OK, I think I’ve found the culprit. Zone Alarm does seem to be blocking UDP between host and client, and I can’t figure out how to stop it without completely disabling my Internet firewall. It thinks that the ethernet adaptor for the LAN is to the internet, and it won’t allow me to edit or change that. It’s the only firewall I have, so I can’t take it down.

I may have to upgrade from the free version to Zone Alarm Pro, because while the Help menu says that there’s an option for setting it up for ICS, it doesn’t seem to display it for the version I have.

[Update a few minutes later]

I finally figured out how to change the zone for the adaptor from “Internet” to “Trusted.” My LAN is working properly now, but clients are still not seeing the internet.

[Late afternoon update]

I’m having trouble thinking that it’s a Zone Alarm problem at this point, because I’m watching the log, and I’ve seen no activity on the LAN being blocked, even when I attempt an internet connection from a client.

I can ping the host machine, but I can’t ping anything on the internet, either by name or IP.

This is most frustrating.

[Update a couple hours later]

At Ian Woollard’s suggestion, I momentarily disabled Zone Alarm, and that was the problem. It seems to work if I reduce the security level for the Internet Zone from “High” to “Medium.”

I’m not sure that I can configure it more specifically without getting the full version, though.

Now the question is, do I spend the forty bucks on Zone Alarm Pro, or on a router…?

I’m inclined to the former, because I can buy it on line, and it will be a good belt-suspenders system for when I get a good hardware firewall up.