The Domain Name System (DNS) is a distributed database system that provides hostname-to-IP resource mapping (usually the IP address) and other information for computers on an internetwork. Any computer on the Internet can use a DNS server to locate any other computer on the Internet.

DNS is made up of two distinct components, the hierarchy and the name service. The DNS hierarchy specifies the structure, naming conventions, and delegation of authority in the DNS service. The DNS name service provides the actual name-to-address mapping mechanism.

Each organization that maintains a computer network will have at least one server which is handling DNS queries. That server is called a name server.
Each computer on each network needs to know the location of only one name server. When your computer requests an IP address, one of three things happens, depending on whether or not the requested IP address is within your local network:

1) If the requested IP address is registered locally (i.e., it’s within your organization’s network), you’ll receive a response directly from one of the local name servers listed in your workstation configuration.

2) If the requested IP address is not registered locally (i.e., outside your organization’s network), but someone within your organization has recently requested the same IP address, then the local name server will retrieve the IP address from its cache. There should be little or no wait for a response.

3) If the requested IP address is not registered locally, and when you are accesing it first time then the local name server will perform a search on behalf of your workstation. This search may involve querying two or more other name servers at potentially very remote locations. These queries can take anywhere from a second or two up to a minute (depending on how well connected you are to the remote network and how many intermediate name servers must be contacted). Sometimes, due to the lightweight protocol used for DNS, you may not receive a response. In these cases, your workstation or client software may continue to repeat the query until a response is received, or you may receive an error message.

When you use an application such as telnet to connect to another computer, you most likely type in the domain name rather than the IP address of that computer. The telnet application takes the domain name and uses one of the above methods to retrieve its corresponding IP address from the name server.