A system for simulating NAT (network address translation) performed in an internet protocol (IP) sharing device to be selectively operated in one of different network address translation (NAT) schemes includes a first network interface linking the IP sharing device with an internal network and a second network interface linking the IP sharing device with an external network. A Network Driver Interface Specification (NDIS) interface hooks packets incoming to the external network or outgoing from the internal network. A NAT unit has a mapping table and performs a network address translation of the hooked packets with reference to the mapping table, wherein the mapping table identifies IP addresses and port numbers of the packets to be translated by the NAT unit.