NGN is a convergence network. The Internet resource naming and addressing is very important to achieve the convergence. ENUM is introduced as the addressing technology between the telephone network and the Internet. It is a special application of DDDS algorithm and adopts the NAPTR type to store the rules in DNS. An introduction of the fundamental theory of ENUM system is provided, discussed the fundamental scope. The DDDS algorithm is analysed, specially expound the NAFFR RR, then puts forward a more strictly classification of the enumservice according the definition in RFCs.