A DNS server contains a "zone file" for each domain, and the zone file is made up of "resource records" (RRs). The most common are described below. See DNS.Forward DNS and Reverse DNS (A and PTR)The Address (A) record associates a domain name or subdomain with an IP address, which is the primary purpose of the DNS system. The @ sign identifies "this domain;" for example, A @ nnn.nnn.nnn.nnn means this is the IP of this domain.The Pointer (PTR) record provides data for reverse DNS, which is used for logging the domain name and verification purposes. Also called "inverse DNS," the PTR record is an option. See reverse DNS.Aliasing Names (CNAME)The Canonical Name (CNAME) record is an alias that points to other names. It is commonly used to map subdomains to the domain without having to enter the IP address again. For example, CNAME mail @ associates the mail subdomain to the this domain.DNS Name Servers (NS)Two Name Server (NS) records identify the authoritative DNS servers for the domain. Required for redundancy, the secondary name server queries the primary server for changes.Mail Servers (MX)The Mail Exchange (MX) record identifies the server to which email is directed. It also contains a priority field so that mail can be directed to multiple servers in a prescribed order.Text Record (TXT)A TXT record can be used for any documentation or identification purpose. It is also used to provide information to the SPF email authentication system. See SPF.Start of Authority (SOA)The Start of Authority (SOA) record contains the name of the primary DNS server, which must correspond to an NS record in the file.COMMON RECORD TYPES SUMMARYA (forward DNS - IPv4) associate subdomain to 32-bit IP PTR (reverse DNS) associate IP address to a subdomain CNAME (aliases) associate an alias to a domain SOA (name of primary nameserver) NS (name server) associate a domain to a DNS server TXT (documentation) DNAME (aliases) associate an alias to a subdomain MX (mail server) associate mail to a mail server AAAA (forward DNS - IPv6) (1st IPv6 record) associate subdomain to 128-bit IP A6 (forward DNS - IPv6) (future IPv6 record) associate subdomain to 128-bit IP.