Mighty 1284p SPI / Ethernet how to get it work ?

Using Arduino Networking, Protocols, and Devices
14

Try changing, in the code I posted:

#if defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
  inline static void initSS()    { DDRB  |=  _BV(4); };
  inline static void setSS()     { PORTB &= ~_BV(4); };
  inline static void resetSS()   { PORTB |=  _BV(4); };

to:

#if defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__) || defined(__AVR_ATmega1284P__)
  inline static void initSS()    { DDRB  |=  _BV(4); };
  inline static void setSS()     { PORTB &= ~_BV(4); };
  inline static void resetSS()   { PORTB |=  _BV(4); };

I didn't look further than that. However it appears that the 1284P is not catered for in w5100.h. Maybe search for similar things in the Ethernet library.