When using two or more EtherTen boards (or other Arduino-Ethernet hardware) on the same network you need unique MAC addresses for each EtherNet device. In the past various solutions have included using random numbers within the sketch to generate a MAC, or working with the serial number from a unique 1-wire or authentication device. However a simpler (and cheaper) method has been created by Freetronics forum member "RedDogInCan".
Their solution uses a sketch that you run once per board - it accepts a MAC address from the user via the serial terminal and writes this to the Etherten microcontroller's EEPROM. The by using the code provided, each board can then retrieve the MAC from EEPROM and pass it to the Ethernet library.
When putting together your next Internet-enabled Arduino project - save time, space and money with the Freetronics EtherTen. Apart from being fully Arduino Uno-compatible, it has onboard Ethernet, microSD socket, full USB interface (so you don't need a costly FTDI cable just to upload a sketch!) and supports optional Power-over-Ethernet.