Binary Conversion for Network Engineers

You don't need to be a binary wizard. You need eight place values memorized and a habit of subtracting. That's the entire skill. Practice for one afternoon and you'll do subnetting in your head faster than most candidates with a calculator.

#The Only Table You Need#link

Each bit in an octet corresponds to one of these values. To convert decimal to binary, start from 128 and ask: is my number ≥ 128? If yes, set bit 8 and subtract 128. Then move to 64. Repeat until you reach 1.

Example: convert 200. Is 200 ≥ 128? Yes → bit 8 = 1, remaining = 72. Is 72 ≥ 64? Yes → bit 7 = 1, remaining = 8. Is 8 ≥ 32? No → bit 6 = 0. Same for 16 (bit 5). Is 8 ≥ 8? Yes → bit 4 = 1, remaining = 0. All remaining bits 0.

Result: `11001000`. Every conversion follows the same pattern. With practice you'll do it in under five seconds.

#Binary to Decimal — Sum the 1s#link

Going the other way is even easier. Add the place values of every bit that's set to 1. `10101000` = 128 + 32 + 8 = 168. `11111111` = 255. `00000000` = 0.

The Mask Patterns You Should Know Cold

• ▪/25 → 10000000 → 128
• ▪/26 → 11000000 → 192
• ▪/27 → 11100000 → 224
• ▪/28 → 11110000 → 240
• ▪/29 → 11111000 → 248
• ▪/30 → 11111100 → 252
• ▪/31 → 11111110 → 254
• ▪/32 → 11111111 → 255

#Bitwise AND — the Operation That Defines a Subnet#link

AND returns 1 only if both inputs are 1. Everything else returns 0. When you AND an IP with a mask, the mask's zeros zero out the host bits, leaving the network address.

#Practice Drill#link