There are many different types available - make sure to get the "X2-Z11-EC-A" (like from SparkFun or Digi)

iDigi™ Developer Cloud

All XBees have to be configured as "ZIGBEE ROUTER AT", which means a firmware with the number 22xx. Change the settings to:

Setting Command Value
PAN ID ID AAAA
Scan Channels SC 1FFE
Channel Verification JV 1
Destination Address High DH 0
Destination Address Low DL 0

XBee Internet Gateway on code.google.com