00000000 = 0
Maximum: 11111111 = 255
Total combinations: 256 (0-255)

Binary Place Values

Just like decimal has place values (ones, tens, hundreds), binary has place values. Each position doubles as you move left.

1286432168421
2⁷2⁶2⁵2⁴2³2²2¹2⁰
1286432168421

Memorize these values: 128, 64, 32, 16, 8, 4, 2, 1. This pattern appears constantly in subnetting.

Converting Decimal to Binary

To convert a decimal number to binary, start from the left and ask: "Can this place value fit?" If yes, put a 1 and subtract. If no, put a 0.

Example: Convert 192 to binary

  1. 192 ≥ 128? Yes → write 1, remainder = 192 - 128 = 64
  2. 64 ≥ 64? Yes → write 1, remainder = 64 - 64 = 0
  3. 0 ≥ 32? No → write 0
  4. 0 ≥ 16? No → write 0
  5. 0 ≥ 8? No → write 0
  6. 0 ≥ 4? No → write 0
  7. 0 ≥ 2? No → write 0
  8. 0 ≥ 1? No → write 0

Result: 192 = 11000000

Example: Convert 85 to binary

  1. 85 ≥ 128? No → write 0
  2. 85 ≥ 64? Yes → write 1, remainder = 85 - 64 = 21
  3. 21 ≥ 32? No → write 0
  4. 21 ≥ 16? Yes → write 1, remainder = 21 - 16 = 5
  5. 5 ≥ 8? No → write 0
  6. 5 ≥ 4? Yes → write 1, remainder = 5 - 4 = 1
  7. 1 ≥ 2? No → write 0
  8. 1 ≥ 1? Yes → write 1, remainder = 1 - 1 = 0

Result: 85 = 01010101

Converting Binary to Decimal

Add up the place values where there's a 1.

Example: Convert 11001100 to decimal

1286432168421
11001100

Add the place values where there's a 1:
128 + 64 + 8 + 4 = 204

Practice Exercises

Do these now. Don't skip ahead until you can solve them quickly.

Quick note

Your answers are saved in your browser so you can come back later. For binary, you can type either 8 bits or fewer bits (we'll pad with leading zeros when checking).

1. Convert to binary:

  1. 255
  2. 128
  3. 64
  4. 224
  5. 10

2. Convert to decimal:

  1. 10110101
  2. 11001111
  3. 00111100
  4. 10011001
  5. 01011010
Show Answers

1. Decimal to Binary:

  1. 255 = 11111111
  2. 128 = 10000000
  3. 64 = 01000000
  4. 224 = 11100000
  5. 10 = 00001010

2. Binary to Decimal:

  1. 10110101 = 181 (128 + 32 + 16 + 4 + 1)
  2. 11001111 = 207 (128 + 64 + 8 + 4 + 2 + 1)
  3. 00111100 = 60 (32 + 16 + 8 + 4)
  4. 10011001 = 153 (128 + 16 + 8 + 1)
  5. 01011010 = 90 (64 + 16 + 8 + 2)

Checkpoint

Before moving to Step 2, make sure you can:

  • Explain why 8 bits gives us 0-255 (256 values)
  • Recite the place values from memory: 128, 64, 32, 16, 8, 4, 2, 1
  • Convert any decimal (0-255) to binary without hesitation
  • Convert any 8-bit binary back to decimal

If any of these feel shaky, practice more. Subnetting builds on this foundation.