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Based on the Wiznet W5100 and directly supported by Arduino official Ethernet library. It adds a micro-SD card slot, which can be used to store files for serving over the network. It is compatible with the Arduino Duemilanove (168 or 328), Uno as well as Mega (1280/2560) and can be accessed using the SD library.
The Wiznet W5100 provides a network (IP) stack capable of both TCP and UDP. It supports up to four simultaneous socket connections.
WARNING: default WebServer sketch does not initialize SD card chipselect, so the card physically present in the socket may interfere with W5100 chipset. Until you add the SD chipselect initialisation, just remove the card from the socket.
Use the SD library included with Arduino IDE to write sketches which read/write to the microSD card. No additional configuration is necessary, the library should work with default configuration.