The original definition of DNS protocol began in the late 1980s, and it used UDP and TCP protocols on its ports.

UDP is usually used for query and response, while TCP is used for regional transmission between master server and slave server. Unfortunately, the maximum packet length to ensure normal operation in all UDP implementations is 5 12 bytes, which is too small for some new DNS functions (such as DNSSEC) that must include a digital signature in each packet.

The limit of 5 12 bytes will also affect the number and name of root servers.

To make all the root server data contained in a 5 12 byte UDP packet, the number of root servers can only be limited to 13, and each server should be named by a single letter in the alphabet.