There is the idea of restricting TSO access by VTAM terminal ID, but it is rarely used. I've never heard of using an IP address for this purpose. I believe others have already said it's a dumb idea, since workstations seldom have fixed IP addresses, and even if they have fixed IP addresses at your site, users often want to be able to logon from other workstations or from home using a VPN type interface to actually access your site's network.
Why does someone believe this "ip address restriction" is worth investigating? If a user is at my desk or i am at some user's desk, why does it matter if one of us log on via the "other person's" terminal.
I believe there is a big difference between security and paranoia. . .