![ethernet shield](https://i0.wp.com/www.exploringarduino.com/wp-content/uploads/2013/06/ethernet-shield.jpg?resize=350%2C262&ssl=1)
Image courtesy of SparkFun
The Arduino ethernet shield can be used to connect your Arduino to the internet. The book explains how to utilize your Arduino as both a client and a server using this shield. You can also use it as an SD card shield.
Buy an ethernet shield from SparkFun, Adafruit, Amazon, or Cooking-Hacks.