Wiznet Ethernet shield - w5100

End-Of-Life Products & Services Tech support before 2012
2

Arduino uses digital pins 10, 11, 12, and 13 (SPI) to communicate with the W5100 on the ethernet shield. These pins cannot be used for general i/o.

please take a look at this page
arduino.cc/en/Main/ArduinoEthernetShield