Task path / input to outcome
How to calculate an IPv4 subnet or CIDR block
- 01
Enter an IPv4 address and prefix
Type a plain IPv4 address such as 10.20.34.77, or paste it with an embedded CIDR prefix like 10.20.34.77/27. If the address carries no prefix, the second field accepts /0 through /32 — the leading slash is optional — or a contiguous dotted-decimal netmask such as 255.255.255.224. Each octet must be a decimal number from 0 to 255, and a mistake is reported with the exact octet or prefix that failed.
- 02
Review the normalized subnet
As soon as the input is valid, the result panel fills in live: the normalized CIDR, network address, broadcast behavior, first and last usable hosts, subnet mask, wildcard mask, and exact total and usable address counts. Every edit recomputes instantly, so you can compare two prefix lengths by changing a single digit and watching the host range move.
- 03
Check edge-case semantics
A /31 is identified as an RFC 3021 point-to-point subnet with two usable endpoints and no broadcast address. A /32 is shown as a single-host route with one usable address. Prefixes from /0 to /30 use standard network and directed-broadcast boundaries, where the usable count is the total minus two. A banner above the results names the semantics of the current prefix, so the special cases are never ambiguous.
- 04
Read the binary and context panels
The binary view renders the address, netmask, and network as dotted binary, which is the fastest way to see exactly which bits a mask covers. The context panel adds a scope label such as private use, loopback, link-local, documentation, or multicast, the address's ordinal position inside its network, and the historical class A through E — all derived from a static standards-based table, not a live registry lookup.
- 05
Copy the values you need
Every result row has its own copy button, and Copy all places the complete label-and-value list on the clipboard for pasting into a change record or ticket. Reset clears both fields for the next calculation. Parsing, mask arithmetic, binary rendering, and clipboard preparation all run inside the current browser tab.