Addressing

There are 3 values that allow you to organize your XBee module address space:

1. Individual module addresses
2. PAN (Presonal Area Network) IDs
3. Channels

For two modules to communicate, they must be on the same channel, have the same PAN ID and the destination address of the sender must match the address of the receiver.

Each XBee module has its own unique address, as well as a destination address that it sends its messages to.  The destination address can specify a single destination or it can be a broadcast address, which will be received by all XBee modules within range.  The broadcast address is 65535 (0xFFFF in hex). 

If you only have a couple modules, you'll probably never need to change the PAN ID or channel.  But if you're in an evironment with a bit more traffic, you might want to be sure that nobody else's messages are getting mixed up with yours, so this offers a nice option for that.

Unique  and Broadcast Addresses

XBee modules can be used with 16-bit addresses or 64-bit addresses.  16 bits already gives you 65536 possible unique values, but hey you never know when you might need number 65537, right?  Any time a module's address is less than 0xFFFE, it will use 16-bit addresses.  When the module's address is set to 0xFFFF or 0xFFFE, it will use a 64-bit address derived from its internal serial number.

There are 2 values that configure the destination address - DH (Destination High) and DL (Destination Low).  If DH is set to 0, as it is by default, the board will send to a 16-bit address.  To send broadcast messages, which will be received by all modules on that channel and PAN, set DH 0x0000FFFF to and DL to 0x00000000 (default value).