Free study tool
Subnetting practice
Work out the network address, broadcast address, usable range, and host count for a new subnet every time. Check your answers field by field, then see the working.
Useful for CompTIA Network+ and CompTIA A+ Core 1
Find the subnet for 172.16.109.109/26
The block sizes worth memorizing
Most exam subnetting lands in the last octet. Knowing these by heart turns each question into one subtraction.
| Prefix | Mask | Block size | Usable hosts |
|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | 255.255.255.128 | 128 | 126 |
| /26 | 255.255.255.192 | 64 | 62 |
| /27 | 255.255.255.224 | 32 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 8 | 6 |
| /30 | 255.255.255.252 | 4 | 2 |
Common questions
- How do I get faster at subnetting?
- Memorize the block sizes for /24 to /30, then practice until finding the block that contains an address is automatic. Most questions only need the octet where the network bits end.
- Where do the practice addresses come from?
- Each question picks a prefix and an address inside one of the RFC 1918 private ranges, so every answer is a realistic internal subnet. The same generator powers the CertLighthouse app.
- Do I need to write the number of hosts with commas?
- No. 16382 and 16,382 are both accepted, and addresses are checked as addresses, not as text.
More free tools
IPv4 subnet calculator
Network, broadcast, usable range, and host count for any address and prefix, with the working shown.Port and protocol reference
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Shorten or expand any IPv6 address the RFC 5952 way, identify its type, and practice.
Practice that remembers where you left off
The CertLighthouse app will have these generators built in, with your progress saved and questions aimed at what you miss most. It's in development now.