طبق اطلاعات سایت رفرنس:
Arduino communicates with both the W5100 and SD card using the SPI bus (through the ICSP header). This is on digital pins 10, 11, 12, and 13 on the Uno and pins 50, 51, and 52 on the Mega.
اینجاست:
https://www.arduino.cc/en/Main/ArduinoEthernetShield