I am having an issue with an Ethernet Shield on an Uno not responding to ARP requests. I have verified the MAC and IP addresses are unique and correct and on the same subnet (no router involved at all). The ethernet 'link' and '100Mbs' LEDs are lit. And I can see the activity LED on the Ethernet shield flash when the ARP request is sent. I have tried this through a switch, and point to point with a crossover cable, with no luck. I have sniffed the segment and I see the arp request being sent, but no response from the ethernet shield.
Is there something special that needs to be done for the shield to reply to ARP requests?
I just modified some of the example code from the IDE.
Just picked one at random, did you?
I know you didn't, and I know nothing about ARP or what is sending the data to the Arduino with Ethernet shield. I just know (some of) the kinds of questions that need to be answered in order to resolve your issue.