Step 3: Get started
Next, open the Arduino development environment. I highly recommend upgrading to Arduino 1.0 or later (if you have not done so already). This version of the software has built in DHCP support, and does not require manually configuring an IP address.
To figure out what IP address has been assigned to your board, open the DhcpAddressPrinter sketch. This can be found at:
File --> Examples --> Ethernet --> DhcpAddressPrinter
Once open, you may need to change the Mac address. On newer versions of the Ethernet shield, you should see this address on a sticker attached to the board. If you are missing a sticker, simply making up a unique mac address should work. If you are using multiple shields, make sure each has a unique mac address.
Once the mac address is properly configured, upload the sketch to your Arduino, and open the serial monitor. It should print out the IP address in use.