A method and a system for inter-working an ANSI type radio access network (RAN) in an Internet Protocol (IP) based core network (CN) is disclosed. The method and a system provide for matching primary network components, such as the mobile switching center (MSC) and the base station controller (BSC), so as to inter-work an ANSI Type RAN in the IP based CN, while causing minimum modifications in the currently operated ANSI type RAN and establishing the IP based CN for accommodating the IP based ANSI type RAN. A unified communication network is thus provided by matching the RAN and the CN based upon the IP using a method and a system for signal connection and traffic transfer connection required for the network matching. As a result, a CN may be established, having the transfer path between a RAN and the CN based upon common technology such as IP. Furthermore, because the present invention accommodates a RAN directly within the IP basis, no separate conversion device is required for the wireless Internet service provision.