Subnet Calculator — Network, Broadcast and Usable Range
Given an address and a prefix length, this returns the network address, the broadcast address, the usable host range, the mask in every notation you might need, and the total host count. It is arithmetic, but it is arithmetic in base two under time pressure, which is where mistakes come from.
Why run it
Because subnet errors are expensive and easy. Two ranges that overlap produce routing behaviour that is intermittent and deeply confusing — some traffic works, some does not, and which is which depends on route specificity nobody is thinking about at the time. A block sized wrongly is discovered when it fills, typically during growth, at which point renumbering is a project rather than a change. Checking before allocation costs nothing; discovering afterwards costs a migration.
Why automate it
The calculation itself is a one-off, but its output belongs in documentation that stays correct. Where automation earns its place is validation: checking that the ranges you have actually allocated do not overlap and do not conflict with what is in use is a check worth repeating as an estate grows, particularly in cloud environments where network creation is self-service and several teams allocate independently. Overlaps introduced that way are found during the first attempt to peer two networks, which is exactly the wrong moment.
What it gives you
Correct address planning before it is expensive to change, and a defence against the overlaps that self-service network creation produces. It is the cheapest possible insurance against a renumbering project.