Keywords: This paper evaluates the performance of the IP protocol having ATM technology as the network support. Two models are approached: "The Classical IP and ARP over ATM" and "LAN Emulation". The theoretical performance was calculated based on the protocol overhead sources and compared to practical experiments. It is shown that: for applications working in IP over ATM environment, when we use short blocks of information transfer, the performance deteriorates considerably; and when we increase its size the performance, on the average. is limited about 86% of the line speed capacity. The influence of MTU size of the IP layer in the performance is also analyzed.