What is the reference used for? The requirements for a reference for a DAC or ADC differ from a reference used for a comparator, for example.

ADC's tend to pull current pulses from the reference so the need for a low impedance reference source is required, whereas a comparator doesn't.

Also, there are issues of accuracy and temperature drift.

If you are using a comparator and need a 100mV reference and the accuracy and tempco of your 2.5v reference is acceptable, a resistor divider may be acceptable.

If you are needing a 100mV reference for an ADC, first check the data on the ADC to see if it will work with such a low reference and read carefully the requirements for the reference input. More information is required to give specific guidance here. I would suggest you review the various app notes on ADC's the various manufacturers have. Try maxim,analog devices,ti,etc