494 PART IV Application Level TTL (Time to Live)--How long in seconds the record can be cached. Many ISPs use 2 or even 3 days for this field (172,800 or 259,000). If no value is entered, the default can be short (as little as 1 hour). Class--Today, the only class that counts is IN for Internet address. This is usu- ally entered only once, in the first record, and is inherited by all subsequent records for that name. Record-Type--There are many record types, usually indicated by a short abbreviation, such as A for address and NS for name server. The types fall into four categories: Table 19.2 Common DNS Resource Record Types and Their Uses and Meanings Use Record Type SOA Zone NS A AAAA A6 Basic PTR DNAME MX KEY Security NXT SIG CNAME Used to map an IP address to a host name in reverse zone lookups. Formerly used for redirection for reverse lookups in IPv6 DNS servers due to longer nature of IPv6 addresses. Now obsolete. Mail Exchanger records point from a name to A records that are the mail exchanger for the name. The public key for the DNS name. Used for negative answers with DNSSec. The signature for an authenticated zone. Maps an alias name to a canonical ("real") name. For example, www.example.com and ftp.example.com might both be running on the host server.example.com. Geographical location. Name Authority Pointer is used to allow regular expression rewrites of the domain name. Contact information for responsible person. Gives locations of well-known services. To add comments and information to the record. Meaning Start of Authority records identify the zone and set parameters. Gives an authoritative name server for the zone, and delegates sub- domains. Not the IP address of the name server, but a text field. Maps the name to the IPv4 address. Each device address requires a separate A record. Used to allow an IPv4 name server to return an IPv6 address. Intended as a transitional type. Now obsolete, these were used to map a name to an IPv6 address. LOC Optional NAPTR RP SRV TXT