All LU and APPLID names are defined to the VTAM software in a control file called SYS1.VTAMLST. When a terminal requests a logical connection in the same network to a program such as CICS, VTAM only allows the connection if both the terminal LU and APPLID are defined in SYS1.VTAMLST. This provides an important security control since no terminal or program could talk over the network without being defined to VTAM.
How TCP/IP Works
TCP/IP consists of two parts: the IP part provides routing of messages among computers; the TCP part provides application support, passing messages to the programs for the various applications. A port number identifies each application. For example:
• Ports 20 and 21 are often used for File Transport Protocol (FTP)
• Port 23 is for remote logons
• Port 25 is for email
• Port 80 is for the httpd server (the program that talks to Internet Explorer over the Internet).
On the mainframe, TCP/IP is a started task that depends on UNIX System Services, the standard z/OS UNIX. From VTAM’s perspective, TCP/IP executes as an APPLID.