Binary to Decimal Conversion
Every IPv4 octet is 8 bits. To convert binary to decimal, write the eight place values 128 64 32 16 8 4 2 1 above the bits and add up wherever there is a 1. That is all subnetting binary really is.
The eight place values
An octet has eight positions, each worth double the one to its right:
| Bit position | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
|---|---|---|---|---|---|---|---|---|
| Value | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
These add up to 255 — the largest value one octet can hold.
Binary → decimal (worked example)
Convert 11000000:
128 64 32 16 8 4 2 1 1 1 0 0 0 0 0 0 128+64 = 192
So 11000000 = 192 — which is why a /26 mask octet is 192.
Convert 11001000: 128 + 64 + 8 = 200.
Decimal → binary (worked example)
Convert 200: go left to right, subtract each place value if it fits.
200 − 128 = 72 → 1 72 − 64 = 8 → 1 8 − 32 <0 → 0 8 − 16 <0 → 0 8 − 8 = 0 → 1 0 → 0 0 0 Result: 11001000
Why it matters for subnetting
Subnet masks are just 1-bits (network) followed by 0-bits (host). Reading a mask in binary tells you instantly how many host bits remain: 11111111.11111111.11111111.11000000 is /26 — 26 ones, 6 host bits, 2^6 − 2 = 62 hosts. Once the place values are automatic, the block-size method becomes pure mental maths.
Frequently asked questions
How do you convert binary to decimal quickly?
Write 128 64 32 16 8 4 2 1 above the eight bits and add the place values wherever there is a 1. For example 11100000 = 128+64+32 = 224.
What is the largest value in one octet?
255 — when all eight bits are 1 (128+64+32+16+8+4+2+1). That is why each part of an IPv4 address ranges from 0 to 255.
Do I need binary for the CCNA?
You need to understand it, but with practice you subnet using the block-size shortcut. Binary is the "why" behind the shortcut.
Why are subnet mask octets only certain numbers?
Because a mask is consecutive 1-bits, only these values can appear: 0, 128, 192, 224, 240, 248, 252, 254, 255.
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