One of the most important protocols used to connect computers together is TCP/IP (Transmission Control Protocol / Internet Protocol). The need arises to analyze and measure the performance of LANs with IP. In the present work an experimental system for the performance evaluation of LAN with IP is described. The present system has been given the acronym EXTIPP: experimental seT-up for Internet Protocol Parameters evaluation. EXTIPP takes into consideration the existence of transmissions in the network made by users outside the environment. This effect is called the Background Load. EXTIPP consists of a number of stations connected together in a certain configuration and transmitting and receiving packets to record the parameters required to perform the evaluation process. EXTIPP shows that the service time (the time required to complete a transmission between two stations) highly increases with the increase in the background load for the TCP Protocol while it slightly increases for the UDP Protocol. The collisions also increase with the increase in the Background Load level for both protocols. The throughput decreases with the increase in the load level. The effect of increasing the packet length was shown to produce better results for the no. of collisions (i.e. less collisions) but with an increase in the service time.