Networking & scheduling tools
The things you end up looking up on some ad-filled site instead.
Two things nobody does in their head correctly: what a CIDR block actually contains, and when a cron expression will next fire. A /26 gives you 59 usable addresses in a VPC rather than 62, because AWS reserves five per subnet. A cron expression with both day-of-month and day-of-week set runs on either, not both, which is the single most misread rule in crontab. These tools show the answer against a real address space and a real timeline.
Free, with no sign-up. Every one runs entirely in your browser: nothing you paste is uploaded, and there is no server to send it to.
VPC Subnet & CIDR Calculator
How many hosts, and does it overlap?
Work out network, broadcast, netmask and usable hosts for any IPv4 CIDR, with the AWS host count that is five lower than the standard one.
Cron Expression Explainer
What it means, and when it next runs
Turn a cron expression into a sentence with its next six run times, and catch the trap: day-of-month and day-of-week are ORed, not ANDed.
VPC Subnet Planner
Lay out the whole VPC at once
Lay out every subnet in an AWS VPC at once, with the five addresses AWS reserves per subnet already taken off the usable count.
Cron Expression Generator
Pick a schedule, get the expression
Build a cron expression from a form, optionally wrapped in a complete Kubernetes CronJob with hardened defaults.
Reading on networking & scheduling
The problems these tools check for, argued out at length.