Visual lab
Learn subnetting visually
11 short lessons, from binary to IPv6. Change the inputs and watch every value update.
Test address
Used by lessons 2 to 4Tip: paste an address with a prefix, like 10.0.5.9/22
/25 · 255.255.255.128
Lesson 3 of 11Beginner
The 60-second method
Skip the 32-bit conversion
Memorize the eight powers of two (128, 64, 32, 16, 8, 4, 2, 1). Then four steps give you every attribute.
1286432168421
1
Find the interesting octet
The prefix tells you which octet the mask boundary falls in.
/1 – /8Octet 1
/9 – /16Octet 2
/17 – /24Octet 3
/25 – /32Octet 4
/25 lands in octet 4
2
Find the block size
Subtract the mask value in the interesting octet from 256. Equivalently, it is 2 raised to the number of host bits left in that octet. Here the mask octet is 128.
256 − 128 = 128
3
Count by the block size
The address has 148 in octet 4. Count up in steps of 128 and stop at the last multiple that does not pass it.
0128…
Network address 10.44.21.128
4
Fill in the range
- First usable = network + 110.44.21.129
- Next network = network + block10.44.22.0
- Broadcast = next network − 110.44.21.255
- Last usable = broadcast − 110.44.21.254
Broadcast 10.44.21.255
Keep the table handy
The cheat sheet lists every mask, block size and prefix side by side.
Lock it in
Drill the “Block size” skill until it is automatic.
Lesson 3 of 11
Next: Bitwise AND