Every prefix from /8 to /32 — mask, wildcard, address count and usable hosts. One page, designed to be pinned above a desk or folded into a laptop bag.
Most of them are a fill-in form wrapped around the same four numbers, and a good few get the edge cases wrong. This one is built around the parts that actually cost you time on site:
10.0.0.5/22, 10.0.0.5 255.255.252.0, 10.0.0.5/255.255.252.0, or a bare address. It also rejects a non-contiguous mask like 255.255.0.255 instead of quietly producing nonsense.100.64.0.0/10 rather than a public address explains a lot of confusing traceroutes.The whole chart, /8 through /32. Same content as the PDF above.
Usable hosts is two fewer than total addresses for anything /30 or larger — one address goes to the network, one to the broadcast. Below that the rules change: a /31 uses both addresses for point-to-point links, and a /32 is a single host route.
The wildcard mask is the inverse of the subnet mask, and it is what Cisco ACLs and OSPF network statements expect. 255.255.255.0 becomes 0.0.0.255. Getting this backwards is one of the most common causes of an ACL that silently matches nothing.
Subnet on nibble boundaries where you can. Prefixes that land on /4 boundaries — /20, /24, /28 — keep the arithmetic readable in hex and make the address plan something a colleague can follow at 2 a.m. without a calculator.
Leave room. The most common addressing mistake is sizing every VLAN exactly to today's device count. Doubling gives you somewhere to grow without a renumber, and renumbering a live network is a genuinely bad day.
Designing an address plan for a real site is a different job from calculating one subnet. If you are laying out VLANs, routing and wireless coverage for a building, that is network design — and we do it across Central Oregon.
We do managed IT, networking and security for businesses across Central Oregon.
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