Subnetris Student Guide

The block picture

The wide band across the middle of the screen is your whole allocation. Its CIDR sits above it. Inside every block are up to three lines: the block's prefix, its usable count underneath, and the name you assigned to it under that.

The usable count is the N-2 rule made visible. A /26 says 62 usable, not 64, because the network number and the broadcast are already spoken for. Cut a block and watch the two halves report a smaller total than the one they came from. As a block gets narrow the word drops off and the bare number stays, and narrower still the whole line goes; the table underneath always has it.

This is the same picture the lecture draws on the board. Nothing on screen is decoration: the width of a block is proportional to how many addresses it holds, so a /25 really is half the width of the /24 it came from.

Blue on the left, red on the right

Every block has a blue left edge and a red right edge.

The blue edge is the Network Number, the first address in the block. Its value is printed along the top of the band.
The red edge is the Broadcast, the last address in the block. Its value is printed along the bottom.

Neither can be given to a machine. That is the N-2 rule, and it is why the two rows are drawn on opposite edges: when you cut a block, the new bar grows a red edge on its left and a blue edge on its right, and you can see the two addresses that just stopped being usable.

The first block's network number and the last block's broadcast are always written out in full. The ones in the middle shorten to just the octets that change, so a /24 shows .128 rather than repeating the first three octets on every label.

Cutting a block

There are three ways, and they all do the same thing.

Drag inside a block. A dashed line follows your pointer and a solid line shows where the cut will actually land, with a label naming the address and how many halvings it takes.
Click a block. Same as a drag that did not move.
Select and press Enter. Point at a block to select it (its row in the table lights up and its details appear at the foot of the left panel), then press Enter to halve it. The diagram has to hold the keyboard for that, which it does once you have clicked in it or tabbed to 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. If you point at .100 in a /24, the cut goes to .128, and the readout says so:

You pointed at .100. Binary Law: a block is only ever cut in half, so the
cut goes at .128, the midpoint of 10.200.5.0/24 (one halving). Nearer cuts
need more halvings: .96 takes 3 halvings (a /27); .100 exactly takes 6
halvings (a /30). Cut again inside the new block to get there.

That second sentence is the useful one. It tells you that .100 is reachable, that it is a /30 boundary, and that getting there means halving six times. Addresses that are not on any boundary at all cannot be reached however many times you cut, and that is the point.

One halving per gesture

A gesture is always exactly one halving, wherever you point. That is the lecture's procedure: to reach .96 in a /24 you halve at .128, halve the left half at .64, then halve the right quarter at .96, three gestures, and the readout counts them for you each time. There is no setting that does several at once, so every cut on the picture is one you made on purpose. Cuts stop at /30, or at /32 when Allow /31 and /32 under Settings in the menu is on.

Rejoining

Click a division bar to merge the two halves under it back into one block. Anything you cut inside those halves disappears with them, and any names assigned inside them are released back to the list. With a block selected you can also press Backspace.

Ctrl+Z undoes and Ctrl+Y redoes. One gesture is one undo step, even when it performed several halvings.

Reading a network need

Each network in the Business needs panel states a host count and, under it, the terms that count comes with. A need might read:

Clinic                              10 hosts
plus 10% growth, gateway included

The number on the right is what the network was given. The line under it says what you still have to do with that number before it is the size you need. Subnetris never shows you the answer here. Working it out is the exercise, and the Check report is where the arithmetic gets spelled out afterwards.

Allowing for growth

plus 10% growth means the network has to hold ten percent more than it holds today. Ten hosts plus ten percent is eleven, so the block has to have at least eleven usable addresses. That makes it a /28, which has fourteen. A /29 has six, which is not enough.

Always round up. Twelve hosts plus ten percent is 13.2, and there is no such thing as a fifth of a computer, so the answer is fourteen. Rounding down would leave a device with nowhere to go.

Do the growth first and the rest afterwards. Growth applies to the hosts, not to the gateway.

Whether the gateway is counted

Every network with a host count says one of two things:

gateway included. The number already covers the router. A network of 110 needs 110 usable addresses, and one of those 110 is the gateway.
gateway not included. The router needs an address of its own on top. A network of 60 needs 61 usable addresses: sixty for the hosts and one more for the gateway.

