DNS Name Server Data Storage: Resource Records and Classes
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Resource Record Classes
Finally, a historical note that needs to be mentioned. When DNS was first created, its inventors wanted it to be as generic as possible. To that end, they designed it so that a DNS server could, theoretically, provide name service for more than one type of underlying protocol. That is, DNS could support TCP/IP as well as other protocols simultaneously.
Of course, protocols have different addressing schemes and also varying needs for name resolution. Therefore, DNS was defined so that each protocol could have a distinct set of resource record types. Each set of resource record types was called a class. Technically, a resource record must be identified using both a class identifier and a resource record type. Like the resource record types, classes have a numeric code number and a text abbreviation. The class for TCP/IP uses the number 1, with the text code IN (for Internet).
In practice, this notion of multiple classes of resource records never took off; DNS is today, to my knowledge, only used for TCP/IP. (There may be some obscure exceptions.) Several other classes have been defined by RFC 1035 and are in the IANA DNS parameters list, but they are for relatively obscure, experimental or obsolete network types, with names such as CSNET, CHAOS and Hesiod. You'll still see this concept of class in the specification of DNS message and resource record formats, but there really is only class today: IN for TCP/IP. For this reason, in most cases, the class name can be omitted in DNS-related commands and data entries, and IN will be assumed by default.
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