W5100 Ethernet Shield is an Arduino  compatible ethernet shiled based on W5100 ethernet module. It has an in built slot for Micro SD card.

This Shield allows an Arduino board to connect to the internet. It is based on the Wiznet W5100 ethernet chip. 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.

Arduino uses digital pins 10, 11, 12, and 13 (SPI) to communicate with the W5100 on the ethernet shield. These pins cannot be used for general i/o.

The shield provides a standard RJ45 ethernet jack.

The reset button on the shield resets both the W5100 and the Arduino board

The shield contains a number of informational LEDs:

• PWR: indicates that the board and shield are powered
  
• LINK: indicates the presence of a network link and flashes when the shield transmits or receives data
  
• FULLD: indicates that the network connection is full duplex
  
• 100M: indicates the presence of a 100 Mb/s network connection (as opposed to 10 Mb/s)
  
• RX: flashes when the shield receives data
  
• TX: flashes when the shield sends data
  
• COLL: flashes when network collisions are detected