# Packet diagram (PacketDiag)

PacketDiag maps bit ranges onto a fixed word width and draws the grid. It removes the entire class of errors that comes from drawing header diagrams by hand.

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## What this example shows

* colwidth, which sets the word size.
* Inclusive bit ranges per field.
* Single-bit flag fields.

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## A worked example

Open it, edit it, or copy the source below.

![Packet diagram (PacketDiag)](https://help.gocapable.com/images/att1423835219.svg)

The source, which you can paste into a new diagram and edit:

```
packetdiag {
  colwidth = 32;
  node_height = 60;

  0-15: "Source port";
  16-31: "Destination port";
  32-63: "Sequence number";
  64-95: "Acknowledgement number";
  96-99: "Offset";
  100-105: "Reserved";
  106: "U";
  107: "A";
  108: "P";
  109: "R";
  110: "S";
  111: "F";
  112-127: "Window";
  128-143: "Checksum";
  144-159: "Urgent pointer";
}
```

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## Making it your own

* colwidth must match the real word size or every offset misleads.
* Ranges are inclusive, so 0-15 is sixteen bits.
* For byte-oriented structures with nesting, use Bytefield.

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## Related

[Rack diagram (RackDiag)An elevation that matches the room.](https://help.gocapable.com/diagrams/rack-diagram-rackdiag.html)

[System context (Structurizr)One model, and views onto it.](https://help.gocapable.com/diagrams/system-context-structurizr.html)

[More languagesBack to the rest of these examples.](https://help.gocapable.com/diagrams/more-languages.html)

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_Specialists. Borrow one when you need it._