There is a restriction on Banknifty options strikes range due to a violation of broker level open interest of 15%.
The range for today, Wednesday, 2 Aug 2017 is 2%.
Banknifty currently is at 25160. You can trade option strikes plus or minus 2% from this ltp, which is +/- 500 points.
Refer to this NSE link for more explanation on open interest violations.