Planning CIDR and subnets without collisions
An IP plan stands or falls with CIDR notation: /24, /22, /27 — the number behind the slash says how much address space a block covers and how many addresses in it are usable. Two blocks that cross each other by one address are no style difference but a collision: traffic does not find the way, and only when enabling routes does the slip show.
The IP planner divides a base network over a list of wishes and shows the blocks it created, the gaps that remained and the blocks that collide with what you filled in yourself — in your browser, your address plan does not leave your machine. The CIDR calculator draws out every single block: mask, broadcast, usable hosts and supernet merging.
This article is about planning on the drawing board: what the notation really means, where the classic slips sit, and when you switch from planning to measuring.
The slash is a boundary, not an estimate
/24 is 256 addresses, of which usable ones are what your subnet policy allows; /27 is 32; a /22 is four /24 blocks adjacent. Those numbers are no guideline but binary — and precisely that is why CIDR planning is checkable: two blocks cross or they do not, there is no "almost". The calculator shows both blocks, so the collision is a fact instead of a suspicion.
Dividing without gaps and without overlap
The planning chore: ten departments, each its own network, in one /16. The planner sorts, divides and — at least as useful — shows what did not add up: the gaps between the blocks. Who wants to use those gaps later (or let them be used by a router that automatically merges) must see them now; a plan without a gap plan is a plan with a future surprise.
Where the slips sit
The classic three: forgetting that the network and broadcast address are not usable (both shown apart in the calculator's outcome); supernets you merge while a block that you wanted kept apart falls inside them; and carrying IPv4 habits to IPv6 — where /64 is the working unit and the IPv6 workbench notes RFC 5952 notation, address types and the ip6.arpa name along.
From plan to measurement
These tools are a drawing board: they measure nothing on your live network and claim nothing about what stands. If you want to know what actually stands, the question is another one: does the address really stand open and reachable — port check — and does the IP address belong to the domain you thought — DNS lookup. Address notation returns in SPF checking: ip4: and ip6: directives are CIDR in a TXT text.