This catches people out, and it is worth being deliberate about. The N-2 rule already took two addresses away, the network number and the broadcast. Convention A then spends one of what is left on the gateway. So a network described as "60 hosts and a gateway" needs a block with 61 usable addresses, which means a /26 with 62, not a /27 with 30.

When a need does not state a host count at all, it says nothing about a gateway either, because there is no number for the gateway to be part of.

Assigning a subnet to a network

An exercise lists the networks it wants in the Business needs panel on the left. Giving one of them a subnet is a drag, and it works from either end:

Press on the block you want to give away, then drag it left onto the network's row in the panel. The row lights up, the line under the diagram says what dropping will do, and releasing assigns it.
Or press on the network's row (its name or its host count, not one of its buttons) and drag it right onto the block. The row you picked up goes faint, the block under the pointer gets a dashed outline and its details appear in the card at the foot of the panel, and releasing assigns it.
The block then fills in, and the network's name appears inside it under the usable count. Its row in the panel shows which subnet it got and how many usable addresses that is.
Clear on that row takes the subnet back and leaves the block empty.
Dropping a different block on a network that already has one moves it; a network only ever holds one subnet at a time.

Two things worth knowing about the gesture. Drag within the diagram and you are cutting, as usual; it only becomes an assignment once the pointer leaves the diagram. And if you release somewhere that is not a network, or a network somewhere that is not a block, nothing happens at all: no cut, no assignment.

Dragging is the only way to assign, so if a network's row is off the bottom of the panel, scroll it into view before you start the drag from either end. On a touch screen, a row still scrolls the panel when you drag it up or down; it is a sideways drag that picks the network up.

Bowie's Hierarchy of Subnetting

Five rules, listed from the one you bend first to the one you can never break. Check reports against them in this order.

Rule	How firm	What it means here
Most elegant design	Change freely	The tidiest plan, the least wasted space. Subnetris notes waste; it never fails you for it.
Business need	Change reluctantly	Every network on the list gets a subnet that fits, and the exercise's own rules are met.
Convention	Follow unless told	The course conventions below. Reported as a note.
N-2 rule	Mostly unbreakable	Two addresses per subnet cannot be hosts. A network that is one address short fails here.
Binary law	Unbreakable	Networks divide in powers of two. The tool cannot draw anything else.

The rule you bend first is the most elegant design. The one you can never bend is binary law.

The conventions above are the lecture's defaults. An instructor can change them from Conventions in the menu (gateway first or last, statics from the start or the end, a pool or none, other DNS servers), and the choice travels in the link, so a link you were sent is read against the conventions it came with. Check's Convention card always states the ones in force.

The table, column by column

Under the diagram is the handout table, one row per subnet in address order.

Column	What it holds
Label	The name you assigned to this block, if any.
Network #	The first address and the prefix together, such as `132.235.0.0/26`. The address is the blue edge; the prefix is the slash notation the diagram prints on each block. They share a cell because those two are what you type into a router together, and the prefix also tells you the size: a /26 is 64 addresses.
Usable IPs	The addresses you can actually assign to machines.
Broadcast Address	The last address. The red edge.
Hosts	The total minus the network number and the broadcast.
Subnet Mask Dotted Decimal	The mask Windows shows you, such as `255.255.255.192`. Hidden by default; see below.
Binary	The same mask in binary. Where the mask has a 1 the bit is network; where it has a 0 the bit is host. Hidden by default; see below.
Gateway	The gateway you entered for that network in its addressing plan. The column appears on its own once you have entered a gateway for any network and goes away when none is left; a network you have not addressed yet reads `not set`. It never shows the gateway the conventions would give.

Pointing at a block on the diagram selects it, and so does clicking its row here: either way the row and the block light up together and the block's details appear at the foot of the left panel. The selection stays put when the pointer leaves the diagram, so you can move over to the panel and read it.

Two checkboxes beside the panel title, Hide binary and Hide dotted decimal, are both ticked to begin with. That leaves the five columns you need for most work, and they fit the width without scrolling sideways. Untick Hide dotted decimal when you want the mask Windows would show you, and untick Hide binary when you want to see where the boundary actually sits in the bits, which is the one thing binary shows you that dotted decimal does not.

