THE URL and DOMAIN NAME SYSTEM
Saturday, November 22, 2008
URL stands for Uniform Resource Locator. The URL specifies the Internet address of a file stored on a host computer connected to the Internet. Every file on the Internet, no matter what its access protocol, has a unique URL. Web browsers use the URL to retrieve the file from the host computer and the specific directory in which it resides. This file is downloaded to the user's client computer and displayed on the monitor connected to the machine.
URLs are translated into numeric addresses using the Domain Name System (DNS). The DNS is a worldwide system of servers that stores location pointers to Web sites. The numeric address, called the IP (Internet Protocol) address, is actually the "real" URL. Since numeric strings are difficult for humans to use, alphneumeric addresses are employed by end users. Once the translation is made by the DNS, the browser can contact the Web server and ask for a specific file located on its site.
Anatomy of a URL
This is the format of the URL:
protocol://host/path/filename
For example, this is a URL on the Web site of the U.S. House of Representatives:
http://www.house.gov/house/2004_House_Calendar.html
This URL is typical of addresses hosted in domains in the
Structure of this URL:
- Protocol: http
- Host computer name: www
- Second-level domain name: house
- Top-level domain name: gov
- Directory name: house
- File name: 2004_House_Calendar_html
Note how much information about the content of the file is present in this well-constructed URL.
Several top-level domains (TLDs) are common in the
| com | commercial enterprise |
| edu | educational institution |
| gov | |
| mil | |
| net | network access provder |
| org | usually nonprofit organizations |
New domain names were approved in November 2000 by the Internet Corporation for Assigned Names and Numbers (ICANN): .biz, .museum, .info, .pro (for professionals) .name (for individuals), .aero (for the aerospace industry), and .coop (for cooperatives). ICANN continues to investigate proposals for addding additional domain names, for example, .mobi for sites designed for mobile devices, and .jobs for the human resources community.
In addition, dozens of domain names have been assigned to identify and locate files stored on host computers in countries around the world. These are referred to as two-letter Internet country codes, and have been standardized by the International Standards Organization as ISO 3166. For example:
| ch | |
| de | |
| jp | |
| | |
As the technology of the Web evolves, URLs have become more complex. This is especially the case when content is retrieved from databases and served onto Web pages. The resulting URLs can have a variety of elaborate structures, for example,
http://spills.incidentnews.gov/incidentnews/FMPro?-db=images&-Format=maps.htm&SpillLink=8&Subject=Waterway%20Closure%20Map&-SortField=EntryDate&-SortOrder=descend&-SortField=EntryTime&-SortOrder=descend&-Token=8&-Max=20&-Find
The first part of this URL looks familiar. What follows are search elements that query the database and determine the order of the results. As a growing number of databases serve content to the Web, these types of URLs will appear more commonly in your browser's address window.
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