Abstract. IPv6 have been examining at the next IP address standard. But IPv4 have to be used for a while by the following reasons: tremendous cost and efforts for converting to IPv6. One of the serious problems of the IPv4 addressing structure is the fact that is a shortage of IP addresses. The address shortage is derived by lots of unused addresses during IP distribution and IP subnetting design. We propose an effective subnet IP address calculation method on VLSM. Also, with the proposed subnet IP address management method, a web based subnet address management system is introduced. The web-based subnet IP management system offers convenience in VLSM-based subnetting. The proposed VLSM calculation method can give a simple and effective IP management.