I2C can be used to connect up to 127 nodes via a bus that only requires two data wires, known as SDA and SCL.

SDA=4 ⇒ D2.
SCL=5 ⇒ D1

SPI is much simpler than I2C. Master and slave are linked by three data wires, usually called MISO, (Master in, Slave out), MOSI (Master out, Slave in) and M-CLK.

M-CLK ⇒ D5
MISO ⇒ D6
MOSI ⇒ D7
(SPI Bus SS (CS)is D8.)