Microchip's ENC28J60 is a 28-pin, 10BASE-T stand alone Ethernet Controller with on board MAC & PHY, 8 Kbytes of Buffer RAM and an SPI serial interface. Target applications include VoIP, Industrial Automation, Building Automation, Home Control, Security and Instrumentation.
The ENC28J60 runs at 3.3V, while the ATmega328 runs at 5V. Normally this is a major problem,
but input are 5v tolerant . That means no level shifting is needed on the 5V lines controlled by the ATmega328 (CS, SCK, SI). The Tristate Buffer is used to shift the lines controlled by the ENC28J60 (SO, INT) from 3.3V to 5V. The lines never need to be put in a high impedance mode, so just tie the output enable line to ground."
Here is the schematic:
The PCB layout:
