By default the IP address of the client is shown in the PXE server. For normal purpose this should do it. But in case you need a reference that can be checked on the machine you can change the value.
Some options are Serial of the machine, Model or Vendor of the machine, Computer ID, CPU or NIC Serial.
You can do this in the file pxe-network.cfg and in below column:
# By default the computer's IP address will be displayed in the
# first column of each row. To display a different identifier,
# you can provide it below. Some possible options are:
# Serial, Model, Vendor, ComputerId, CPU, NICSERIAL
If something is not clear or you need help withthe setup of this, please let us know at firstname.lastname@example.org