The authors present a study of the performance achieved by user processes when using the IPC mechanisms as implemented in Berkeley Unix 4.2BSD in Ethernet based environments. The authors assess not only the impact that different processors, network hardware interfaces, and Ethernets have on the communication across machines, but also the effect of the loading of the hosts and communication media that participate in the interprocess communication mechanism. The measurements highlight the ultimate bounds on performance that may be achieved by user process applications communicating across machines, and serve as a guide in designing performance-critical applications. A detailed timing analysis is presented of the dynamic behavior of the TCP/IP and the UDP/IP network communication protocols' implementation in Berkeley Unix 4.2BSD.>