It is important to understand the search order for configuration files used by TCP/IP functions, and when you can override the default search order with environment variables, JCL, or other variables you provide. This knowledge allows you to accommodate your local data set and file naming standards, and it is helpful to know the configuration data set or file in use when diagnosing problems.
It is important to note that the z/OS® Communications Server environment consists of the TCP/IP address space, z/OS Communications Server applications, and the TCP/IP MVS™ applications. The TCP/IP address space functions are also referred to as the stack. The z/OS Communications Server applications refer to those applications using the z/OS UNIX socket API. The TCP/IP MVS applications refer to those applications written to the MVS APIs (for example, C, Sockets-Extended, CICS®, IMS™, and REXX). The TCP/IP stack and both sets of applications have some common (or global) configuration files, but they also use configuration files that are different.
Another important point to note is that when a search order is applied for any configuration file, the search ends with the first file found. Therefore, unexpected results are possible if you place configuration information in a file that never gets found, either because other files exist earlier in the search order, or because the file is not included in the search order chosen by the application.