The CIP/SCIP connection is initiated by the secondary device to the primary device. When you give a TSW an IP ID of 91 and point it to the IP address, hostname, or FQDN of the primary device, it doesn't matter what else is on the network with the same IP ID, it matters that the primary device's network status is up and it has a populated IP table and programming to establish what gets communicated between the devices.
The only time this is an issue, using your example, is if you were to have one processor, therefore one IP/hostname/FQDN, running multiple programs and multiple secondary devices (TSW's) trying to connect using the same IP ID. An IP ID can only be used once per primary device, not once per program, with the exception being things like EISC.
