The Arduino Ethernet Shield allows an Arduino board to connect to the internet. It is based on
the Wiznet W5100 ethernet chip (datasheet). The Wiznet W5100 provides a network (IP) stack
capable of both TCP and UDP. It supports up to four simultaneous socket connections. Use the
Ethernet library to write sketches which connect to the internet using the shield. The ethernet
shield connects to an Arduino board using long wire-wrap headers which extend through the
shield. This keeps the pin layout intact and allows another shield to be stacked on top.