Would that not depend upon the site and therefore be something you have to get from the TCP/IP parameters, your coworkers, your team leader, your managers, or your site support group?
A 3270 emulator does not provide any access to a mainframe. That either comes with the job, or in some cases you can rent access to one (the cost to do this can be substantial, however). All a 3270 emulator does is provide a way for a microcomputer to emulate the 3270 terminal that is used with a mainframe.