Subnetris | Interactive Zone @ ECT

Subnetris is a practice tool for IPv4 subnetting. It draws a network the way the lecture draws it, as a block you cut in half over and over until it holds the networks you need, and it fills in the handout table as you go.

Why bother

Subnetting is a blocking topic. If it does not click, DHCP, NAT, DNS, routing and firewalls all stay out of reach, because every one of them assumes you can split a network and say what is in it. The way it clicks is by doing it: cut a block, watch the numbers move, and get told when a rule is broken and which rule it was.

The one rule underneath everything: a subnet is a block, and a block can only be cut in half. That is Binary Law, and this tool cannot draw anything that breaks it. Point anywhere you like and the cut still lands on a legal boundary, with a line of prose telling you why.

What you are looking at

The band across the middle is your whole allocation. Its left edge is drawn blue, because that is the Network Number; its right edge is red, because that is the Broadcast. Neither can be given to a machine, which is the N-2 rule. Inside each block is its prefix and how many addresses it can actually use.
The table underneath is the handout table, one row per subnet in address order. The three rows at the foot are the ones to watch: total addresses never change, and the N-2 tax grows by two every time you make another subnet.
The panel on the left is what the network has to hold. In an exercise it lists the networks you have been asked for; on your own you type your own. Point at any block and its details (size, usable range, broadcast, mask, gateway) appear at the foot of the panel.

What you do here

Cut a block. Drag inside it, or just click it. You can also point at a block and press Enter. Click a division bar to undo a cut and rejoin the two halves.
Name the pieces. Drag a block onto one of the networks in the left panel, or drag the network onto the block. Either way round assigns it.
Check your work. Check reports against Bowie's Hierarchy of Subnetting and names the rule any failure broke. It will tell you when a subnet is two addresses short, and it will point out waste without failing you for it.
Practise by hand. Drill hides the numbers and turns the table into blanks for you to fill in, then marks them. That is the version a quiz asks for.

Where to start

Exercises holds the lecture problems, the handout's five walkthrough tables, and the study-guide questions.
Random, a category inside Exercises, builds a fresh problem from a code such as SN1-DPK7M3Q. The same code always builds the same problem, so a code posted to a class gives everyone the same work.
Help, under the menu, has the cheat sheet, the course conventions and the keyboard shortcuts.

Nothing is saved to an account and nothing is sent anywhere: your work travels in the address bar, so a bookmark or a copied link brings the whole plan back.

What you are looking at

The wide band is your whole allocation. Its left edge, drawn blue, is the Network Number; its right edge, drawn red, is the Broadcast. The number in the middle is the prefix. Every cut you make puts a bar in the block, greys out the parent prefix above the bar, and gives each half its own prefix. At the bar a new red and blue pair appears: the broadcast of the left half and the network number of the right half.

How to cut

Drag inside a block. A ghost bar follows the pointer and the readout shows where the cut will actually land.
Click a block. Same thing without the drag.
Keyboard: pointing at a block selects it, and so do the arrow keys; Enter halves the selection, Backspace rejoins it, F zooms into it.
Click a bar to rejoin the two halves under it.

Why the bar snapped

Because of Binary Law. A block is divided into two equal halves, so the only place a first cut can go is the midpoint. Every gesture is exactly one halving, so a nearer boundary is reached by cutting again inside the new block. Cuts stop at /30, or at /32 with Allow /31 and /32 on under Settings in the menu. The readout always names the address you pointed at, the address the cut used, and how many halvings a nearer boundary would take.

Bowie's Hierarchy of Subnetting

Five rules, from the one you bend first to the one you can never break.

Most elegant design, change freely. The tidiest plan, the least wasted space.
Business need, change reluctantly. What the organization actually requires.
Convention, follow unless told otherwise. The course conventions below.
N-2 rule, mostly unbreakable. Two addresses per subnet cannot be hosts.
Binary law, unbreakable. Networks divide in powers of two.

Check opens a report against these in that order, and names the rule any failure broke. Close it, fix what it names, and check again; the badge on each need keeps the last result.

The table and the N-2 rule

Under the diagram is the handout table, one row per subnet in address order. The Network # column carries the network number and the prefix together, the way you would type them into a router, and the prefix tells you how big the subnet is. The Hosts column has the N-2 rule applied: the network number and the broadcast are subtracted because neither can be assigned to a machine. Watch the wording on a quiz: "how big is the subnet" means the total, "how many hosts will it hold" means subtract two. A Gateway column appears once you have entered a gateway for any network, showing what you entered. Under the rows, Total IPs and USABLE IPs sit side by side: the total never changes, the usable count falls every time you cut, and the gap between them is what your cutting has cost so far.

Course conventions

A. The default gateway uses the last usable address in the network.
B. All other static addresses start at the beginning of the range.
C. DHCP pools sit between the static addresses and the gateway.
D. DNS servers are 132.235.9.75 and 132.235.200.41 unless you are told otherwise.

Those are the lecture's defaults. Conventions in the menu changes any of them (gateway first or last, statics from the start or the end, a pool or none, other DNS servers), and the choice travels in the share link, so a link your instructor sends carries the conventions it should be read against.

Reading a network need

A network in the Business needs panel states a host count, and under it the terms that count comes with. plus 10% growth means it has to hold ten percent more than it does today, rounded up, so 10 hosts becomes 11. gateway included means the router is already one of those addresses; gateway not included means it needs one more on top.

The size that works out to is not shown here on purpose. Working it out is the exercise, and Check spells the arithmetic out afterwards.

The addressing plan

Once a network has a subnet, its row in Business needs grows an Addressing section: the gateway, the first and last address of the DHCP pool, and a named row for each device that needs a fixed address. Type a full address or just the part that changes, such as .126. Nothing is filled in for you. Where the gateway goes is yours to work out from the conventions, and a network with a subnet but no gateway fails Check, because a network without a default gateway does not work. After you commit an address, Check says what the conventions would have done it should be.

The badge tells you where you stand. A problem is a plan that cannot work, such as an address outside its own subnet or two things on one address, and it fails the exercise. A thing to check is a plan that works but breaks a convention, and it does not.

Exercises, random codes and drill

Exercises opens a picker with a category pull-down, and a sentence under it says what each category is for: the lecture problems, the five walkthrough tables from the handout, the study-guide checks, and Random. Random builds a fresh problem from a code such as SN1-DPK7M3Q: pick a difficulty and roll, or paste the code you were given. The same code always produces the same problem, so one code posted to a class gives everyone the same work. Drill hides the numbers and turns the table into blanks for you to fill in, then grades them.

Sharing

The address bar carries your work, so a bookmark or a pasted link brings the whole plan back. Share gives you that link, and Open in IPCalc opens the same network in the ECT Visual Subnet Calculator.

Block Display in the menu gives the diagram the whole window by closing the needs panel and the table; choose it again and they come back exactly as they were.

Cheat sheet

Learn the /24 line and derive the rest by halving or doubling.

The usable counts here follow the course handout. RFC 3021 allows a /31 to be used as a two-address point-to-point link, and a /32 names a single host; the lecture table treats both as having no usable addresses, and so does this tool.

Keyboard

Left and Right move the selection, Home and End jump to the ends.
Enter halves, Backspace rejoins, F focuses, Esc clears the selection or zooms out.
Ctrl+Z undoes and Ctrl+Y redoes.