Every computer on the Internet is assigned a unique address, known as an Internet Protocol (IP) address. An IP address is 32 bits long and is written in the form aaa.bbb.ccc.ddd, where each value represents a number in the range of 0 to 255. When one computer sends a piece of information to another, it sends the message to the other computer’s IP address.
Since IP addresses are difficult to remember, people started assigning names to their computers. However, since the Internet was formed in a university environment where people don’t like giving up control of their domain, a hierarchical naming convention was eventually developed. This gave people the ability to name their computers whatever they wanted, yet provided a method to guarantee that the name would be unique across the Internet.
This technique is known as domain names. Domain names are mapped to an IP address, which in turn is used in the actual communications with the computer. Domain names are built from groups of characters separated by periods. For instance, www.JustPC.com is a valid domain name. The .com is the top-level domain. It is used to indicate that the computer is commercial. Some other top-level domains are .edu for education, .gov for government, .net for network, and .org for nonprofit organizations.
Other top-level domains may reflect geographic locations, such as .us for the United States, .ca for Canada, and .uk for United Kingdom. Within a country, mid-level domains may be used to further refine the address. For example, mail.bcpl.lib.md.us is a valid domain name for the Baltimore County Public Library in the state of Maryland in the United States.
The remainder of the address for both domains is up for grabs. An organization called InterNIC is currently responsible for managing the .com and .net top-level domains. Organizations may request any mid-level name and will be granted permission to use that name if it is not in use by someone else (and as long as they pay the appropriate registration fees).
Once you have a mid- and top-level name, you can choose your own machine names. In my case, I use www.JustPC.com as my web server, ftp.JustPC.com as my FTP server, and JustPC.com as my mail server.
Since all information sent via the Internet must use a valid IP address, a process must exist that will translate a domain name into an IP address. This process is called a Domain Name Server (DNS). The DNS takes a domain name and returns a valid IP address. If your computer is attached to the Internet, it either has a hard-coded IP address of a DNS or it receives this information dynamically when a connection is established, via a modem or when the computer is first turned on.