To work with this project, you must know your Arduino Ethernet Shield's MAC address and IP address to communicate properly over the Internet.
Current Arduino Ethernet Shields come with a dedicated and uniquely assigned 48-bit MAC (Media Access Control) address which is printed on the sticker. Write down your Ethernet shield's MAC address so you can refer to it later. The following image shows an Ethernet shield with the MAC address of 90-A2-DA-0D-E2-CD:
You can rewrite your Arduino Ethernet Shield's MAC address using hexadecimal notations, as in