ATM is one of the leading high-speed networking technologies which has found wide acceptance in the last few years. In order to allow conventional protocols such as IP to run over ATM, various standards have been defined by the ATM Forum. These standards allow traditional applications such as those using TCP/IP or UDP/IP protocols and those applications running over broadcast networks to run unchanged over ATM networks. In this paper, we compare the performances of two common IP implementations over ATM namely, Classical IP and LANE, using local area ATM network testbeds consisting of DEC Alpha workstations and Intel Pentium machines. All hosts run windows NT 4.0 and are connected to a DEC GIGAswitch/ATM. We use metrics such as application throughput, latency, and CPU usage for comparing performances since they have a direct impact on the Quality of Service that is delivered to end-user applications. In addition, we also explore experimentally the effects of ATM adapters, protocols (TCP or UDP), host architectures, and processor speeds on application performance when using Classical IP and LANE over ATM.