As mentioned on the Networking page, every system on the Internet must have a unique IP address. (This does not include systems that are behind a NAT firewall because they are not directly on the Internet.) DNS acts as a directory service for all of these systems, allowing you to specify each one by its hostname. A telephone book allows you to look up an individual person by name and get their telephone number, their unique identifier on the telephone system's network. DNS allows you to look up individual server by name and get its IP address, its unique identifer on the Internet.
There are other hostname-to-IP directory services in use, mainly for LANs. Windows LANs can use WINS. UNIX LANs can use NIS. But because DNS is the directory service for the Internet (and can also be used for LANs) it is the most widely used. UNIX LANs could always use DNS instead of NIS, and starting with Windows 2000 Server, Windows LANs could use DNS instead of, or in addition to, WINS. And on small LANs where there are only a few machines you could just use HOSTS files on each system instead of setting up a server running DNS, NIS, or WINS.
As a service, DNS is critical to the operation of the Internet. When you enter www.some-domain.com in a Web browser, it's DNS that takes the www host name and translates it to an IP address. Without DNS, you could be connected to the Internet just fine, but you ain't goin' no where. Not unless you keep a record of the IP addresses of all of the resources you access on the Internet and use those instead of host/domain names.
Finding a single server out of all of the servers on the Internet is like trying to find a single file on drive with thousands of files. In both cases it helps to have some hierarchy built into the directory to logically group things. The DNS "namespace" is hierarchical in the same type of upside-down tree structure seen with file systems. Just as you have the root of a partition or drive, the DNS namespace has a root which is signified by a period.
There are other hostname-to-IP directory services in use, mainly for LANs. Windows LANs can use WINS. UNIX LANs can use NIS. But because DNS is the directory service for the Internet (and can also be used for LANs) it is the most widely used. UNIX LANs could always use DNS instead of NIS, and starting with Windows 2000 Server, Windows LANs could use DNS instead of, or in addition to, WINS. And on small LANs where there are only a few machines you could just use HOSTS files on each system instead of setting up a server running DNS, NIS, or WINS.
As a service, DNS is critical to the operation of the Internet. When you enter www.some-domain.com in a Web browser, it's DNS that takes the www host name and translates it to an IP address. Without DNS, you could be connected to the Internet just fine, but you ain't goin' no where. Not unless you keep a record of the IP addresses of all of the resources you access on the Internet and use those instead of host/domain names.
| DNS Basics |  |
Finding a single server out of all of the servers on the Internet is like trying to find a single file on drive with thousands of files. In both cases it helps to have some hierarchy built into the directory to logically group things. The DNS "namespace" is hierarchical in the same type of upside-down tree structure seen with file systems. Just as you have the root of a partition or drive, the DNS namespace has a root which is signified by a period.
