Extension was moved to: https://lab.civicrm.org/extensions/belgianpostcodes
This extension adds functionality to lookup city and province after entering a postcode.
Copy the files to your extension directory afterwards you can install the extension through the CiviCRM extension manager.
On the inline address edit form and on the contact edit form a piece of javascript is added. The javascript adds a new input field on the form. This field is an autocomplete field which autocomplete the postcode. After a postcode is selected or autocompleted the fields zipcode and city are filled.
The autocomplete calls civicrm/ajax/zipcodes/autocomplete which returns a list of matching postcodes.
Upopn installation the Belgium postcode database is inserted in the table civicrm_zipcodes