Ethernet.MACAddress should be a getter, not setter https://github.com/arduino-libraries/Ethernet/blob/39103da0e1bc569023625ee4693272773397dbb6/src/Ethernet.cpp#L147-L153 ``` void EthernetClass::MACAddress(uint8_t *mac_address) { SPI.beginTransaction(SPI_ETHERNET_SETTINGS); W5100.getMACAddress(mac_address); SPI.endTransaction(); } ```