By default, your Adobe Campaign Classic instance is not accessible from various IP addresses.
If your IP address has not been whitelisted, you will not be able to login to the instance from this address. The same way, you may not be able to connect an API to your Message Center or Marketing instance if the IP address has not been whitelisted with the instance explicitly.
Control Panel allows you to set up new connections to your instances by whitelisting IP addresses ranges!
Once IP addresses whitelisted, you can create and link Campaign operators to them so that the users can access the instance.