In the old country, the most important number to remember is your personal identification number. It is easy to memorize because it contains your birth date and the magical “four digits”, the unique combination that newborn babies are allocated automatically and resident aliens on application. To receieve your four digits is, for an immigrant, to be accepted as a rightful community member. Many people complain that personal number is an intervention into their integrity, a sign of the Big Brother, but it proves very convenient. You are asked the number in all contacts with authorities, from car insurance to blood test.
Here in England you are instead asked for your postcode. Since many people obviously share a postcode I cannot really imagine how it works but it does. And apparently your identity changes every time you move.