Both settings are remembered, and Copy follows them, so what you paste is what you were looking at. A drill overrides them: if the drill asks you for the mask, that column appears whatever the checkbox says, because otherwise the question would vanish with the column.

The N-2 rule and the tax rows

Three rows sit at the foot of the table.

USABLE IPs: how many addresses across the whole allocation can still be given to a machine.
The gap between the two is the tax: what you have spent on network numbers and broadcasts. It grows by two every time you make another subnet, which is why USABLE IPs falls while Total IPs does not.
Total IPs: the size of the allocation. This never changes, no matter how you cut it.

Work the handout's five walkthrough tables in order and watch those three numbers. A whole /24 is 254 usable and a tax of 2. Four /26 networks out of the same /24 are 248 usable and a tax of 8. You did not lose addresses to arithmetic; you spent them on having four networks.

Watch the wording on a quiz. "How big is the subnet" means the total, which you read off the prefix in the Network # column: a /26 is 64 addresses. "How many hosts will it hold" means the Hosts column.

Course conventions

These apply to every subnetting assignment for the whole semester.

A. The default gateway uses the last usable address in the network. Subnetris shows it on the selection card and can ask for it in a drill.
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.

Filling in the addressing plan

Cutting the block and handing it to a network answers "how big". The addressing plan answers "which address goes where", which is the half you will actually type into a router.

Once a network has a subnet, its row in the Business needs panel grows an Addressing section. Open it and there are three things to fill in:

Gateway. One address, the router's own address on this network.
DHCP pool. Two addresses, the first and the last of the range the server may hand out.
Static addresses. A named row for each device that gets a fixed address. An exercise may already list the ones it wants, and you can add your own with the box at the bottom.

Every box takes a full address such as 10.200.5.126, or just the part that changes, such as .126. Shorthand is filled in from the block the network sits in, so you are never retyping an octet the diagram already shows you. An empty DHCP pool or static is unanswered, not wrong. An empty gateway is wrong; see below.

The badge beside the word Addressing says where you stand at a glance: not started, ok, a count of things to check, or a count of problems.

Remove deletes a static you added. On a static the exercise requires, the button reads Clear instead and only empties the address, because the exercise still wants that device addressed.

The gateway is yours to place

Nothing in the panel is filled in for you, and nothing tells you where the gateway goes. The box shows a.b.c.d until you type an address. Work it out from the conventions: under the lecture's defaults the gateway is the last usable address, so for 10.200.5.0/25 that is .126.

A network that has a subnet but no gateway fails Check under Business need, because a network without a default gateway does not work. This is the one addressing box that is not optional. Once you have committed an address, the panel and the report tell you where it departs from the conventions, so you can check yourself after working it out, never instead of working it out.

Problems and things to check

Subnetris separates the two, and the difference is the whole point.

A problem is a plan that cannot work. An address outside its own subnet, an address that is the network number or the broadcast, the same address given to two things, a pool that runs backwards, or a static sitting inside the pool where the server could hand it out again. Problems fail the exercise.
A thing to check is a plan that works but disagrees with a course convention. A gateway that is not the last usable address, statics that do not start at the beginning, a pool that does not fill the gap. These are notes, and the exercise still passes.

That split is Bowie's Hierarchy doing its job. A convention is a rule you follow unless you are told otherwise, so breaking one is worth a sentence rather than a failure. The N-2 rule is nearly unbreakable, so putting the gateway on the broadcast address is a failure.

Your plan travels in the share link along with everything else, so a link you send to a classmate or hand in carries the addresses you typed.

Exercises and the Check button

Exercises in the toolbar opens the library. It has three sections.

Exercises: the six small-group problems, escalating from two equal parts up to five departments in a /20.
Walkthrough: the handout's five tables on 10.200.5.0/24, already cut, so you can compare the tax rows across them.
Check yourself: the study-guide questions.

An exercise loads its network and its list of needs into the left panel. Cut the block, then drag each piece onto the network that should live there, as described under Assigning a subnet to a network. When every need has a home, press Check.

What each message means

Check opens its report in a window over the diagram: one card per rule, in the order of the table above, from the rule you bend first at the top to Binary Law at the bottom, and each card is either OK, FAIL or NOTE. Close it, fix what it names, and press Check again. The badge on each need in the left panel keeps the last result after the window closes.

