They are simple PhoneNET to Ethernet bridges, and they work fairly well. They only support MacIP, and will not work with anything higher than 10.3.9 (10.4 killed off Classic AppleTalk).
A good thing to keep in the toolbox. It's really simple to setup: plug in the wall wart, plug in the PhoneNet cable (you'll need an adaptor for that if you don't have one, if it's a splitter you'll also need a terminator) and plug in the Ethernet cable. I don't know if a straight or cross over cable is used, I'll have to find out -- I just used the Ethernet cable it comes with (it has a couple of pins omitted).
A good thing to keep in the toolbox. It's really simple to setup: plug in the wall wart, plug in the PhoneNet cable (you'll need an adaptor for that if you don't have one, if it's a splitter you'll also need a terminator) and plug in the Ethernet cable. I don't know if a straight or cross over cable is used, I'll have to find out -- I just used the Ethernet cable it comes with (it has a couple of pins omitted).