Provinces of Belgium
The country of Belgium is divided into three regions. Two of these regions, the Flemish Region or Flanders, and Walloon Region, or Wallonia, are each subdivided into five provinces. The third region, the Brussels-Capital Region, is not divided into provinces, as it was originally only a small part of a province itself.
Postal codes in Belgium
Postal codes in Belgium are numeric and consist of 4 numbers. The first digit indicates the province (except for the 3xxx numbers that are shared by the eastern part of Flemish Brabant and Limburg, the 6xxx that are shared between the Hainaut and Luxembourg province, and the 1xxx that are shared by the Brussels Capital Region, the western part of Flemish Brabant and Walloon Brabant). The more zeros though the higher the number of inhabitants of that city in the province. For example: Brugge is the capital and largest urban centre of the coastal province of West Flanders so it gets the 8000 code, the second city is Kortrijk and gets 8500. 
When writing the address, the postal code is put in front of the town name.