I have an [Arduino Uno](http://arduino.cc/en/Main/ArduinoBoardUno) with Ethernet Shield as server, and I make requests on the Arduino through the Internet. I use two libraries to do it (Ethernet.h and SPI.h).

I want to check the client [IP address](http://en.wikipedia.org/wiki/IP_address), so I accept only [HTTP](http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol) requests from a known IP address (for example, 50.50.50.50) which is the static IP address in my office. How can I get the the client IP address on the Arduino?