TCP/IP Tutorial and Technical Overview - IBM Redbooks

The following figure shows the general format of a resource record (Figure 12-4).
Figure 12-4 DNS general resource record format
Where:
Name The domain name to be defined. The Domain Name System is
very general in its rules for the composition of domain names.
However, it recommends a syntax for domain names that
minimizes the likelihood of applications that use a DNS
resolver (that is, nearly all TCP/IP applications) from
misinterpreting a domain name. A domain name adhering to
this recommended syntax will consist of a series of labels
consisting of alphanumeric characters or hyphens, each label
having a length of between 1 and 63 characters, starting with
an alphabetic character. Each pair of labels is separated by a
dot (period) in human-readable form, but not in the form used
within DNS messages. Domain names are not case-sensitive.
Type Identifies the type of the resource in this record. There are
numerous possible values, but some of the more common
ones, along with the RFCs which define them, are listed in
Table 12-2 on page 438.
Class Identifies the protocol family. The only commonly used value is
IN (the Internet system), though other values are defined by
RFC 1035 and include:
CS (value 2): The CSNET class. This has been obsoleted.
CH (value 3): The CHAOS class.
HS (value 4): The Hesiod class.
Chapter 12. Directory and naming protocols 437