DNS (Domain Name Service) is the mechanism by which a program running on your host computer can locate the address of other hosts on the Internet, and by which other hosts on the Internet can locate you. The DNS essentially provides a map of the structure of the Internet.
Organizations must register a domain name with the InterNIC and obtain addresses to use for the hosts in their domain. For example, ipswitch.com is a registered domain name, and some addresses assigned to ipswitch.com are 156.21.50.1 through 156.21.50.255. For information about registering a domain name, see the InterNIC’s Web site at http://www.internic.net.
All hosts on the Internet must have a host name and an IP (Internet Protocol) address. You can give a host any host name you want, as long as it is unique within your domain. For example, some host names and addresses in the ipswitch.com network are:
test1.ipswitch.com 156.21.50.1
test2.ipswitch.com 156.21.50.2
test3.ipswitch.com 156.21.50.3
DNS servers provide the mapping of host names to their addresses. The DNS server for ipswitch.com lists each Ipswitch host and its corresponding address. Thus, any host outside of ipswitch.com can query the DNS server for ipswitch.com to find the address of a particular host. Once it has the address, the requesting application can communicate directly with the host. Note that querying a DNS server is also called a “DNS lookup” or a “lookup.”
When a host outside ipswitch.com wants to send mail to a user on the ipswitch.com network, it queries the DNS server for ipswitch.com to find the mail server for users on ipswitch.com. The host can then send mail to the mail server, which will deliver it to the appropriate user.