What is 4 digit number in nomenclature of enzymes?

What is 4 digit number in nomenclature of enzymes?

      Reference - ncert 

                         Class 11

                         Page no 158

In ncert it is given that enzymes are named according to a 4 digit no. Now what is this four digit no and how it is assigned. 

Actually each enzymes have it's unique 4 digit no.By these numbers one by one we can conclude that this enzymes belong to which class, which subclass etc. 

For example for enzyme hexokinase 4 digit no is 2.7.1.1

Here 1st no indicates class ,2nd no to subclass,3rd no to sub-subclass,4th no to individual enzymes. 

At ncert level only classes are mentioned. Nothing is given about subclass etc.

Classes - There are six classes of enzymes hence first no is from 1-6. In ncert these classes are given sequence wise.(this is literature of ncert. it gives things,we think it's normal but there is reason behind each line written in ncert ).

So number given to classes - 

1. Oxidoreductase/dehydrogenases
2. Transferase
3. Hydrolases
4. Lyases
5. Isomerases 
6. Ligases.

Hence we can say that enzyme hexokinase belongs to Class transferases.