Around 1996-97, there was a Linux-based firewall/router that booted from a single 1.44MB diskette. iirc, it needed a very small amount of RAM too, something like only 8MB. The diskette could be used with write-protect enabled to prevent accidental (or malicious for that matter) modifications to the diskette contents. I believe it also had an SMTP relay. It was particularly useful for sharing dial-up and DSL connections at a time when commercial solutions would’ve been much more expensive than a stripped down repurposed spare computer with no hard drive. A bit hard to imagine now, but back then it was pretty fantastic to have an Internet connection shared between computers on a home or small business network.

Edit: Found its Wikipedia page! [0]

[0]: https://en.m.wikipedia.org/wiki/Linux_Router_Project

Boy, does that bring back memories. I used precisely such a repurposed PC with this back in the day, to juggle two DSL circuits into our office.

Of course, a thumb drive can host a far more sophisticated router these days.