Just as the configuration directory is the iPlanet Directory Server 5.1 that is used for iPlanet server administration, the user directory is the iPlanet Directory Server 5.1 that contains the entries for users and groups in your enterprise.
For most directory configurations, the user directory and the configuration directory should be two separate server instances. These server instances can be installed on the same machine, but for best results you should consider placing the configuration directory on a separate machine.
Between your user directory and your configuration directory, it is your user directory that will receive the overwhelming percentage of the directory traffic. For this reason, you should give the user directory the greatest computing resources. Because the configuration directory should receive very little traffic, it can be installed on a machine with very low-end resources.
Also, you should use the default directory ports (389 and 636) for the user directory. If your configuration directory is managed by a server instance dedicated to that purpose, you should use some non-standard port for the configuration directory.
You cannot install a user directory until you have installed a configuration directory somewhere on your network.