Today, in class we were discussing about how s-box is used in feistal ciphers. Our sir went on to say that for s-box substitution, we take the 1st and last bits as one part and the remaining bits as another, following which we find the matching 4 bits.
So if we were to have the 6bits as: "100100", it would map to 1110 (left column: 10 , right column: 0010)
My doubt now is if it possible for us to take different positions of bits together, like the 1st and 3rd bit? If not, is there a reason why we can only take the 1st and last bits?