draw.io is my go to for most engineering type drawings.
I just want to use something which is free and open source if that exists.
You can use Inkscape (free and open source), this one I did using Inkscape 1.2, so you can have an idea.
Here is the Inkscape svg file. Be aware that there is no "auto" nothing, you have to draw all this by yourself.
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Mermaid is great for diagrams in markdown style:
"Generation of diagram and flowchart from text in a similar manner as markdown"
See https://github.com/mermaid-js/mermaid
Github has also added mermaid support in markdown recently:
See https://github.blog/2022-02-14-include-diagrams-markdown-files-mermaid/
Online playground: https://mermaid-js.github.io/mermaid-live-editor
There is an open issue regarding network diagram support in mermaid: https://github.com/mermaid-js/mermaid/issues/1227
Alternatives:
Awesome! That piece of software is 37 years old and still maintained 
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Another great open source alternative: