Let’s start by going back in time to the 1980s. This was when one of the most widely used protocols on the Internet – DNS –was developed. In case you are new to DNS or need a refresher, take a look at this detailed post on DNS before reading further.
DNS uses UDP as the transport layer protocol, except in some cases where it can switch to TCP. Thus, the size of the DNS message is limited to 512 bytes when using UDP. The basic DNS message begins with a fixed 12-byte header, followed by four variable-length sections: