DNS service for a domain is managed on the set of name servers first introduced "in.named and DNS Name Servers". Name servers can manage a single domain, or multiple domains, or domains and some or all of their corresponding subdomains. The part of the namespace that a given name server controls is called a zone; thus, the name server is said to be authoritative for the zone. If you are responsible for a particular name server, you may be given the title zone administrator.
The data in a name server's database are called zone files. One type of zone file stores IP addresses and host names. When someone attempts to connect to a remote host using a host name by a utility like ftp or telnet, DNS performs name-to-address mapping, by looking up the host name in the zone file and converting it into its IP address.
For example, the Ajax domain shown in Figure 28-7 contains a top domain (Ajax), four subdomains, and five sub-subdomains. It is divided into four zones shown by the thick lines. Thus, the Ajax name server administers a zone composed of the Ajax, Sales, Retail, and Wholesale domains. The Manf and QA domains are zones unto themselves served by their own name servers, and the Corp name server manages a zone composed of the Corp, Actg, Finance, and Mktg domains.
The DNS database also include zone files that use the IP address as a key to find the host name of the machine, enabling IP address to host name resolution. This process is called reverse resolution or more commonly, reverse mapping. Reverse mapping is used primarily to verify the identity of the machine that sent a message or to authorize remote operations on a local host.
The in-addr.arpa domain is a conceptual part of the DNS namespace that uses IP addresses for its leaves, rather than domain names. It is the part of your zone that enables address-to-name mapping.
Just as DNS domain names are read with the lowest level subdomain occupying the furthest left position and the root at the far right, in-addr.arpa domain IP addresses are read from lowest level to the root. Thus, the IP addresses are read backward. For example, suppose a host has the IP address 18.104.22.168. In the in-addr.arpa zone files, its address is listed as 22.214.171.124.in-addr.arpa. with the dot at the end indicating the root of the in-addr.arpa domain.