What is the best way for using routerboard 750 in a place where only have 48v power ? please suggest cheapest and most reliable way . thanks
adding a resistor in a series to RB? =)
Is it a question or answer 
Please explain details . thanks .
24VDC .33A Active 802.3af POE Splitter with isolation
SKU: POE-24s-afi
It will accept 48v poe input.
Chupaka is telling a joke. Resistor is not a serious answer. You need a DC to DC convertor as tgrand said. Laird makes them. Here is a link to the data sheet.
http://www.roc-noc.com/pdf/laird/POE-xxs-afi_DataSheet.pdf
Tom
maybe not a serious, but the cheapest =) and your one is the most reliable 
Well, both cheap and reliable would be a voltage regulator circuit, a few external components and a heatsink. The LM117HV/LM317HV are adjustable 3-terminal positive voltage regulators capable of supplying in excess of 1.5A over a 1.2V to 57V output range. They are exceptionally easy to use and require only two external resistors to set the output voltage.
Normally, no capacitors are needed unless the device is situated more than 15 centimeters from the input filter capacitors in which case an input bypass is needed. An optional output capacitor can be added to improve transient response. The adjustment terminal can be bypassed to achieve very high ripple rejections ratios which are difficult to achieve with standard 3-terminal regulators.