Binary Law is always OK, and says why: the tool cannot draw anything but halvings.
N-2 rule fails when an assigned block is too small once the network number and broadcast are set aside. The message names the block you used, the usable count it really has, and the prefix you need instead.
Convention is a note. Once you have started a network's addressing plan it says where that plan departs from the conventions in force, and when nothing departs it states the conventions in one line. It never fails you.
Business need fails when a network has no subnet, when a network has a subnet but no gateway, or when the exercise asked for something specific that you have not done: exactly four equal parts, all of the space used, or the largest block going to the first name.
Most elegant design is only ever a note. It tells you when a block is bigger than the need, what would have fitted, and how many addresses you could reclaim. You can pass an exercise with a design note outstanding, and often that is the right answer.

Changing the tree clears the report, so a message on screen always describes what you are looking at.

Random exercises and codes

Open Exercises and choose Random from the category pull-down. It builds a fresh problem: pick a difficulty, decide whether you want public or private address space, and press Start. If your instructor posted a code, paste it into the Code box instead and press Start.

Tier	What you get
A	A /24 into two equal parts.
B	A /24 into four equal parts.
C	A /24 or /25 into three networks using all the space, largest first.
D	A /24 for three or four LANs by host count.
E	A /22 for four or five sites by host count.
F	A /20 for five departments plus a router-to-router link.

Each problem has a code such as SN1-DPK7M3Q. The same code always builds the same problem, so if your instructor posts one code, everyone in the class works the same numbers, and you can quote the code when you ask a question.

Drill mode

Drill in the menu hides the addresses on the diagram and turns the table into blanks. You fill in the network number, the broadcast, the mask and the usable count for each subnet, then press Grade.

Addresses accept the shorthand: .128 is fine.
The mask must be written in dotted decimal. Answering /26 is marked wrong with a hint, because that column is the dotted-decimal column.
Reveal shows the expected value beside every blank, Clear answers empties them, and End drill puts the numbers back.

Some exercises start in drill mode by themselves. Your typed answers never travel in a share link, so a drill link you send to a classmate arrives blank.

The address bar carries your whole plan: the allocation, every cut, the names you assigned and the exercise you are working. Bookmark the page or copy the link from Share and it comes back exactly as you left it.

The menu also opens the same plan in the ECT Visual Subnet Calculator, and a link from that calculator opens here with its divisions and labels intact, so you can move between the two tools freely.

The cheat sheet

Help carries the mask table from /22 to /32. Learn the /24 line and derive the rest by halving or doubling: 256 total and 254 usable at /24, halve to 128 and 126 at /25, halve again to 64 and 62 at /26. If you can rebuild that table from the /24 row you never have to memorise the whole thing, which matters because you can write it out on blank paper at the start of a quiz.

The table follows the course handout, where a /31 and a /32 have no usable addresses. Real equipment can use a /31 as a two-address point-to-point link under RFC 3021, but that is not what this course asks for.

Keyboard and accessibility

Left and Right move the selection along the blocks in address order; Home and End jump to the ends.
Enter halves the selection, Backspace rejoins it, F zooms into it, Esc clears the selection or zooms back out.
Ctrl+Z and Ctrl+Y undo and redo.
Cutting, rejoining, selecting and zooming can all be done from the keyboard. Assigning a subnet to a network is a drag only.
High-Visibility under Settings enlarges the type, thickens the edges and hardens the contrast, for projection or for reading at a distance.
Dark Mode starts on Light and cycles to Auto, then Dark, then back to Light. Auto follows whatever your computer is set to. Whichever you pick is remembered.

Tips that pay off

Say the size out loud before you cut. "LAN1 needs 110 hosts, so a /25 holds 126, that fits." Then cut once and check the table.
Sort the needs largest first. Place the biggest network before the small ones and the small ones always find room. Place a small one in the middle of the space and you can strand the big one.
Use the tax row as a smell test. If you have four subnets the tax must be 8. If it is not, you have more subnets than you meant to make.
Zoom in when the blocks get thin. Double-click a block to fill the screen with it, and use the breadcrumb above the diagram to come back out.
Count your halvings. One cut per gesture is the procedure you will be asked to reproduce by hand, and the readout tells you how many a nearer boundary would take.