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Is Rendezvous Sharing More Than You'd Like?
Gropo asks: "I just got an email from my father who has just recently upgraded from OS 9 to Mac OS X on his PowerMac. He's connected to the 'net via Adelphia Cable and shares his TCP connection with my Mother's iMac via Software Base Station. He got a call from his neighbor (also running Mac OS X) who noticed 'My Father's Computer' show up on his network. My first thought was: 'He's picking up your AirPort signal' - alas the neighbor has no AirPort card. The neighbor *does* however also have an Adelphia cable modem. I asked him to scan for available afp:// servers and sure enough, a foreign machine showed up. What's the easiest way (if at all possible) to enable auto-detection for the local wireless LAN (useful for file and printer sharing within the household) yet remain invisible to other people also behind the cable companies' local DHCP box?"
In this particular case the problem is appletalk routing. Since you are creating a local subnet using the PowerMac as the router, you probably have appletalk activated on the wrong network interface. It needs to be on the Airport ethernet only and not on the wired connection. It can only be on one at a time so just switch it. - incidentally this won't change your ability to share info with the airported computer. You could also try blocking access to the appletalk port (548 IIRC) on your built in firewall. Alternatively, get a proper hardware firewall and use that to mask your subnet. Ultimately you need to be careful what services you enable on which interface as one of them is visible to the world and one isn't.
Cable modems are notorious for creating security openings. In many cases, you and all the other computers in your neighborhood are bridged onto a single network. So it's the same as if you were on one big LAN.
This issue affects your dad's computer whether or not your mom's computer is connected via it (the in-house network is just an extra wrinkle).
So you need to do a careful job of insulating your dad's computer from the outside network. Start by turning off all unnecessary services that could be carried on the Ethernet adapter. (i.e., make sure these services are not allowed to communicate over the Ethernet adapter. It's fine to let them run over the Airport adapter if your software base station is configured correctly, but you will have to discriminate between the two). OS X does a pretty good job of not loading too many services in the default configuration. But you can fine tune what's going on using OS X's internal firewall. You should also turn off any file or printer sharing on the Ethernet adapter (using the Sharing preference panel). I'm not sure whether you can turn off Rendezvous on one particular adapter, but if you can, that would be a good idea too.
Another way to restrict data from being sent over the Ethernet connection out to your neighbors, would be to install firewall or routing hardware between your Dad's computer and the cable modem. Then you won't really have to worry about reconfiguring your dad's computer at all. Anything that is labeled for "cable modem sharing" or "DSL connection sharing" should work fine for you. However, if you're going to get a connection sharing box, you might as well get one that can provide a connection directly to both your dad's computer and your mom's, so hers doesn't have to go through his to get to the Internet. There are plenty of cable modem routers out there that also include 802.11b support, and any of these should solve all your problems at once (i.e., they will hide your computers from your neighbors, and they will allow both of your computers to connect to the Internet independently via Airport or Ethernet). Apple's Airport base station is particularly nice, but there are other boxes in the $100 range that will work fine.