Is there a difference between the pins in the Arduino UNO Ethernet and the Ethernet shield?
Arduino Ethernet doesn't have a USB connector, so you must program the board via external ISP; the principle is the same. Using an Ethernet shield on top of an Arduino board has many advantages; e.g., we can have more pins when the shield removed and the device has a USB connector. Remember that pins 10-13 are used by the Mega328 to communicate with the W5100, so these pins cannot be used when Ethernet mode is on.