What is the purpose of the S-boxes in DES?

            The role of the S-boxes in the function  F is that the substitution consists of a set of eight S-boxes ,each of which accepts 6 bits as input and produces 4 bits as follows: The first and last bits of the input to box Si form a 2-bit binary number to select one of four substitutions defined by the four rows in the table for Si.The middle four bits select one of the sixteen columns.The decimal value in the cell selected by the row and column is then converted to its 4-bit representation to produce the output. For example, in S1,for input 011001,the row is 01 and the column is 1100.The value in row 1,column 12 is 9,so the output is 1001.