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Discussion of directories on the Internet
There are many discussions relating to the Internet that involve directories, such as the DNS and Napster. This essay is a general discussion about such directories.
There are many discussions relating to the Internet that involve directories, such as the DNS and Napster. This essay is a general discussion about such directories.

About directories
There are many types of directories on the Internet. The Internet as most of us use it works because of directories.

Directories of IP addresses
How do you find a particular piece of data or a particular service on the Internet? Well, in most cases, the data or service resides on another computer, and you need to communicate with that other computer.

Communication in most systems on the Internet is between computers that communicate with each other "directly" through special routing computers and communications lines. Those computers can get data to each other using the IP protocol that routes the data in packets addressed with IP addresses.

Diagram showing Server with Storage and Bandwidth, ISPs with Bandwidth, and Personal Computers, all connected
The IP addresses are related to the routing of messages to a particular computer. Depending on various situations, a particular computer's IP address can change. For example, it may be moved to another building, or the ISP to which it is connected may change how it is connected to the rest of the Internet. In addition, the particular computer that may be associated with what you are trying to connect to may change, so you'll need another IP address. (It is actually more complicated, because the connection to the computer is broken down further into sockets, which allows for multiple services on the same computer, and a given computer may have more than one IP address, etc., etc., but for our purposes we'll just look at IP addresses. That's complicated enough.)

When we choose which computer to connect our personal computer to for a particular reason we need to specify the current IP address of that computer. Because that address is not usually fixed, we usually use some means to find the address each time we want to connect (or at least each day or so, or after an error). The most common means is to use some sort of directory.

Once you find a particular computer, there are all sorts of ways to find the data on it.

Directories are just special-purpose servers on the Internet
Most commonly, the directories we use to find a particular computer are themselves hosted on other computers on the Internet. We just try to put those directories on computers that have (by convention) IP addresses that are easier to find, or are more stable, than what we are looking up. The knowledge of how to find those directories is built into many tools, such as browsers (the DNS directory) or Napster (its directory), or AOL Instant Messaging (AOL's directory). There is nothing magic about any specific system. Your browser could be built to look at other directories than the DNS (as IE was with RealNames and Netscape with its directory).

Directories are used on the Internet just like in the "real" world
This is no different than the "real" world. You communicate with people through means such as telephone, mail/delivery service, visiting them in person, etc. You find a person or business' current phone number or address through directories like the Yellow Pages, or your own address book. Nothing is permanent. People move and their number and address changes. Area codes and even Zip codes are changed without changing the physical address.

Sometimes the "key" you use to search a directory is unique, like a phone number. Sometimes it isn't unique, like a name in a phone book, and you need to use other information to choose which entry is appropriate. Different directories are set up for different purposes, and often there are multiple directories you can use to find the address you want. You often choose the directory to use based on what information you have as a key to search the directory and other properties of the directory.

For example, the Yellow Pages is sorted by business category, and then alphabetically by business name. Other information, such as address and optional marketing information (brands carried, hours of operation, years in business, etc.), is associated with each entry. You can find a business if you know what category it's in. You may or may not know (or care about) the name, location, hours, etc. The White Pages is different and is sorted purely by name, with extra information only about phone number and location, sometimes a minor field to say purpose (office, children's, etc.) when there are more than one number at the same location.

When you give someone information so they can find you, you may indicate which directory to use. For example, a national advertisement with "Our local office is listed in your Yellow Pages under 'Pest Control'". Sometimes you have choices. For example, if I know your name it may be faster to use the White Pages, but if I'm unsure of the spelling I might use the Yellow Pages.

We use many directories all the time on the Internet
On the Internet there are many types of directories. There is the DNS that is used to associate a (hopefully) permanent typing-friendly label with a current IP address. There are search engines and directories, like Google and Yahoo!, that let you find web pages or sites based on imprecise queries. There is the directory of current IP addresses of users of ICQ and AOL Instant Messaging.

Napster maintains a dynamic directory of the IP addresses of the users currently connected to their system, as well as a list of the names of the files each of those users offers to share. It needs the IP addresses to tell users how to connect to each other for sharing, since most user machines do not have IP address listings in other general directories. It does not host the files themselves, just information for accessing them.

In many P2P systems, there is a directory somewhere.

Directories are built on the Internet like any other application
All of these directories are separate from the Internet itself. They are just conventions we use to get addressing information for resources.

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