A top-level domain (TLD) is the final part of a domain name -- the letters that come after the dot. The most famous is .com, but there are others -- .net, .org, .tv and .us, for example. But none holds as much allure as the .com extension.

As of May 2008, there were 76,007,285 active and registered .com domain names. This compares to 11,397,594 .net names, 6,772,308 .org names, 5,037,335 .info names, 1,968,760 .biz names, and 1,412,141 .us names. The .com extension clearly dominates.

There are a number of factors that have given the .com domain name its leadership spot among the top-level domains. One issue, of course, is that it was one of the first domain name extensions available (along with .net, .gov and .edu). Other extensions were added later, including .aero, .biz, .coop, .info, .museum, .name, .pro (Nov. 2000), and .mobi (2005). The .tv extension has existed since 1996 as a country code for Tuvalu. In 2000, the country struck a deal to make the extension widely available to people outside of the country, and in 2006, .tv was first widely marketed as an extension for the entertainment industry. So .com had a pretty significant head start.

But the biggest factor in the popularity of the .com extension is that .com was created to be the top-level domain for businesses. Domain name registrars -- the companies that register domain names -- typically offer a set of guidelines for choosing a domain name extension. The following descriptions are taken from the Network Solutions Web site, but any domain name registrar will offer a similar list:

• .com represents the word "commercial," and is the most widely used extension in the world. Most businesses prefer a .com domain name because it is a highly recognized symbol for having a business presence on the Internet.
• .net represents the word "network," and is most commonly used by Internet service providers, Web-hosting companies or other businesses that are directly involved in the infrastructure of the Internet. Additionally, some businesses choose domain names with a .net extension for their intranet Websites.
• .org represents the word "organization," and is primarily used by non-profits groups or trade associations.
• .biz is used for small business Web sites.
• .info is for credible resource Web sites and signifies a "resource" web site.
• .mobi (short for "mobile") is reserved for Web sites built for easy viewing on mobile devices.
• .us is for American Web sites and is the newest extension.
• .tv is for rich content/multimedia Web sites, commonly used within the entertainment or media industry.

Any commercial business, after reading this list, had to have a .com domain name. And as more businesses registered .com domains and marketed those names to consumers, more and more consumers began to identify .com as the de facto TLD.

At this point, the term "dotcom" has made it into the general lexicon in the U.S.

Dotcom: A company that embraces the internet as the key component in its business.

This definition has secured the .com extension’s top spot in the list of top-level domains.

Melissa Chang is the founder of Pure Incubation, an Internet incubator based in the Boston area. She blogs at 16th Letter.

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