Asking Someone Else to Look Up the Phone Number (IP Address) for You

Instead of using your local phone book, you can always call the phone company and ask for help. Phone companies call this service directory assistance, and almost everyone else in the U.S. calls the service information. You just call directory assistance (dialing 411 in the U.S.), tell the operator the name of the person or business that you want to call, and the operator tells you the number.

The TCP/IP protocol called DNS behaves something like a phone company's directory assistance service. DNS defines protocols for the purpose of discovering which names correspond to which IP address. DNS also defines the structure and format of TCP/IP host names. Similar to the way that you can dial 411 in the U.S. and get help finding a phone number, a TCP/IP host can send a message to a DNS server to get help finding an IP address.

DNS defines how to figure out names and IP addresses for the entire Internet, as well as inside a single site at a single company. In the next sections, you'll first walk through the simpler processes used inside a single company; then you'll learn about DNS in the Internet and the conventions for what names look like.