DougMorgan:
If you mean the DHT22 temperature/humidity sensors they do not have a mac address. Each sensor needs it's own single bi-directional digital I/O line to the Arduino. Adafruit has libraries for this sensor available on line with examples. I've never tested it but you should be able to create multiple instances of the DHT object connected to different pins.
Doug
That is correct.
I have four DHT11 and two DHT22 sensors on my system at http://219.88.69.69/2WG/ - check the Recent Climate and Climate for Week web pages. You can identify the DHT22 sensor readings because they provide temperature and humidity readings to one decimal place.
All of the DHT sensors on my system simply have pin 3 plugged into separate and consecutive pins on my Arduino board. This allows all the sensors to be read within a do loop and the results to be stored within arrays. The DHT sensors all share one common power and ground connection to the Arduino board.
My only use of a MAC address is to assign one to the Arduino board and then let my router assign the board a fixed internal LAN IP address when the Ethernet.begin() procedure is invoked within the setup() procedure. By using a fixed LAN IP address on the board my router can route www http port 80 traffic coming into my broadband modem (via my external/public IP address) to the Arduino board to enable my www user interface to operate using the EthernetClient object.
On my Arduino web site as above I am publishing some of the underlying code - more will be released over the new few days (weekend).
Cheers
Catweazle NZ.