Joined: 06 Jun 2008 Posts: 8218 Location: Dubuque, Iowa, USA
Go to IBM's web site. Find the Communications Server bookshelf on that web site. Read in the bookshelf until you've found out what you want to know about TCP/IP on the mainframe.
In general, connections between subsystems on a single machine are not done via TCP/IP but rather cross-memory services in one fashion or another. Connections from your PC's terminal emulator to the mainframe are accomplished via the mainframe's TCP/IP stack. So it's not just things like DB2, MQ, or CICS that use TCP/IP.
And the mainframe can, and does, act as client in some cases, server in other cases -- often at the same time for different processes. For example, if you use your desktop machine to FTP to the mainframe, the mainframe is acting as the server to your client. At the very same time, your coworker could be using FTP on the mainframe to send a file to a Unix machine -- in which case the mainframe is the client and the Unix machine the server.