Online Use
Many South Korean websites require the submission of a valid resident registration number in order for users to register an account. This practice ties each registered account to a unique online identity, rather than allowing anonymous registration. Because only a few large websites allow alternate means of identification, such as an alien registration number or passport number, foreigners cannot use many South Korean websites.
The principal means of authenticating a valid resident registration number is to use an algorithm to check the last digit against what it should be based upon the rest of the digits entered.
After SK Communications(Nate ON, most popular messenger at that time in Korea) was hacked in 26 July 2011, which about 70% of all Korean person's number was hacked, all sites of South Korea must delete and can't use the number anymore except payment use. From that time, this number has replaced with receiving identification SMS(use phone owner's info), Public key certificate(must visit one of banks once to receive), i-PIN(Internet Personal Identification Number, register governmental site with RRN and receive as alternative number-requires Active X), etc.
Read more about this topic: Resident Registration Number