The FujiNet is a very cool peripheral available for 8-bit Atari systems.
Somewhere along the way, I learned of the ability to telnet over the FujiNet.
The requirements are simple:
For the first requirement, I have a FujiNet device.
For the second requirement, the terminal program can be found on the fujinet.online TNFS server at /networking/modem-programs.atr. As you can see, I mounted that image to drive 1 in the FujiNet configuration on my Atari 800:

For the third requirement, I added telnetd/xinetd to one of my Ubuntu VMs, and booted the Atari 800 into Ice-T XE.
To use telnet, you have to issue the ATNET1 command to the FujiNet. After that,
you use ATDT to telnet, such as ATDT192.168.1.37 (or a hostname; DNS works as well):

I think this is very cool -- I now can use my Atari 800 as an 80-column terminal connected to my home network!