From a user perspective, hostname lookups are done just like they are with HOSTS files: An application calls a name-to-address lookup function, which rather than parse through the HOSTS file, it instead queries a nameserver, which returns the IP address of the destination system to the calling application. The difference between the two is that rather than be restricted to data in the local HOSTS file, the query can be distributed across the entire Internet.
Name-to-address lookup queries can be for local hosts, hosts in a remote domain, or hosts in a remote domain in another container domain (such as a host in the NOVELL.COM domain, or a host in the NOVELL.CO.UK domain). If a query is for a local host, the client will ask the local DNS server for the IP address of the destination system. Figure C.4 below illustrates this process.
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In order for this process to work, each client must be configured to support DNS, and must also be provided with two pieces of information. They must be given the name of the local domain that they are in ("UNOVERICA.COM" in this example), and the IP address of at least one nameserver responsible for that domain ("SERVER1" in our case).
Additionally, the local DNS server must have information about the hosts in the local domain in order for this to work properly. There are literally dozens of DNS server products on the market that each implement the underlying database technology differently, preventing any discussion of this technology here. It is sufficient to say that the DNS servers must have access to a database of the hostnames and corresponding IP addresses within the local domain in order for them to fulfill any queries they receive.
For more information about how DNS works across the Internet, refer to section C.3.1.2 External Lookups. For more information on the data that is stored in DNS, refer to section C.3.2 DNS Resource Records.
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