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”
Restart the iMac.