Subnetting under exam pressure
You do not have time to convert to binary in the exam. You need a procedure that runs in fifteen seconds and does not fail when you are nervous.
Subnetting is the only CCNA topic where speed is part of the skill. It appears inside questions about routing, ACLs and troubleshooting, so every second you spend on it is a second you are not spending on the actual question being asked.
Memorise these, and nothing else
The whole method rests on one table. Learn it until it is reflex.
| Prefix | Mask octet | Block size | Usable hosts |
|---|---|---|---|
| /24 | 0 | 256 | 254 |
| /25 | 128 | 128 | 126 |
| /26 | 192 | 64 | 62 |
| /27 | 224 | 32 | 30 |
| /28 | 240 | 16 | 14 |
| /29 | 248 | 8 | 6 |
| /30 | 252 | 4 | 2 |
Block size is the number that does all the work. It is 256 - mask octet, and
subnets always start at multiples of it. Everything below is just applying that.
The procedure
- Find the interesting octet — the one the prefix lands in. /25 to /30 is the fourth octet, /17 to /24 is the third, and so on.
- Get the block size from the table.
- Count up in block-size steps until you pass the address you were given. The step before is your network address.
- Broadcast is the next network minus one.
- Usable range is everything between them.
Worked example: what is the network for 172.16.44.77/26?
- /26 lands in the fourth octet. Block size 64.
- Count: 0, 64, 128. 77 sits between 64 and 128.
- Network: 172.16.44.64
- Broadcast: 172.16.44.127 (next block 128, minus one)
- Usable: .65 to .126
No binary. Roughly eight seconds once it is reflex.
Worked example: a third-octet prefix
10.5.130.200/19. /19 is in the third octet. Mask octet 224, block size 32. Count in the third octet: 0, 32, 64, 96, 128, 160. 130 sits between 128 and 160.
- Network: 10.5.128.0
- Broadcast: 10.5.159.255
- Usable: 10.5.128.1 to 10.5.159.254
The question shapes that actually appear
"How many subnets and hosts?"
Subnets = 2 to the power of bits borrowed. Hosts = 2 to the power of bits left, minus two. The minus two is the network and broadcast addresses — and it is the single most common careless error on the exam.
"Which mask gives at least N hosts?"
Read the hosts column upward until you find the first number that is big enough. Do not pick the exact fit if it is not there — 60 hosts needs a /26 (62 usable), not a /27.
"Which of these addresses is on the same subnet as X?"
Work out X's network and broadcast, then check which candidate falls inside. Do not compare the addresses to each other; compare each one to the range.
Point-to-point links
A /30 gives exactly two usable addresses, which is why router-to-router links use it. If a question mentions a serial or point-to-point link and asks for the most efficient mask, the answer is almost always /30.
How to practise so it sticks
Ten questions a day, timed, every day until the exam. Not fifty questions once a week — subnetting is a motor skill, and motor skills come from frequency, not volume.
Track the ones you get wrong. Almost everyone has one specific failure: forgetting the minus two, mis-identifying the interesting octet, or losing track when the prefix crosses into the third octet. Find yours and drill that shape specifically.
Practise this on a real simulator
CCST Networking and CCNA are free on ITWars — every lab, incident and practice exam, with no trial timer and no card.
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