On a linux box, plug into only the yellow network.
In Firefox, turn off the web proxy settings (via Edit --> Preferences).
Start up wireshark undo sudo.
Select the interface your machine is connected on.
Try to go to some site, like cnn.com or calvin.edu.
Q1: What destinations do you see in the packets in wireshark? What ports are the packets destined for?
Q2: Why can't you get the site you chose?
Q3: Can you do DNS? Could the name "cnn.com" (or whatever you chose) be resolved to an IP address? If not, why not? If so, why?
Now, turn your web proxy settings back on: 192.168.0.1, port 80, for all protocols.
Clear your capture in wireshark.
In firefox, go to some junk URL: I typed "junk.asdlfkjasdf".
Stop the capture and see what you see.
Q4: Report on the details of what you see? Is junk.asdfkjasdf converted to an IP address? Where are the http packets going to? Where does the error message you see in the browser come from?
Start a new capture.
Now, go to a good site: calvin.edu or cnn.com or slashdot.org or something. (Don't go to google.com as that always uses https now, and that makes things confused.)
Q5: Describe what you see in your capture. Where is the traffic to/from? How is the URL of the destination communicated?
Q6: Describe what a web proxy must do to the http requests it gets.