Networking
Cheat sheet
IPv4 Subnetting on One Card
Before it appears: how many usable hosts does a /24 give you?
Quick check · tap an answer to continue
Lesson notes
Key points
- CIDR /n: The slash number counts how many leading bits belong to the network portion.
- Hosts = 2^h − 2: Usable hosts equal two to the host bits, minus network and broadcast addresses.
- /24 → 254 hosts: Eight host bits give 256 addresses, so 254 are usable for devices.
- Block size = 256 − mask: Subtract the interesting mask octet from 256 to find where each subnet starts.
- Broadcast = next − 1: The broadcast address is one less than the next subnet's network address.
- Private ranges: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16 are private, not internet-routable.
Takeaway: Usable hosts = 2^(host bits) − 2; block size = 256 − mask.
Quick check: How many usable host addresses does a /26 subnet provide?
- 64
- 62
- 30
A /26 leaves 6 host bits: 2^6 = 64, minus 2 gives 62.
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