I have Arduino Uno + Ethernet Shield w5100 (few years old). Recently my program stopped working and after some experiments I have found that Ethernet shield is not working correctly.
This simple code:
My question: is it really the Ethernet Shield, what is broken? Cannot be there some problem with my Arduino itself, e.g. its communication with the Ethernet Shield?
I've had that problem before using the W5100 and its generally due to a bad solder joint. One thing you might want to do is reset the board, push down on the W5100 chip, and see if it comes up with the proper IP address. If it does, then its a bad solder joint and you might want to go over it with a soldering iron.
Hope that helps
Yay! Its possible the pins on the ethernet board had an oxidation coating or were not making good contact with the Arduino. I guess that brute forcing it into the connector or perhaps jiggling it around a bit got the connection going. Congratulations!