The DNS RFCs specify a set of standard resource record types and their syntax (for example, SOA, NS, A, MX, CNAME, PTR, HINFO, TXT, etc).

The DNS is designed to be extensible with new kinds of data, and new RR types are frequently defined whenever the need arises.

Different resource record types are used for different purposes. For example, SOA and NS are resource records used internally by DNS itself, whereas A and CNAME records are used by application programs, and MX records are used in e-mail delivery.