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Subnetting Cheat Sheet (IPv4)
Quick reference for CIDR, subnet masks, usable hosts, wildcard masks, and the “magic number” block size.
Fast rules to remember
Common small LAN subnets (last octet changes)
Aggregating / larger subnets (3rd octet changes)
Binary weights (quick mental math)
Examples:
How to use the “magic number”
Find the first mask octet that isn’t 255. Subnets start at multiples of 256 − mask_octet in that octet.
Example: 192.168.10.0/26 → mask octet = 192 → block size = 64 → networks at .0, .64, .128, .192
Example: 10.20.0.0/20 → mask = 255.255.240.0 → block size = 16 in the 3rd octet → networks at 10.20.0,16,32,48.0/20
/31 and /32 special cases
Mini-exercises
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Quick reference for CIDR, subnet masks, usable hosts, wildcard masks, and the “magic number” block size.
Fast rules to remember
- Usable hosts = 2^(32 − prefix) − 2 (except /31 and /32 special cases)
- Block size (magic number) = 256 − (mask value in the first non-255 octet)
- Wildcard mask = 255.255.255.255 − (subnet mask) (handy for ACLs)
- /31 is point-to-point (both addresses usable per RFC 3021). /32 = single host.
Common small LAN subnets (last octet changes)
Code:
CIDR Mask Usable Block Wildcard /30 255.255.255.252 2 4 0.0.0.3 /29 255.255.255.248 6 8 0.0.0.7 /28 255.255.255.240 14 16 0.0.0.15 /27 255.255.255.224 30 32 0.0.0.31 /26 255.255.255.192 62 64 0.0.0.63 /25 255.255.255.128 126 128 0.0.0.127 /24 255.255.255.0 254 256 0.0.0.255
Code:
CIDR Mask Usable Block (3rd octet) Wildcard /23 255.255.254.0 510 2 0.0.1.255 /22 255.255.252.0 1022 4 0.0.3.255 /21 255.255.248.0 2046 8 0.0.7.255 /20 255.255.240.0 4094 16 0.0.15.255 /19 255.255.224.0 8190 32 0.0.31.255 /18 255.255.192.0 16382 64 0.0.63.255 /17 255.255.128.0 32766 128 0.0.127.255 /16 255.255.0.0 65534 256 (2nd octet) 0.0.255.255
Code:
128 64 32 16 8 4 2 1 ← bit values in an octet
- Mask octet 224 → bits 128+64+32 → /27 → block size 32
- Mask octet 252 → bits 128+64+32+16+8+4 → /30 → block size 4
How to use the “magic number”
Find the first mask octet that isn’t 255. Subnets start at multiples of 256 − mask_octet in that octet.
Example: 192.168.10.0/26 → mask octet = 192 → block size = 64 → networks at .0, .64, .128, .192
Example: 10.20.0.0/20 → mask = 255.255.240.0 → block size = 16 in the 3rd octet → networks at 10.20.0,16,32,48.0/20
/31 and /32 special cases
Code:
/31 255.255.255.254 → 2 addresses, both usable (point-to-point links) /32 255.255.255.255 → single host route (no network/broadcast)
- How many usable hosts in /27? What’s the block size? (Answer: 30, block 32)
- What networks does 172.16.40.0/21 cover? (Hint: block 8 in 3rd octet → 172.16.40–47.0/21)
- You need ~60 hosts. Which prefix fits best? (Answer: /26)
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