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A Sysadmin’s Tools

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A sysadmin’s tools

Five things you usually do in your head, re-Google every time, or install a separate app for. Four of the five run entirely in the browser — not a single byte leaves it; the breach check talks to an external API, but in a way that never exposes the password itself or its hash — details in that tab.

Accepts an IP with a prefix (/24) or a decimal-form mask (/255.255.255.0). Handles the edge cases /31 (point-to-point, RFC 3021 — both addresses usable, no broadcast) and /32 (a single host).

Calculated over a 365.25-day calendar year (this is where the classic “8h 45m/year” for 99.9% and “52m 35.7s/year” for 99.99% figures come from — a plain 365-day year doesn’t match them as closely).

Runs on the k-anonymity API from Have I Been Pwned: the password is hashed right in your browser (SHA-1), and only the first 5 characters of that hash are sent to the server — never the password or the full hash. The response contains hundreds of possible matches for that prefix, and the comparison against your actual hash happens locally. There is no way to recover the password from this request.

Quick builder

Standard 5-field cron format: minute · hour · day-of-month · month · day-of-week. Understands *, comma-separated lists, ranges a-b, and step */N, plus @daily/@hourly/@weekly/@monthly/@yearly/@reboot. If both day-of-month and day-of-week are set to something other than “*”, it fires when either one matches (that’s how cron itself behaves, not a bug in this calculator). The next 5 run times are computed from your browser’s current time and don’t account for daylight saving transitions.

Four of the five tools send nothing anywhere: calculations run locally in your browser, no API calls, nothing saved. The breach check talks to an external API — exactly how and why that’s still private is explained in its own tab. If what you’re calculating is part of a bigger task (network design, password policy, a backup schedule, an SLA with a client) — happy to discuss: services and pricing.

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