Why do you need separate databases/files/tables?
You can put all this data in a single table and use a find to pull up all matching records that start with a given IP address fragment.
Define your drop down list formatted field as a global field.
Put a button next to your IP address drop down and use it to perform a script such as:
Enter find mode
Set Field [Yourtable::YourIPAddressField; YourTable::YourGlobalDropdownField & "*"]
If [get (foundcount) > 0]
Go to Layout [select a layout set up to display results in a table or list view]
Ugh - you are right! I am making this way more complicated then it needs to be! Thanks for reminding me!