IP applications use specific IP ports for sending and receiving data and listening for incoming connect requests. If multiple applications attempt to use the same port, this results in unpredictable behavior and possibly security violations. For applications to perform as expected, you should reserve the ports on behalf of the applications. The port number, the protocol (TCP or UDP), and the job name are required. Once the port is reserved for a specific job name, IP knows to route the inbound data to the application identified by the job name.
Ports reserved for UDP applications are separate and distinct from ports reserved for TCP applications. Therefore, it is legitimate to reserve the same port for a TCP application and a UDP application.
You cannot reserve a UDP port for more than one job name. In general, you should not reserve the same TCP port number for two different job names. This may result in undesirable behavior. There are two exceptions:
Similar applications may use the same port for load balancing. In this case the Advanced option, "Share ports across multiple listeners" must be enabled.
Dissimilar applications may use the same port, but listen on unique incoming IP addresses. In this case the Advanced option, "Bind an IP address" must be enabled. The most common use for this option is with the TN3270 server and the TELNET server. Both are defined by the IETF to use the well known port 23. To allow for this, you would reserve port 23 for both job names, and enable the "Bind an IP address" function for the TELNET server.
You are allowed to reserve the same TCP port for different job names without enabling the "Bind an IP address" or "Share ports across multiple listeners" functions. This is because applications may have the intelligence to handle this at run time. However, reserving the same TCP port for multiple applications is not common and should be done with caution.