Either store the "known IP's" in a configuration file, and read it into a ComboBox when you start the application, or you can pick up all the IP addresses on your LAN segment and load them into a ComboBox if you want to use only local ones.
For LAN IP's, see here:
Retrieving IP and MAC addresses for a LAN[
^] For obvious reasons, you cannot retreive all IP addresses on the internet ...