There is the recipe for connecting an a600 to the net over a serial link:
Ingredients:
- A fonera 2.0g router (in developer mode)
- A serial to USB adapter and the corresponding module (driver).
- Extra slip module for the fonera (compiled by me).
- Some slip tools & scripts for the fonera (compiled and wrote by me).
- Some extra iptables rules for the fonera's firewall (added by me, included on the scripts above).
- A null modem cable.
- An unexpanded Amiga 600. I've installed a 64Mb CF as HD, but you can built It from floppies.
- Workbench 2.x+
- AmiTCP 4 (demo version, of course).
- If you want a webserver, add the arexxwebserver package...
Preparation:
- Plug all cables and machines together.
(fonera usb port<---> USB to serial dongle <---> Null modem cable <---> Amiga 600 serial port)
- Turn on the machines.
- Install all the stuff into the fonera and restart It.
- Run Workbench.
- Install and configure AmiTCP to match your network settings using slip protocol.
- Configure arexxwebserver if you want It
- Run AmiTCP.

That's what I did, and It works like a charm.
Of course, It's very sloooooooooooooow... But I's amazing what you can do with a 1Mb Chip Ram machine.
Now you can do networking betwen your slip machine (the amiga) and your wired and wifi machines, and/or internet too...
The only advice:
The most reliable speed I got with serial.device was 9600 bps. You can use 19200 bps if you want, but you'll lose a lot of packets (about 25% on my tests).
You can use also cslip instead of plain slip, but It gives no much speed increase, and a bit more of cpu power load...
Extra tip:
Baudrate: 9600, hardware handsake (7 wires or CTS/RTS)
MTU: 576
I'll post all the details later (I'm at work now).