user20324: I currently have an Arduino Ethernet Shield which is Mega Compatible. I've found on the Arduino Ethernet Library https://www.arduino.cc/en/Reference/Ethernet that the Shield uses different Pins on the Arduino Uno and Arduino Mega. How is this possible? The Ethernet shield isn't connected to those pins mentioned. 
Arduino Uno:
Pin: 13 - SCK
Pin: 12 - MISO
Pin: 11 - MOSI
Pin: 10 - SS for Ethernet Controller
Pin: 4 - SS for SD Card
Arduino Mega:
Pin: 52 - SCK
Pin: 50 - MISO
Pin: 51 - MOSI
Pin: 53 - SS for Ethernet Controller
Pin: 4 - SS for SD Card


user48790: Those 4 pins D50 to D53 are the ICSP pins which appear twice on the MEGA; the Ethernet Shield uses the second set of connections located in the middle of the Mega board.
The header files gets told (by the Arduino IDE) that the board is a MEGA so the compiler uses those pin numbers to connect.

20324: <p>I currently have an Arduino Ethernet Shield which is Mega Compatible. I've found on the Arduino Ethernet Library <a href="https://www.arduino.cc/en/Reference/Ethernet" rel="nofollow noreferrer">https://www.arduino.cc/en/Reference/Ethernet</a> that the Shield uses different Pins on the Arduino Uno and Arduino Mega. How is this possible? The Ethernet shield isn't connected to those pins mentioned. </p>

<p>Arduino Uno:
Pin: 13 - SCK
Pin: 12 - MISO
Pin: 11 - MOSI
Pin: 10 - SS for Ethernet Controller</p>

<p>Pin: 4 - SS for SD Card</p>

<p>Arduino Mega:
Pin: 52 - SCK
Pin: 50 - MISO
Pin: 51 - MOSI
Pin: 53 - SS for Ethernet Controller</p>

<p>Pin: 4 - SS for SD Card</p>

<p><a href="https://i.stack.imgur.com/HZUsw.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HZUsw.png" alt="Arduino Mega Pins"></a></p>
 48790: <p>Those 4 pins D50 to D53 are the ICSP pins which appear twice on the MEGA; the Ethernet Shield uses the second set of connections located in the middle of the Mega board.</p>

<p>The header files gets told (by the Arduino IDE) that the board is a MEGA so the compiler uses those pin numbers to connect.</p>
