In the past years several operational <i>D<sub>st</sub></i> forecasting algorithms, based on both IMF and solar wind plasma parameters, have been developed and used. We describe an Artificial Neural Network (ANN) algorithm which calculates the <i>D<sub>st</sub></i> index on the basis of IMF data only and discuss its performance for several individual storms. Moreover, we briefly comment on the physical grounds which allow the <i>D<sub>st</sub></i> forecasting based on IMF only.