How to assign IP manually for an MS connected to OpenBTS?

Hi all, 

By default, once an MS is connected to the OpenBTS network it assigns and IP of subnet `192.168.99.X` automatically.

I want to assign IP manually to all the devices connecting to the OpenBTS network.

What part of the code do I need to modify to do the same?

Or is there any option in GPRS / SGSN for the same? 

Regards.