How to play Net
A grid of tiles, each holding a piece of network cable: a straight run, a corner, a T-junction or an end with a computer on it. The red server sits in the middle. Turn the tiles until every computer is wired to the server.
The puzzle goes by many names: Net, NetWalk, FreeNet, Pipes. Boards come in three sizes, 5 × 5 to 9 × 9. Every one is generated in your browser and has exactly one answer, which you can reach without guessing.
Rules
- Each tap turns a tile a quarter turn. Tiles never move or swap places; only the way they point changes.
- The puzzle is solved when every tile is connected to the server. Then every cable end meets another one, nothing points off the edge of the board, and there are no loops.
- Tiles connected to the server fill with ink and the rest stay hollow, so you can always see how far the network reaches.
- Lock a tile you are sure of and it can’t be turned until you unlock it. Locked tiles are shaded.
- The clock starts with your first turn and stops when the last computer comes online. Each size remembers its own board, and the clock pauses while the tab is hidden.
- Par is the fewest quarter turns from the deal to the solution, a turn either way counting as one. Undone turns and locks don’t count.
Controls
- Click / tap
- Turn a tile clockwise
- Right-click
- Turn a tile anticlockwise
- Long-press / middle-click
- Lock or unlock a tile
- Arrows / WASD
- Move the keyboard cursor
- Space / Enter / E
- Turn the tile under the cursor clockwise
- Q / Shift+Space
- Turn it anticlockwise
- L
- Lock or unlock it
- Z / Ctrl+Z
- Undo
- R
- Back to the deal
- P
- Pause
Tips
- Start at the edges. No cable may point off the board, so a corner piece in a corner of the board has only one way to sit, and a T on an edge must keep its stem pointing inwards.
- Two computers side by side can never be joined: they would make a little network of their own. The same goes for any group of tiles that would close itself off from the server.
- A straight has only two ways to sit, and next to an edge it has to run along it.
- Lock each tile as soon as you are sure of it. The locks build a frame of settled tiles to reason from, and a stray tap can’t undo them.
Net strategy
Work in from the edges
Nothing may point off the board, so the border settles first. In a corner of the board a corner piece has one way to sit; on an edge a T has one (bar along the edge, stem in), and a straight has to run along the edge. Each settled tile tells its neighbours whether a cable comes in from that side, and the border often fills in all the way round before you reach the middle.
No little islands
Every tile must reach the server, so two computers side by side never point at each other. More generally, if joining two groups of tiles would leave them with no free cable end, that join is wrong: the group would be cut off for good. This settles many tiles in the middle of the board, where the edges no longer help.
No loops
The cables form a tree. There is one cable fewer than there are tiles, so a single loop would leave some tile cut off. When two tiles are already joined by a path through the network, a cable straight between them is out.
A short what-if on 9 × 9
On the largest board the generator sometimes relies on a one-step test: turn an unsettled tile one way and follow the direct rules. If that leaves a cable end with nowhere to go, a loop or a closed-off group, that orientation is wrong. Since each board has exactly one solution, a contradiction is proof. Lock tiles as you settle them so the board shows you what is known.
About Net
Turning tiles until the cables join into one network is an old computer puzzle. One of its earliest known versions is KPlumber, which was ported to Windows in 2000 as Linkz. It spread as FreeNet and NetWalk, and in 2005 Simon Tatham made a version for the web that he called Net. In 2004 researchers showed that solving such boards in general is NP-complete.
Here a random tree of cables is grown out from the server, never more than three cables to a tile and with long straight runs made rarer, and then every tile is turned at random. A board is kept only if a logic solver can rebuild the network from the turned tiles. It settles the sides every remaining orientation agrees on and rules out cables that would close a loop or cut a group off from the rest; on 9 × 9 it may also try an orientation and drop it when that runs into a contradiction. Whatever it finishes has exactly one solution.
Questions
Can I solve it without guessing?
Yes. The generator keeps a board only if its logic solver can finish it, and that solver never guesses. On 5 × 5 and 7 × 7 every step follows directly from the edges, loops and closed-off groups. On 9 × 9 you may sometimes need a short what-if: turn a tile one way, follow what that forces, and turn it back if something breaks.
What does par mean?
Par is the fewest quarter turns that take the dealt board to the solution, a turn either way counting as one. Locks and undone turns don’t count. It is a target for tidy solving; your time is what goes on the boards.
Is this the same as Pipe Mania?
No. In Pipe Mania you lay new pipe pieces against the clock before the flooz arrives. In Net every piece is already on the board, and you only turn them until the network is whole.
Is my board saved if I leave?
Yes, in your browser, one board per size. The clock pauses while the tab is hidden.
