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This paper describes the system design and implementation of a demonstrator prototype, running at University of Bremen, which implements Mobile IP handoffs between GPRS and WLAN systems. Mobile IP requires public IP addresses but there is no support of such IP addresses by most mobile operators. To solve the problem of NAT (network address translation) and firewalls resulting from this, several concepts are available. This paper presents a practical implementation of one such concept where Mobile IP handoffs are realised using tunneling functions and an additional Mobile IP foreign agent to provide GPRS network access. Furthermore, this paper presents measurement results of the test bed and shows the quality of the Mobile IP handoffs.