For a couple of weeks, about the same time every day, I would get a call from the lab coordinator that the computers were having problems connecting to the Internet.

The network pref pane said that the Ethernet connection was Connected.

Symptoms: A user would try to get to a website, like ixl.com or pbskids.org and the progress bar would stall about mid-way, and the browser window was blank.

I could remote in via ARD and look at the Network settings. All were correct. They all had a unique IP address, within the range of 192.168.0.2-192.168.1.253.

Using Terminal, I could ping outside servers.

I could launch VMWare Fusion, and using a browser, running on Windows, I could reach those same sites.

I tried resetting Safari, clearing the caches in Firefox, reinstalling the browsers.

Then I found this thread: https://discussions.apple.com/thread/4093878?start=0&tstart=0

Solution: Turn off IP6 (!)

Open the Network Pref Pane, unlock using the pref pane

Click on "Advanced..."

Change the value for "Configure IPv6" from "Automatic" to "Off"

Click "OK"

Restart the iMac.

It works!