June 1970 saw the establishment of “zip” numbers in Korea. The provinces are divided up into postal areas as can be seen by the map. Seoul and Kyong-Gi Province have postal numbers from 100 to 180. Kang-Won Province uses numbers from 200 to 250, The two Chung-Chong Provinces use 300 to 390. The Chollas and Cheju Island number from 500 to 590. Kyong-Sang Provinces go from 600 to 690).
The lowest regular postal number is 100 for Seoul Central Post Office and the highest listed in the official zip number book is 690-84 of Ulsan Korea (famous for cement and refinery). Each official post office has its zip number visible at the entrance in a large rectangular sign made up of white and blue plastic.